kernel/

process_standard.rs

// Licensed under the Apache License, Version 2.0 or the MIT License.
// SPDX-License-Identifier: Apache-2.0 OR MIT
// Copyright Tock Contributors 2022.

//! Tock default Process implementation.
//!
//! `ProcessStandard` is an implementation for a userspace process running on
//! the Tock kernel.

use core::cell::Cell;
use core::cmp;
use core::fmt::Write;
use core::mem::MaybeUninit;
use core::num::NonZeroU32;
use core::ptr::NonNull;
use core::{mem, ptr, slice, str};

use crate::collections::queue::Queue;
use crate::collections::ring_buffer::RingBuffer;
use crate::config;
use crate::debug;
use crate::errorcode::ErrorCode;
use crate::init_uninit_struct;
use crate::kernel::Kernel;
use crate::platform::chip::Chip;
use crate::platform::mpu::{self, MPU};
use crate::process::ProcessBinary;
use crate::process::{BinaryVersion, ReturnArguments};
use crate::process::{Error, FunctionCall, FunctionCallSource, Process, Task};
use crate::process::{FaultAction, ProcessCustomGrantIdentifier, ProcessId};
use crate::process::{ProcessAddresses, ProcessSizes, ShortId};
use crate::process::{State, StoppedState};
use crate::process_checker::AcceptedCredential;
use crate::process_loading::ProcessLoadError;
use crate::process_policies::ProcessFaultPolicy;
use crate::process_policies::ProcessStandardStoragePermissionsPolicy;
use crate::processbuffer::{ReadOnlyProcessBuffer, ReadWriteProcessBuffer};
use crate::storage_permissions::StoragePermissions;
use crate::syscall::{self, Syscall, SyscallReturn, UserspaceKernelBoundary};
use crate::upcall::UpcallId;
use crate::utilities::capability_ptr::{CapabilityPtr, CapabilityPtrPermissions};
use crate::utilities::cells::{MapCell, NumericCellExt, OptionalCell};

use tock_tbf::types::CommandPermissions;

/// Gets a mutable (unique) reference to the contained value.
///
/// TODO: this is copied from the standard library, where it is available under
/// the `maybe_uninit_slice` nightly feature. Remove and switch to the core
/// library variant once that is stable.
///
/// # Safety
///
/// Calling this when the content is not yet fully initialized causes undefined
/// behavior: it is up to the caller to guarantee that every `MaybeUninit<T>` in the
/// slice really is in an initialized state. For instance, `.assume_init_mut()` cannot
/// be used to initialize a `MaybeUninit` slice.
#[inline(always)]
const unsafe fn maybe_uninit_slice_assume_init_mut<T>(src: &mut [MaybeUninit<T>]) -> &mut [T] {
    // SAFETY: similar to safety notes for `slice_get_ref`, but we have a
    // mutable reference which is also guaranteed to be valid for writes.
    #[allow(clippy::ref_as_ptr)]
    unsafe {
        &mut *(src as *mut [MaybeUninit<T>] as *mut [T])
    }
}

/// Divides one mutable raw slice into two at an index.
///
/// This method implementation is copied from the standard library, where it is
/// available with `raw_slice_split` nightly feature. TODO: switch to the
/// standard library function once that is stable.
///
/// The first will contain all indices from `[0, mid)` (excluding the index
/// `mid` itself) and the second will contain all indices from `[mid, len)`
/// (excluding the index `len` itself).
///
/// # Panics
///
/// Panics if `mid > len`.
///
/// # Safety
///
/// `mid` must be [in-bounds] of the underlying [allocation].  Which means
/// `self` must be dereferenceable and span a single allocation that is at least
/// `mid * size_of::<T>()` bytes long. Not upholding these requirements is
/// *[undefined behavior]* even if the resulting pointers are not used.
///
/// Since `len` being in-bounds is not a safety invariant of `*mut [T]` the
/// safety requirements of this method are the same as for
/// [`split_at_mut_unchecked`].  The explicit bounds check is only as useful as
/// `len` is correct.
///
/// [`split_at_mut_unchecked`]: https://doc.rust-lang.org/stable/std/primitive.pointer.html#method.split_at_mut_unchecked
/// [in-bounds]: https://doc.rust-lang.org/stable/std/primitive.pointer.html#method.add-1
/// [allocation]: https://doc.rust-lang.org/stable/std/ptr/index.html#allocation
/// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
unsafe fn raw_slice_split_at_mut<T>(slice: *mut [T], mid: usize) -> (*mut [T], *mut [T]) {
    assert!(mid <= slice.len());

    let len = slice.len();
    let ptr = slice.cast::<T>();

    // SAFETY: Caller must pass a valid pointer and an index that is in-bounds.
    let tail = unsafe { ptr.add(mid) };
    (
        core::ptr::slice_from_raw_parts_mut(ptr, mid),
        core::ptr::slice_from_raw_parts_mut(tail, len - mid),
    )
}

/// Interface supported by [`ProcessStandard`] for recording debug information.
///
/// This trait provides flexibility to users of [`ProcessStandard`] to determine
/// how debugging information should be recorded, or if debugging information
/// should be recorded at all.
///
/// Platforms that want to only maintain certain debugging information can
/// implement only part of this trait.
///
/// Tock provides a default implementation of this trait on the `()` type.
/// Kernels that wish to use [`ProcessStandard`] but do not need process-level
/// debugging information can use `()` as the `ProcessStandardDebug` type.
pub trait ProcessStandardDebug: Default {
    /// Record the address in flash the process expects to start at.
    fn set_fixed_address_flash(&self, address: u32);
    /// Get the address in flash the process expects to start at, if it was
    /// recorded.
    fn get_fixed_address_flash(&self) -> Option<u32>;
    /// Record the address in RAM the process expects to start at.
    fn set_fixed_address_ram(&self, address: u32);
    /// Get the address in RAM the process expects to start at, if it was
    /// recorded.
    fn get_fixed_address_ram(&self) -> Option<u32>;
    /// Record the address where the process placed its heap.
    fn set_app_heap_start_pointer(&self, ptr: *const u8);
    /// Get the address where the process placed its heap, if it was recorded.
    fn get_app_heap_start_pointer(&self) -> Option<*const u8>;
    /// Record the address where the process placed its stack.
    fn set_app_stack_start_pointer(&self, ptr: *const u8);
    /// Get the address where the process placed its stack, if it was recorded.
    fn get_app_stack_start_pointer(&self) -> Option<*const u8>;
    /// Update the lowest address that the process's stack has reached.
    fn set_app_stack_min_pointer(&self, ptr: *const u8);
    /// Get the lowest address of the process's stack , if it was recorded.
    fn get_app_stack_min_pointer(&self) -> Option<*const u8>;
    /// Provide the current address of the bottom of the stack and record the
    /// address if it is the lowest address that the process's stack has
    /// reached.
    fn set_new_app_stack_min_pointer(&self, ptr: *const u8);

    /// Record the most recent system call the process called.
    fn set_last_syscall(&self, syscall: Syscall);
    /// Get the most recent system call the process called, if it was recorded.
    fn get_last_syscall(&self) -> Option<Syscall>;
    /// Clear any record of the most recent system call the process called.
    fn reset_last_syscall(&self);

    /// Increase the recorded count of the number of system calls the process
    /// has called.
    fn increment_syscall_count(&self);
    /// Get the recorded count of the number of system calls the process has
    /// called.
    ///
    /// This should return 0 if
    /// [`ProcessStandardDebug::increment_syscall_count()`] is never called.
    fn get_syscall_count(&self) -> usize;
    /// Reset the recorded count of the number of system calls called by the app
    /// to 0.
    fn reset_syscall_count(&self);

    /// Increase the recorded count of the number of upcalls that have been
    /// dropped for the process.
    fn increment_dropped_upcall_count(&self);
    /// Get the recorded count of the number of upcalls that have been dropped
    /// for the process.
    ///
    /// This should return 0 if
    /// [`ProcessStandardDebug::increment_dropped_upcall_count()`] is never
    /// called.
    fn get_dropped_upcall_count(&self) -> usize;
    /// Reset the recorded count of the number of upcalls that have been dropped
    /// for the process to 0.
    fn reset_dropped_upcall_count(&self);

    /// Increase the recorded count of the number of times the process has
    /// exceeded its timeslice.
    fn increment_timeslice_expiration_count(&self);
    /// Get the recorded count of the number times the process has exceeded its
    /// timeslice.
    ///
    /// This should return 0 if
    /// [`ProcessStandardDebug::increment_timeslice_expiration_count()`] is
    /// never called.
    fn get_timeslice_expiration_count(&self) -> usize;
    /// Reset the recorded count of the number of the process has exceeded its
    /// timeslice to 0.
    fn reset_timeslice_expiration_count(&self);
}

/// A debugging implementation for [`ProcessStandard`] that records the full
/// debugging state.
pub struct ProcessStandardDebugFull {
    /// Inner field for the debug state that is in a [`MapCell`] to provide
    /// mutable access.
    debug: MapCell<ProcessStandardDebugFullInner>,
}

/// Struct for debugging [`ProcessStandard`] processes that records the full set
/// of debugging information.
///
/// These pointers and counters are not strictly required for kernel operation,
/// but provide helpful information when an app crashes.
#[derive(Default)]
struct ProcessStandardDebugFullInner {
    /// If this process was compiled for fixed addresses, save the address
    /// it must be at in flash. This is useful for debugging and saves having
    /// to re-parse the entire TBF header.
    fixed_address_flash: Option<u32>,

    /// If this process was compiled for fixed addresses, save the address
    /// it must be at in RAM. This is useful for debugging and saves having
    /// to re-parse the entire TBF header.
    fixed_address_ram: Option<u32>,

    /// Where the process has started its heap in RAM.
    app_heap_start_pointer: Option<*const u8>,

    /// Where the start of the stack is for the process. If the kernel does the
    /// PIC setup for this app then we know this, otherwise we need the app to
    /// tell us where it put its stack.
    app_stack_start_pointer: Option<*const u8>,

    /// How low have we ever seen the stack pointer.
    app_stack_min_pointer: Option<*const u8>,

    /// How many syscalls have occurred since the process started.
    syscall_count: usize,

    /// What was the most recent syscall.
    last_syscall: Option<Syscall>,

    /// How many upcalls were dropped because the queue was insufficiently
    /// long.
    dropped_upcall_count: usize,

    /// How many times this process has been paused because it exceeded its
    /// timeslice.
    timeslice_expiration_count: usize,
}

impl ProcessStandardDebug for ProcessStandardDebugFull {
    fn set_fixed_address_flash(&self, address: u32) {
        self.debug.map(|d| d.fixed_address_flash = Some(address));
    }
    fn get_fixed_address_flash(&self) -> Option<u32> {
        self.debug.map_or(None, |d| d.fixed_address_flash)
    }
    fn set_fixed_address_ram(&self, address: u32) {
        self.debug.map(|d| d.fixed_address_ram = Some(address));
    }
    fn get_fixed_address_ram(&self) -> Option<u32> {
        self.debug.map_or(None, |d| d.fixed_address_ram)
    }
    fn set_app_heap_start_pointer(&self, ptr: *const u8) {
        self.debug.map(|d| d.app_heap_start_pointer = Some(ptr));
    }
    fn get_app_heap_start_pointer(&self) -> Option<*const u8> {
        self.debug.map_or(None, |d| d.app_heap_start_pointer)
    }
    fn set_app_stack_start_pointer(&self, ptr: *const u8) {
        self.debug.map(|d| d.app_stack_start_pointer = Some(ptr));
    }
    fn get_app_stack_start_pointer(&self) -> Option<*const u8> {
        self.debug.map_or(None, |d| d.app_stack_start_pointer)
    }
    fn set_app_stack_min_pointer(&self, ptr: *const u8) {
        self.debug.map(|d| d.app_stack_min_pointer = Some(ptr));
    }
    fn get_app_stack_min_pointer(&self) -> Option<*const u8> {
        self.debug.map_or(None, |d| d.app_stack_min_pointer)
    }
    fn set_new_app_stack_min_pointer(&self, ptr: *const u8) {
        self.debug.map(|d| {
            match d.app_stack_min_pointer {
                None => d.app_stack_min_pointer = Some(ptr),
                Some(asmp) => {
                    // Update max stack depth if needed.
                    if ptr < asmp {
                        d.app_stack_min_pointer = Some(ptr);
                    }
                }
            }
        });
    }

    fn set_last_syscall(&self, syscall: Syscall) {
        self.debug.map(|d| d.last_syscall = Some(syscall));
    }
    fn get_last_syscall(&self) -> Option<Syscall> {
        self.debug.map_or(None, |d| d.last_syscall)
    }
    fn reset_last_syscall(&self) {
        self.debug.map(|d| d.last_syscall = None);
    }

    fn increment_syscall_count(&self) {
        self.debug.map(|d| d.syscall_count += 1);
    }
    fn get_syscall_count(&self) -> usize {
        self.debug.map_or(0, |d| d.syscall_count)
    }
    fn reset_syscall_count(&self) {
        self.debug.map(|d| d.syscall_count = 0);
    }

    fn increment_dropped_upcall_count(&self) {
        self.debug.map(|d| d.dropped_upcall_count += 1);
    }
    fn get_dropped_upcall_count(&self) -> usize {
        self.debug.map_or(0, |d| d.dropped_upcall_count)
    }
    fn reset_dropped_upcall_count(&self) {
        self.debug.map(|d| d.dropped_upcall_count = 0);
    }

    fn increment_timeslice_expiration_count(&self) {
        self.debug.map(|d| d.timeslice_expiration_count += 1);
    }
    fn get_timeslice_expiration_count(&self) -> usize {
        self.debug.map_or(0, |d| d.timeslice_expiration_count)
    }
    fn reset_timeslice_expiration_count(&self) {
        self.debug.map(|d| d.timeslice_expiration_count = 0);
    }
}

impl Default for ProcessStandardDebugFull {
    fn default() -> Self {
        Self {
            debug: MapCell::new(ProcessStandardDebugFullInner::default()),
        }
    }
}

impl ProcessStandardDebug for () {
    fn set_fixed_address_flash(&self, _address: u32) {}
    fn get_fixed_address_flash(&self) -> Option<u32> {
        None
    }
    fn set_fixed_address_ram(&self, _address: u32) {}
    fn get_fixed_address_ram(&self) -> Option<u32> {
        None
    }
    fn set_app_heap_start_pointer(&self, _ptr: *const u8) {}
    fn get_app_heap_start_pointer(&self) -> Option<*const u8> {
        None
    }
    fn set_app_stack_start_pointer(&self, _ptr: *const u8) {}
    fn get_app_stack_start_pointer(&self) -> Option<*const u8> {
        None
    }
    fn set_app_stack_min_pointer(&self, _ptr: *const u8) {}
    fn get_app_stack_min_pointer(&self) -> Option<*const u8> {
        None
    }
    fn set_new_app_stack_min_pointer(&self, _ptr: *const u8) {}

    fn set_last_syscall(&self, _syscall: Syscall) {}
    fn get_last_syscall(&self) -> Option<Syscall> {
        None
    }
    fn reset_last_syscall(&self) {}

    fn increment_syscall_count(&self) {}
    fn get_syscall_count(&self) -> usize {
        0
    }
    fn reset_syscall_count(&self) {}
    fn increment_dropped_upcall_count(&self) {}
    fn get_dropped_upcall_count(&self) -> usize {
        0
    }
    fn reset_dropped_upcall_count(&self) {}
    fn increment_timeslice_expiration_count(&self) {}
    fn get_timeslice_expiration_count(&self) -> usize {
        0
    }
    fn reset_timeslice_expiration_count(&self) {}
}

/// Entry that is stored in the grant pointer table at the top of process
/// memory.
///
/// One copy of this entry struct is stored per grant region defined in the
/// kernel. This type allows the core kernel to lookup a grant based on the
/// driver_num associated with the grant, and also holds the pointer to the
/// memory allocated for the particular grant.
#[repr(C)]
struct GrantPointerEntry {
    /// The syscall driver number associated with the allocated grant.
    ///
    /// This defaults to 0 if the grant has not been allocated. Note, however,
    /// that 0 is a valid driver_num, and therefore cannot be used to check if a
    /// grant is allocated or not.
    driver_num: usize,

    /// The start of the memory location where the grant has been allocated, or
    /// null if the grant has not been allocated.
    grant_ptr: *mut u8,
}

/// A type for userspace processes in Tock.
///
/// As its name implies, this is the standard implementation for Tock processes
/// that exposes the full support for processes running on embedded hardware.
///
/// [`ProcessStandard`] is templated on two parameters:
///
/// - `C`: [`Chip`]: The implementation must know the [`Chip`] the kernel is
///   running on to properly store architecture-specific and MPU state for the
///   process.
/// - `D`: [`ProcessStandardDebug`]: This configures the debugging mechanism the
///   process uses for storing optional debugging data. Kernels that do not wish
///   to store per-process debugging state can use the `()` type for this
///   parameter.
pub struct ProcessStandard<'a, C: 'static + Chip, D: 'static + ProcessStandardDebug + Default> {
    /// Identifier of this process and the index of the process in the process
    /// table.
    process_id: Cell<ProcessId>,

    /// An application ShortId, generated from process loading and
    /// checking, which denotes the security identity of this process.
    app_id: ShortId,

    /// Pointer to the main Kernel struct.
    kernel: &'static Kernel,

    /// Pointer to the struct that defines the actual chip the kernel is running
    /// on. This is used because processes have subtle hardware-based
    /// differences. Specifically, the actual syscall interface and how
    /// processes are switched to is architecture-specific, and how memory must
    /// be allocated for memory protection units is also hardware-specific.
    chip: &'static C,

    /// Application memory layout:
    ///
    /// ```text
    ///     ╒════════ ← memory_start + memory_len
    ///  ╔═ │ Grant Pointers
    ///  ║  │ ──────
    ///  ║  │ Upcall Queue
    ///  ║  │ ──────
    ///     │ Process Control Block
    ///  D  │ ──────
    ///  Y  │ Grant Regions
    ///  N  │
    ///  A  │   ↓
    ///  M  │ ──────  ← kernel_memory_break
    ///  I  │
    ///  C  │ ──────  ← app_break               ═╗
    ///     │                                    ║
    ///  ║  │   ↑                                  A
    ///  ║  │  Heap                              P C
    ///  ╠═ │ ──────  ← app_heap_start           R C
    ///     │  Data                              O E
    ///  F  │ ──────  ← data_start_pointer       C S
    ///  I  │ Stack                              E S
    ///  X  │   ↓                                S I
    ///  E  │                                    S B
    ///  D  │ ──────  ← current_stack_pointer      L
    ///     │                                    ║ E
    ///  ╚═ ╘════════ ← memory_start            ═╝
    /// ```
    ///
    /// The start of process memory. We store this as a pointer and length and
    /// not a slice due to Rust aliasing rules. If we were to store a slice,
    /// then any time another slice to the same memory or an ProcessBuffer is
    /// used in the kernel would be undefined behavior.
    memory_start: *const u8,
    /// Number of bytes of memory allocated to this process.
    memory_len: usize,

    /// Reference to the slice of `GrantPointerEntry`s stored in the process's
    /// memory reserved for the kernel. These driver numbers are zero and
    /// pointers are null if the grant region has not been allocated. When the
    /// grant region is allocated these pointers are updated to point to the
    /// allocated memory and the driver number is set to match the driver that
    /// owns the grant. No other reference to these pointers exists in the Tock
    /// kernel.
    grant_pointers: MapCell<&'static mut [GrantPointerEntry]>,

    /// Pointer to the end of the allocated (and MPU protected) grant region.
    kernel_memory_break: Cell<*const u8>,

    /// Pointer to the end of process RAM that has been sbrk'd to the process.
    app_break: Cell<*const u8>,

    /// Pointer to high water mark for process buffers shared through `allow`
    allow_high_water_mark: Cell<*const u8>,

    /// Process flash segment. This is the region of nonvolatile flash that
    /// the process occupies.
    flash: &'static [u8],

    /// The footers of the process binary (may be zero-sized), which are metadata
    /// about the process not covered by integrity. Used, among other things, to
    /// store signatures.
    footers: &'static [u8],

    /// Collection of pointers to the TBF header in flash.
    header: tock_tbf::types::TbfHeader<'static>,

    /// Credential that was approved for this process, or `None` if the
    /// credential was permitted to run without an accepted credential.
    credential: Option<AcceptedCredential>,

    /// State saved on behalf of the process each time the app switches to the
    /// kernel.
    stored_state:
        MapCell<<<C as Chip>::UserspaceKernelBoundary as UserspaceKernelBoundary>::StoredState>,

    /// The current state of the app. The scheduler uses this to determine
    /// whether it can schedule this app to execute.
    ///
    /// The `state` is used both for bookkeeping for the scheduler as well as
    /// for enabling control by other parts of the system. The scheduler keeps
    /// track of if a process is ready to run or not by switching between the
    /// `Running` and `Yielded` states. The system can control the process by
    /// switching it to a "stopped" state to prevent the scheduler from
    /// scheduling it.
    state: Cell<State>,

    /// How to respond if this process faults.
    fault_policy: &'a dyn ProcessFaultPolicy,

    /// Storage permissions for this process.
    ///
    /// This is stored in a `Cell` because we need to create the
    /// [`ProcessStandard`] first to then later determine the storage
    /// permissions.
    storage_permissions: Cell<StoragePermissions>,

    /// Configuration data for the MPU
    mpu_config: MapCell<<<C as Chip>::MPU as MPU>::MpuConfig>,

    /// MPU regions are saved as a pointer-size pair.
    mpu_regions: [Cell<Option<mpu::Region>>; 6],

    /// Essentially a list of upcalls that want to call functions in the
    /// process.
    tasks: MapCell<RingBuffer<'a, Task>>,

    /// Count of how many times this process has entered the fault condition and
    /// been restarted. This is used by some `ProcessRestartPolicy`s to
    /// determine if the process should be restarted or not.
    restart_count: Cell<usize>,

    /// The completion code set by the process when it last exited, restarted,
    /// or was terminated. If the process is has never terminated, then the
    /// `OptionalCell` will be empty (i.e. `None`). If the process has exited,
    /// restarted, or terminated, the `OptionalCell` will contain an optional 32
    /// bit value. The option will be `None` if the process crashed or was
    /// stopped by the kernel and there is no provided completion code. If the
    /// process called the exit syscall then the provided completion code will
    /// be stored as `Some(completion code)`.
    completion_code: OptionalCell<Option<u32>>,

    /// Flag that stores whether this process has a task that is ready when
    /// the process is in the [`State::YieldedFor`] state.
    is_yield_wait_for_ready: Cell<bool>,

    /// Values kept so that we can print useful debug messages when apps fault.
    debug: D,
}

impl<C: Chip, D: 'static + ProcessStandardDebug> Process for ProcessStandard<'_, C, D> {
    fn processid(&self) -> ProcessId {
        self.process_id.get()
    }

    fn short_app_id(&self) -> ShortId {
        self.app_id
    }

    fn binary_version(&self) -> Option<BinaryVersion> {
        let version = self.header.get_binary_version();
        match NonZeroU32::new(version) {
            Some(version_nonzero) => Some(BinaryVersion::new(version_nonzero)),
            None => None,
        }
    }

    fn get_credential(&self) -> Option<AcceptedCredential> {
        self.credential
    }

    fn enqueue_task(&self, task: Task) -> Result<(), ErrorCode> {
        // If this app is in a `Fault` state then we shouldn't schedule
        // any work for it.
        if !self.is_running() {
            return Err(ErrorCode::NODEVICE);
        }

        let ret = self.tasks.map_or(Err(ErrorCode::FAIL), |tasks| {
            match tasks.enqueue(task) {
                true => {
                    // If the process is yielded-for this task, set the ready flag.
                    if let State::YieldedFor(yielded_upcall_id) = self.state.get() {
                        if let Some(upcall_id) = match task {
                            Task::FunctionCall(FunctionCall {
                                source: FunctionCallSource::Driver(upcall_id),
                                ..
                            }) => Some(upcall_id),
                            Task::ReturnValue(ReturnArguments { upcall_id, .. }) => Some(upcall_id),
                            _ => None,
                        } {
                            self.is_yield_wait_for_ready
                                .set(upcall_id == yielded_upcall_id);
                        }
                    }
                    // The task has been successfully enqueued.
                    Ok(())
                }
                false => {
                    // The task could not be enqueued as there is
                    // insufficient space in the ring buffer.
                    Err(ErrorCode::NOMEM)
                }
            }
        });

        if ret.is_err() {
            // On any error we were unable to enqueue the task. Record the
            // error, but importantly do _not_ increment kernel work.
            self.debug.increment_dropped_upcall_count();
        }

        ret
    }

    fn ready(&self) -> bool {
        match self.state.get() {
            State::Running => true,
            State::YieldedFor(_) => self.is_yield_wait_for_ready.get(),
            State::Yielded => self.tasks.map_or(false, |ring_buf| ring_buf.has_elements()),
            _ => false,
        }
    }

    fn remove_pending_upcalls(&self, upcall_id: UpcallId) -> usize {
        self.tasks.map_or(0, |tasks| {
            let count_before = tasks.len();
            tasks.retain(|task| match task {
                // Remove only tasks that are function calls with an id equal
                // to `upcall_id`.
                Task::FunctionCall(function_call) => match function_call.source {
                    FunctionCallSource::Kernel => true,
                    FunctionCallSource::Driver(id) => id != upcall_id,
                },
                _ => true,
            });
            let count_after = tasks.len();
            if config::CONFIG.trace_syscalls {
                debug!(
                    "[{:?}] remove_pending_upcalls[{:#x}:{}] = {} upcall(s) removed",
                    self.processid(),
                    upcall_id.driver_num,
                    upcall_id.subscribe_num,
                    count_before - count_after,
                );
            }
            count_before - count_after
        })
    }

    fn is_running(&self) -> bool {
        match self.state.get() {
            State::Running | State::Yielded | State::YieldedFor(_) | State::Stopped(_) => true,
            _ => false,
        }
    }

    fn get_state(&self) -> State {
        self.state.get()
    }

    fn set_yielded_state(&self) {
        if self.state.get() == State::Running {
            self.state.set(State::Yielded);
        }
    }

    fn set_yielded_for_state(&self, upcall_id: UpcallId) {
        if self.state.get() == State::Running {
            self.state.set(State::YieldedFor(upcall_id));

            // Verify if the process has a task that this yield waits for
            self.is_yield_wait_for_ready
                .set(self.tasks.map_or(false, |tasks| {
                    tasks
                        .find_first_matching(|task| match task {
                            Task::ReturnValue(ReturnArguments { upcall_id: id, .. }) => {
                                upcall_id == *id
                            }
                            Task::FunctionCall(FunctionCall {
                                source: FunctionCallSource::Driver(id),
                                ..
                            }) => upcall_id == *id,
                            _ => false,
                        })
                        .is_some()
                }));
        }
    }

    fn stop(&self) {
        match self.state.get() {
            State::Running => self.state.set(State::Stopped(StoppedState::Running)),
            State::Yielded => self.state.set(State::Stopped(StoppedState::Yielded)),
            State::YieldedFor(upcall_id) => self
                .state
                .set(State::Stopped(StoppedState::YieldedFor(upcall_id))),
            State::Stopped(_stopped_state) => {
                // Already stopped, nothing to do.
            }
            State::Faulted | State::Terminated => {
                // Stop has no meaning on a inactive process.
            }
        }
    }

    fn resume(&self) {
        if let State::Stopped(stopped_state) = self.state.get() {
            match stopped_state {
                StoppedState::Running => self.state.set(State::Running),
                StoppedState::Yielded => self.state.set(State::Yielded),
                StoppedState::YieldedFor(upcall_id) => self.set_yielded_for_state(upcall_id),
            }
        }
    }

    fn set_fault_state(&self) {
        // Use the per-process fault policy to determine what action the kernel
        // should take since the process faulted.
        let action = self.fault_policy.action(self);
        match action {
            FaultAction::Panic => {
                // process faulted. Panic and print status
                self.state.set(State::Faulted);
                panic!("Process {} had a fault", self.get_process_name());
            }
            FaultAction::Restart => {
                self.try_restart(None);
            }
            FaultAction::Stop => {
                // This looks a lot like restart, except we just leave the app
                // how it faulted and mark it as `Faulted`. By clearing
                // all of the app's todo work it will not be scheduled, and
                // clearing all of the grant regions will cause capsules to drop
                // this app as well.
                self.terminate(None);
                self.state.set(State::Faulted);
            }
        }
    }

    fn start(&self, _cap: &dyn crate::capabilities::ProcessStartCapability) {
        // `start()` can only be called on a terminated process.
        if self.get_state() != State::Terminated {
            return;
        }

        // Reset to start the process.
        if let Ok(()) = self.reset() {
            self.state.set(State::Yielded);
        }
    }

    fn try_restart(&self, completion_code: Option<u32>) {
        // `try_restart()` cannot be called if the process is terminated. Only
        // `start()` can start a terminated process.
        if self.get_state() == State::Terminated {
            return;
        }

        // Terminate the process, freeing its state and removing any
        // pending tasks from the scheduler's queue.
        self.terminate(completion_code);

        // If there is a kernel policy that controls restarts, it should be
        // implemented here. For now, always restart.
        if let Ok(()) = self.reset() {
            self.state.set(State::Yielded);
        }

        // Decide what to do with res later. E.g., if we can't restart
        // want to reclaim the process resources.
    }

    fn terminate(&self, completion_code: Option<u32>) {
        // A process can be terminated if it is running or in the `Faulted`
        // state. Otherwise, you cannot terminate it and this method return
        // early.
        //
        // The kernel can terminate in the `Faulted` state to return the process
        // to a state in which it can run again (e.g., reset it).
        if !self.is_running() && self.get_state() != State::Faulted {
            return;
        }

        // And remove those tasks
        self.tasks.map(|tasks| {
            tasks.empty();
        });

        // Clear any grant regions this app has setup with any capsules.
        unsafe {
            self.grant_ptrs_reset();
        }

        // Save the completion code.
        self.completion_code.set(completion_code);

        // Mark the app as stopped so the scheduler won't try to run it.
        self.state.set(State::Terminated);
    }

    fn get_restart_count(&self) -> usize {
        self.restart_count.get()
    }

    fn has_tasks(&self) -> bool {
        self.tasks.map_or(false, |tasks| tasks.has_elements())
    }

    fn dequeue_task(&self) -> Option<Task> {
        self.tasks.map_or(None, |tasks| tasks.dequeue())
    }

    fn remove_upcall(&self, upcall_id: UpcallId) -> Option<Task> {
        self.tasks.map_or(None, |tasks| {
            tasks.remove_first_matching(|task| match task {
                Task::FunctionCall(fc) => match fc.source {
                    FunctionCallSource::Driver(upid) => upid == upcall_id,
                    _ => false,
                },
                Task::ReturnValue(rv) => rv.upcall_id == upcall_id,
                Task::IPC(_) => false,
            })
        })
    }

    fn pending_tasks(&self) -> usize {
        self.tasks.map_or(0, |tasks| tasks.len())
    }

    fn get_command_permissions(&self, driver_num: usize, offset: usize) -> CommandPermissions {
        self.header.get_command_permissions(driver_num, offset)
    }

    fn get_storage_permissions(&self) -> StoragePermissions {
        self.storage_permissions.get()
    }

    fn number_writeable_flash_regions(&self) -> usize {
        self.header.number_writeable_flash_regions()
    }

    fn get_writeable_flash_region(&self, region_index: usize) -> (usize, usize) {
        self.header.get_writeable_flash_region(region_index)
    }

    fn update_stack_start_pointer(&self, stack_pointer: *const u8) {
        if stack_pointer >= self.mem_start() && stack_pointer < self.mem_end() {
            self.debug.set_app_stack_start_pointer(stack_pointer);
            // We also reset the minimum stack pointer because whatever
            // value we had could be entirely wrong by now.
            self.debug.set_app_stack_min_pointer(stack_pointer);
        }
    }

    fn update_heap_start_pointer(&self, heap_pointer: *const u8) {
        if heap_pointer >= self.mem_start() && heap_pointer < self.mem_end() {
            self.debug.set_app_heap_start_pointer(heap_pointer);
        }
    }

    fn setup_mpu(&self) {
        self.mpu_config.map(|config| {
            // # Safety
            //
            // `configure_mpu` is unsafe, as invoking it with an incorrect
            // configuration can allow an untrusted application to access
            // kernel-private memory.
            //
            // This call is safe given we trust that the implementation of
            // `ProcessStandard` correctly provisions a set of MPU regions that
            // does not grant access to any kernel-private memory, and
            // `ProcessStandard` does not provide safe, publically accessible
            // APIs to add other arbitrary MPU regions to this configuration.
            unsafe {
                self.chip.mpu().configure_mpu(config);
            }
        });
    }

    fn add_mpu_region(
        &self,
        unallocated_memory_start: *const u8,
        unallocated_memory_size: usize,
        min_region_size: usize,
    ) -> Option<mpu::Region> {
        self.mpu_config.and_then(|config| {
            let new_region = self.chip.mpu().allocate_region(
                unallocated_memory_start,
                unallocated_memory_size,
                min_region_size,
                mpu::Permissions::ReadWriteOnly,
                config,
            )?;

            for region in self.mpu_regions.iter() {
                if region.get().is_none() {
                    region.set(Some(new_region));
                    return Some(new_region);
                }
            }

            // Not enough room in Process struct to store the MPU region.
            None
        })
    }

    fn remove_mpu_region(&self, region: mpu::Region) -> Result<(), ErrorCode> {
        self.mpu_config.map_or(Err(ErrorCode::INVAL), |config| {
            // Find the existing mpu region that we are removing; it needs to match exactly.
            if let Some(internal_region) = self.mpu_regions.iter().find(|r| r.get() == Some(region))
            {
                self.chip
                    .mpu()
                    .remove_memory_region(region, config)
                    .or(Err(ErrorCode::FAIL))?;

                // Remove this region from the tracking cache of mpu_regions
                internal_region.set(None);
                Ok(())
            } else {
                Err(ErrorCode::INVAL)
            }
        })
    }

    fn sbrk(&self, increment: isize) -> Result<CapabilityPtr, Error> {
        // Do not modify an inactive process.
        if !self.is_running() {
            return Err(Error::InactiveApp);
        }

        let new_break = self.app_break.get().wrapping_offset(increment);
        self.brk(new_break)
    }

    fn brk(&self, new_break: *const u8) -> Result<CapabilityPtr, Error> {
        // Do not modify an inactive process.
        if !self.is_running() {
            return Err(Error::InactiveApp);
        }

        self.mpu_config.map_or(Err(Error::KernelError), |config| {
            if new_break < self.allow_high_water_mark.get() || new_break >= self.mem_end() {
                Err(Error::AddressOutOfBounds)
            } else if new_break > self.kernel_memory_break.get() {
                Err(Error::OutOfMemory)
            } else if let Err(()) = self.chip.mpu().update_app_memory_region(
                new_break,
                self.kernel_memory_break.get(),
                mpu::Permissions::ReadWriteOnly,
                config,
            ) {
                Err(Error::OutOfMemory)
            } else {
                let old_break: *const u8 = self.app_break.get();
                self.app_break.set(new_break);

                // # Safety
                //
                // `configure_mpu` is unsafe, as invoking it with an incorrect
                // configuration can allow an untrusted application to access
                // kernel-private memory.
                //
                // This call is safe given we trust that the implementation of
                // `ProcessStandard` correctly provisions a set of MPU regions
                // that does not grant access to any kernel-private memory, and
                // `ProcessStandard` does not provide safe, publically
                // accessible APIs to add other arbitrary MPU regions to this
                // configuration.
                unsafe {
                    self.chip.mpu().configure_mpu(config);
                }

                if new_break > old_break {
                    // We need to initialize (zero) the newly accessible memory
                    // region at `[old_break; new_break)`. This serves two
                    // purposes:
                    //
                    // 1. It prevents a process from accessing any information
                    //    still contained in this memory from prior kernel
                    //    instances or processes.
                    //
                    // 2. It satisfies Rust's requirements that all
                    //    dereferencable memory be properly initialized. This is
                    //    important, as we'll be creating references into this
                    //    process-accessible memory region through the process
                    //    buffer infrastructure.
                    let old_break_mut_ptr: *mut u8 = old_break.cast_mut();
                    unsafe {
                        core::ptr::write_bytes(
                            old_break_mut_ptr,
                            // Set the newly app-accessible memory to `0`:
                            0_u8,
                            new_break.addr() - old_break.addr(),
                        );
                    }
                }

                let base = self.mem_start() as usize;
                let old_break_unit_ptr: *const () = old_break.cast();
                // # Safety
                // The passed range [base, new_break) exactly matches the process' memory range,
                // and a process should have RW access to its own memory.
                let break_result = unsafe {
                    CapabilityPtr::new_with_authority(
                        old_break_unit_ptr,
                        base,
                        (new_break as usize) - base,
                        CapabilityPtrPermissions::ReadWrite,
                    )
                };

                Ok(break_result)
            }
        })
    }

    #[allow(clippy::not_unsafe_ptr_arg_deref)]
    fn build_readwrite_process_buffer(
        &self,
        buf_start_addr: *mut u8,
        size: usize,
    ) -> Result<ReadWriteProcessBuffer, ErrorCode> {
        if !self.is_running() {
            // Do not operate on an inactive process
            return Err(ErrorCode::FAIL);
        }

        // A process is allowed to pass any pointer if the buffer length is 0,
        // as to revoke kernel access to a memory region without granting access
        // to another one
        if size == 0 {
            // Clippy complains that we're dereferencing a pointer in a public
            // and safe function here. While we are not dereferencing the
            // pointer here, we pass it along to an unsafe function, which is as
            // dangerous (as it is likely to be dereferenced down the line).
            //
            // Relevant discussion:
            // https://github.com/rust-lang/rust-clippy/issues/3045
            //
            // It should be fine to ignore the lint here, as a buffer of length
            // 0 will never allow dereferencing any memory in a safe manner.
            //
            // ### Safety
            //
            // We specify a zero-length buffer, so the implementation of
            // `ReadWriteProcessBuffer` will handle any safety issues.
            // Therefore, we can encapsulate the unsafe.
            Ok(unsafe { ReadWriteProcessBuffer::new(buf_start_addr, 0, self.processid()) })
        } else if self.in_app_owned_memory(buf_start_addr, size) {
            // TODO: Check for buffer aliasing here

            // Valid buffer, we need to adjust the app's watermark
            // note: `in_app_owned_memory` ensures this offset does not wrap
            let buf_end_addr = buf_start_addr.wrapping_add(size);
            let new_water_mark = cmp::max(self.allow_high_water_mark.get(), buf_end_addr);
            self.allow_high_water_mark.set(new_water_mark);

            // Clippy complains that we're dereferencing a pointer in a public
            // and safe function here. While we are not dereferencing the
            // pointer here, we pass it along to an unsafe function, which is as
            // dangerous (as it is likely to be dereferenced down the line).
            //
            // Relevant discussion:
            // https://github.com/rust-lang/rust-clippy/issues/3045
            //
            // It should be fine to ignore the lint here, as long as we make
            // sure that we're pointing towards userspace memory (verified using
            // `in_app_owned_memory`) and respect alignment and other
            // constraints of the Rust references created by
            // `ReadWriteProcessBuffer`.
            //
            // ### Safety
            //
            // We encapsulate the unsafe here on the condition in the TODO
            // above, as we must ensure that this `ReadWriteProcessBuffer` will
            // be the only reference to this memory.
            Ok(unsafe { ReadWriteProcessBuffer::new(buf_start_addr, size, self.processid()) })
        } else {
            Err(ErrorCode::INVAL)
        }
    }

    #[allow(clippy::not_unsafe_ptr_arg_deref)]
    fn build_readonly_process_buffer(
        &self,
        buf_start_addr: *const u8,
        size: usize,
    ) -> Result<ReadOnlyProcessBuffer, ErrorCode> {
        if !self.is_running() {
            // Do not operate on an inactive process
            return Err(ErrorCode::FAIL);
        }

        // A process is allowed to pass any pointer if the buffer length is 0,
        // as to revoke kernel access to a memory region without granting access
        // to another one
        if size == 0 {
            // Clippy complains that we're dereferencing a pointer in a public
            // and safe function here. While we are not dereferencing the
            // pointer here, we pass it along to an unsafe function, which is as
            // dangerous (as it is likely to be dereferenced down the line).
            //
            // Relevant discussion:
            // https://github.com/rust-lang/rust-clippy/issues/3045
            //
            // It should be fine to ignore the lint here, as a buffer of length
            // 0 will never allow dereferencing any memory in a safe manner.
            //
            // ### Safety
            //
            // We specify a zero-length buffer, so the implementation of
            // `ReadOnlyProcessBuffer` will handle any safety issues. Therefore,
            // we can encapsulate the unsafe.
            Ok(unsafe { ReadOnlyProcessBuffer::new(buf_start_addr, 0, self.processid()) })
        } else if self.in_app_owned_memory(buf_start_addr, size)
            || self.in_app_flash_memory(buf_start_addr, size)
        {
            // TODO: Check for buffer aliasing here

            if self.in_app_owned_memory(buf_start_addr, size) {
                // Valid buffer, and since this is in read-write memory (i.e.
                // not flash), we need to adjust the process's watermark. Note:
                // `in_app_owned_memory()` ensures this offset does not wrap.
                let buf_end_addr = buf_start_addr.wrapping_add(size);
                let new_water_mark = cmp::max(self.allow_high_water_mark.get(), buf_end_addr);
                self.allow_high_water_mark.set(new_water_mark);
            }

            // Clippy complains that we're dereferencing a pointer in a public
            // and safe function here. While we are not dereferencing the
            // pointer here, we pass it along to an unsafe function, which is as
            // dangerous (as it is likely to be dereferenced down the line).
            //
            // Relevant discussion:
            // https://github.com/rust-lang/rust-clippy/issues/3045
            //
            // It should be fine to ignore the lint here, as long as we make
            // sure that we're pointing towards userspace memory (verified using
            // `in_app_owned_memory` or `in_app_flash_memory`) and respect
            // alignment and other constraints of the Rust references created by
            // `ReadWriteProcessBuffer`.
            //
            // ### Safety
            //
            // We encapsulate the unsafe here on the condition in the TODO
            // above, as we must ensure that this `ReadOnlyProcessBuffer` will
            // be the only reference to this memory.
            Ok(unsafe { ReadOnlyProcessBuffer::new(buf_start_addr, size, self.processid()) })
        } else {
            Err(ErrorCode::INVAL)
        }
    }

    unsafe fn set_byte(&self, addr: *mut u8, value: u8) -> bool {
        if self.in_app_owned_memory(addr, 1) {
            // # Safety
            //
            // We verify that this will only write process-accessible memory,
            // but this can still be undefined behavior if something else holds
            // a reference to this memory. The caller must ensure nothing else
            // holds a reference to this memory.
            unsafe {
                *addr = value;
            }
            true
        } else {
            false
        }
    }

    fn grant_is_allocated(&self, grant_num: usize) -> Option<bool> {
        // Do not modify an inactive process.
        if !self.is_running() {
            return None;
        }

        // Update the grant pointer to the address of the new allocation.
        self.grant_pointers.map_or(None, |grant_pointers| {
            // Implement `grant_pointers[grant_num]` without a chance of a
            // panic.
            grant_pointers
                .get(grant_num)
                .map(|grant_entry| !grant_entry.grant_ptr.is_null())
        })
    }

    fn allocate_grant(
        &self,
        grant_num: usize,
        driver_num: usize,
        size: usize,
        align: usize,
    ) -> Result<(), ()> {
        // Do not modify an inactive process.
        if !self.is_running() {
            return Err(());
        }

        // Verify the grant_num is valid.
        if grant_num >= self.kernel.get_grant_count_and_finalize() {
            return Err(());
        }

        // Verify that the grant is not already allocated. If the pointer is not
        // null then the grant is already allocated.
        if let Some(is_allocated) = self.grant_is_allocated(grant_num) {
            if is_allocated {
                return Err(());
            }
        }

        // Verify that there is not already a grant allocated with the same
        // `driver_num`.
        let exists = self.grant_pointers.map_or(false, |grant_pointers| {
            // Check our list of grant pointers if the driver number is used.
            grant_pointers.iter().any(|grant_entry| {
                // Check if the grant is both allocated (its grant pointer is
                // non null) and the driver number matches.
                (!grant_entry.grant_ptr.is_null()) && grant_entry.driver_num == driver_num
            })
        });
        // If we find a match, then the `driver_num` must already be used and
        // the grant allocation fails.
        if exists {
            return Err(());
        }

        // Use the shared grant allocator function to actually allocate memory.
        // Returns `None` if the allocation cannot be created.
        if let Some(grant_ptr) = self.allocate_in_grant_region_internal(size, align) {
            // Update the grant pointer to the address of the new allocation.
            self.grant_pointers.map_or(Err(()), |grant_pointers| {
                // Implement `grant_pointers[grant_num] = grant_ptr` without a
                // chance of a panic.
                grant_pointers
                    .get_mut(grant_num)
                    .map_or(Err(()), |grant_entry| {
                        // Actually set the driver num and grant pointer.
                        grant_entry.driver_num = driver_num;
                        grant_entry.grant_ptr = grant_ptr.as_ptr();

                        // If all of this worked, return true.
                        Ok(())
                    })
            })
        } else {
            // Could not allocate the memory for the grant region.
            Err(())
        }
    }

    fn allocate_custom_grant(
        &self,
        size: usize,
        align: usize,
    ) -> Result<(ProcessCustomGrantIdentifier, NonNull<u8>), ()> {
        // Do not modify an inactive process.
        if !self.is_running() {
            return Err(());
        }

        // Use the shared grant allocator function to actually allocate memory.
        // Returns `None` if the allocation cannot be created.
        if let Some(ptr) = self.allocate_in_grant_region_internal(size, align) {
            // Create the identifier that the caller will use to get access to
            // this custom grant in the future.
            let identifier = self.create_custom_grant_identifier(ptr);

            Ok((identifier, ptr))
        } else {
            // Could not allocate memory for the custom grant.
            Err(())
        }
    }

    fn enter_grant(&self, grant_num: usize) -> Result<NonNull<u8>, Error> {
        // Do not try to access the grant region of an inactive process.
        if !self.is_running() {
            return Err(Error::InactiveApp);
        }

        // Retrieve the grant pointer from the `grant_pointers` slice. We use
        // `[slice].get()` so that if the grant number is invalid this will
        // return `Err` and not panic.
        self.grant_pointers
            .map_or(Err(Error::KernelError), |grant_pointers| {
                // Implement `grant_pointers[grant_num]` without a chance of a
                // panic.
                match grant_pointers.get_mut(grant_num) {
                    Some(grant_entry) => {
                        // Get a copy of the actual grant pointer.
                        let grant_ptr = grant_entry.grant_ptr;

                        // Check if the grant pointer is marked that the grant
                        // has already been entered. If so, return an error.
                        if (grant_ptr as usize) & 0x1 == 0x1 {
                            // Lowest bit is one, meaning this grant has been
                            // entered.
                            Err(Error::AlreadyInUse)
                        } else {
                            // Now, to mark that the grant has been entered, we
                            // set the lowest bit to one and save this as the
                            // grant pointer.
                            grant_entry.grant_ptr = (grant_ptr as usize | 0x1) as *mut u8;

                            // And we return the grant pointer to the entered
                            // grant.
                            Ok(unsafe { NonNull::new_unchecked(grant_ptr) })
                        }
                    }
                    None => Err(Error::AddressOutOfBounds),
                }
            })
    }

    fn enter_custom_grant(
        &self,
        identifier: ProcessCustomGrantIdentifier,
    ) -> Result<*mut u8, Error> {
        // Do not try to access the grant region of an inactive process.
        if !self.is_running() {
            return Err(Error::InactiveApp);
        }

        // Get the address of the custom grant based on the identifier.
        let custom_grant_address = self.get_custom_grant_address(identifier);

        // We never deallocate custom grants and only we can change the
        // `identifier` so we know this is a valid address for the custom grant.
        Ok(custom_grant_address as *mut u8)
    }

    unsafe fn leave_grant(&self, grant_num: usize) {
        // Do not modify an inactive process.
        if !self.is_running() {
            return;
        }

        self.grant_pointers.map(|grant_pointers| {
            // Implement `grant_pointers[grant_num]` without a chance of a
            // panic.
            if let Some(grant_entry) = grant_pointers.get_mut(grant_num) {
                // Get a copy of the actual grant pointer.
                let grant_ptr = grant_entry.grant_ptr;

                // Now, to mark that the grant has been released, we set the
                // lowest bit back to zero and save this as the grant
                // pointer.
                grant_entry.grant_ptr = (grant_ptr as usize & !0x1) as *mut u8;
            }
        });
    }

    fn grant_allocated_count(&self) -> Option<usize> {
        // Do not modify an inactive process.
        if !self.is_running() {
            return None;
        }

        self.grant_pointers.map(|grant_pointers| {
            // Filter our list of grant pointers into just the non-null ones,
            // and count those. A grant is allocated if its grant pointer is
            // non-null.
            grant_pointers
                .iter()
                .filter(|grant_entry| !grant_entry.grant_ptr.is_null())
                .count()
        })
    }

    fn lookup_grant_from_driver_num(&self, driver_num: usize) -> Result<usize, Error> {
        self.grant_pointers
            .map_or(Err(Error::KernelError), |grant_pointers| {
                // Filter our list of grant pointers into just the non null
                // ones, and count those. A grant is allocated if its grant
                // pointer is non-null.
                match grant_pointers.iter().position(|grant_entry| {
                    // Only consider allocated grants.
                    (!grant_entry.grant_ptr.is_null()) && grant_entry.driver_num == driver_num
                }) {
                    Some(idx) => Ok(idx),
                    None => Err(Error::OutOfMemory),
                }
            })
    }

    fn is_valid_upcall_function_pointer(&self, upcall_fn: *const ()) -> bool {
        let ptr: *const u8 = upcall_fn.cast();
        let size = mem::size_of::<*const u8>();

        // It is okay if this function is in memory or flash.
        self.in_app_flash_memory(ptr, size) || self.in_app_owned_memory(ptr, size)
    }

    fn get_process_name(&self) -> &'static str {
        self.header.get_package_name().unwrap_or("")
    }

    fn get_completion_code(&self) -> Option<Option<u32>> {
        self.completion_code.get()
    }

    fn set_syscall_return_value(&self, return_value: SyscallReturn) {
        match self.stored_state.map(|stored_state| unsafe {
            // Actually set the return value for a particular process.
            //
            // The UKB implementation uses the bounds of process-accessible
            // memory to verify that any memory changes are valid. Here, the
            // unsafe promise we are making is that the bounds passed to the UKB
            // are correct.
            self.chip
                .userspace_kernel_boundary()
                .set_syscall_return_value(
                    self.mem_start(),
                    self.app_break.get(),
                    stored_state,
                    return_value,
                )
        }) {
            Some(Ok(())) => {
                // If we get an `Ok` we are all set.

                // The process is either already in the running state (having
                // just called a nonblocking syscall like command) or needs to
                // be moved to the running state having called Yield-WaitFor and
                // now needing to be resumed. Either way we can set the state to
                // running.
                self.state.set(State::Running);
                // The task is running, if it was yielded-for an upcall,
                // the upcall must have been scheduled, unset
                // the ready flag.
                self.is_yield_wait_for_ready.set(false);
            }

            Some(Err(())) => {
                // If we get an `Err`, then the UKB implementation could not set
                // the return value, likely because the process's stack is no
                // longer accessible to it. All we can do is fault.
                self.set_fault_state();
            }

            None => {
                // We should never be here since `stored_state` should always be
                // occupied.
                self.set_fault_state();
            }
        }
    }

    fn set_process_function(&self, callback: FunctionCall) {
        // See if we can actually enqueue this function for this process.
        // Architecture-specific code handles actually doing this since the
        // exact method is both architecture- and implementation-specific.
        //
        // This can fail, for example if the process does not have enough memory
        // remaining.
        match self.stored_state.map(|stored_state| {
            // Let the UKB implementation handle setting the process's PC so
            // that the process executes the upcall function. We encapsulate
            // unsafe here because we are guaranteeing that the memory bounds
            // passed to `set_process_function` are correct.
            unsafe {
                self.chip.userspace_kernel_boundary().set_process_function(
                    self.mem_start(),
                    self.app_break.get(),
                    stored_state,
                    callback,
                )
            }
        }) {
            Some(Ok(())) => {
                // If we got an `Ok` we are all set and should mark that this
                // process is ready to be scheduled.

                // Move this process to the "running" state so the scheduler
                // will schedule it.
                self.state.set(State::Running);
            }

            Some(Err(())) => {
                // If we got an Error, then there was likely not enough room on
                // the stack to allow the process to execute this function given
                // the details of the particular architecture this is running
                // on. This process has essentially faulted, so we mark it as
                // such.
                self.set_fault_state();
            }

            None => {
                // We should never be here since `stored_state` should always be
                // occupied.
                self.set_fault_state();
            }
        }
    }

    fn switch_to(&self) -> Option<syscall::ContextSwitchReason> {
        // Cannot switch to an invalid process
        if !self.is_running() {
            return None;
        }

        let (switch_reason, stack_pointer) =
            self.stored_state.map_or((None, None), |stored_state| {
                // Switch to the process. We guarantee that the memory pointers
                // we pass are valid, ensuring this context switch is safe.
                // Therefore we encapsulate the `unsafe`.
                unsafe {
                    let (switch_reason, optional_stack_pointer) = self
                        .chip
                        .userspace_kernel_boundary()
                        .switch_to_process(self.mem_start(), self.app_break.get(), stored_state);
                    (Some(switch_reason), optional_stack_pointer)
                }
            });

        // If the UKB implementation passed us a stack pointer, update our
        // debugging state. This is completely optional.
        if let Some(sp) = stack_pointer {
            self.debug.set_new_app_stack_min_pointer(sp);
        }

        switch_reason
    }

    fn debug_syscall_count(&self) -> usize {
        self.debug.get_syscall_count()
    }

    fn debug_dropped_upcall_count(&self) -> usize {
        self.debug.get_dropped_upcall_count()
    }

    fn debug_timeslice_expiration_count(&self) -> usize {
        self.debug.get_timeslice_expiration_count()
    }

    fn debug_timeslice_expired(&self) {
        self.debug.increment_timeslice_expiration_count();
    }

    fn debug_syscall_called(&self, last_syscall: Syscall) {
        self.debug.increment_syscall_count();
        self.debug.set_last_syscall(last_syscall);
    }

    fn debug_syscall_last(&self) -> Option<Syscall> {
        self.debug.get_last_syscall()
    }

    fn get_addresses(&self) -> ProcessAddresses {
        ProcessAddresses {
            flash_start: self.flash_start() as usize,
            flash_non_protected_start: self.flash_non_protected_start() as usize,
            flash_integrity_end: ((self.flash.as_ptr() as usize)
                + (self.header.get_binary_end() as usize))
                as *const u8,
            flash_end: self.flash_end() as usize,
            sram_start: self.mem_start() as usize,
            sram_app_brk: self.app_memory_break() as usize,
            sram_grant_start: self.kernel_memory_break() as usize,
            sram_end: self.mem_end() as usize,
            sram_heap_start: self.debug.get_app_heap_start_pointer().map(|p| p as usize),
            sram_stack_top: self.debug.get_app_stack_start_pointer().map(|p| p as usize),
            sram_stack_bottom: self.debug.get_app_stack_min_pointer().map(|p| p as usize),
        }
    }

    fn get_sizes(&self) -> ProcessSizes {
        ProcessSizes {
            grant_pointers: mem::size_of::<GrantPointerEntry>()
                * self.kernel.get_grant_count_and_finalize(),
            upcall_list: Self::CALLBACKS_SIZE,
            process_control_block: Self::PROCESS_STRUCT_SIZE,
        }
    }

    fn print_full_process(&self, writer: &mut dyn Write) {
        if !config::CONFIG.debug_panics {
            return;
        }

        self.stored_state.map(|stored_state| {
            // We guarantee the memory bounds pointers provided to the UKB are
            // correct.
            unsafe {
                self.chip.userspace_kernel_boundary().print_context(
                    self.mem_start(),
                    self.app_break.get(),
                    stored_state,
                    writer,
                );
            }
        });

        // Display grant information.
        let number_grants = self.kernel.get_grant_count_and_finalize();
        let _ = writer.write_fmt(format_args!(
            "\
            \r\n Total number of grant regions defined: {}\r\n",
            self.kernel.get_grant_count_and_finalize()
        ));
        let rows = number_grants.div_ceil(3);

        // Access our array of grant pointers.
        self.grant_pointers.map(|grant_pointers| {
            // Iterate each grant and show its address.
            for i in 0..rows {
                for j in 0..3 {
                    let index = i + (rows * j);
                    if index >= number_grants {
                        break;
                    }

                    // Implement `grant_pointers[grant_num]` without a chance of
                    // a panic.
                    grant_pointers.get(index).map(|grant_entry| {
                        if grant_entry.grant_ptr.is_null() {
                            let _ =
                                writer.write_fmt(format_args!("  Grant {:>2} : --        ", index));
                        } else {
                            let _ = writer.write_fmt(format_args!(
                                "  Grant {:>2} {:#x}: {:p}",
                                index, grant_entry.driver_num, grant_entry.grant_ptr
                            ));
                        }
                    });
                }
                let _ = writer.write_fmt(format_args!("\r\n"));
            }
        });

        // Display the current state of the MPU for this process.
        self.mpu_config.map(|config| {
            let _ = writer.write_fmt(format_args!("{}", config));
        });

        // Print a helpful message on how to re-compile a process to view the
        // listing file. If a process is PIC, then we also need to print the
        // actual addresses the process executed at so that the .lst file can be
        // generated for those addresses. If the process was already compiled
        // for a fixed address, then just generating a .lst file is fine.

        if self.debug.get_fixed_address_flash().is_some() {
            // Fixed addresses, can just run `make lst`.
            let _ = writer.write_fmt(format_args!(
                "\
                    \r\nTo debug libtock-c apps, run `make lst` in the app's\
                    \r\nfolder and open the arch.{:#x}.{:#x}.lst file.\r\n\r\n",
                self.debug.get_fixed_address_flash().unwrap_or(0),
                self.debug.get_fixed_address_ram().unwrap_or(0)
            ));
        } else {
            // PIC, need to specify the addresses.
            let sram_start = self.mem_start() as usize;
            let flash_start = self.flash.as_ptr() as usize;
            let flash_init_fn = flash_start + self.header.get_init_function_offset() as usize;

            let _ = writer.write_fmt(format_args!(
                "\
                    \r\nTo debug libtock-c apps, run\
                    \r\n`make debug RAM_START={:#x} FLASH_INIT={:#x}`\
                    \r\nin the app's folder and open the .lst file.\r\n\r\n",
                sram_start, flash_init_fn
            ));
        }
    }

    fn get_stored_state(&self, out: &mut [u8]) -> Result<usize, ErrorCode> {
        self.stored_state
            .map(|stored_state| {
                self.chip
                    .userspace_kernel_boundary()
                    .store_context(stored_state, out)
            })
            .unwrap_or(Err(ErrorCode::FAIL))
    }
}

impl<C: 'static + Chip, D: 'static + ProcessStandardDebug> ProcessStandard<'_, C, D> {
    /// Alignment requirement for each `GrantPointerEntry` in the grant pointers
    /// slice.
    const GRANT_POINTERS_ALIGNMENT: usize = mem::align_of::<GrantPointerEntry>();

    /// Number of upcalls stored in the upcall ring buffer (10 element length).
    const CALLBACK_LEN: usize = 10;
    /// Size of the upcall storage buffer.
    ///
    /// As of June 2026, we cannot do
    ///
    /// ```ignore
    /// const CALLBACKS_SIZE usize = mem::size_of::<MaybeUninit<[Task; Self::CALLBACK_LEN]>>();
    /// ```
    ///
    /// because of the error:
    ///
    /// ```ignore
    /// error: generic `Self` types are currently not permitted in anonymous constants
    ///     --> kernel/src/process_standard.rs:1712:70
    ///      |
    /// 1712 |     const CALLBACKS_SIZE: usize = mem::size_of::<MaybeUninit<[Task; Self::CALLBACK_LEN]>>();
    ///      |                                                                      ^^^^
    ///      |
    /// ```
    const CALLBACKS_SIZE: usize = mem::size_of::<Task>() * Self::CALLBACK_LEN;
    /// Alignment requirement of the upcall storage buffer.
    const CALLBACKS_ALIGNMENT: usize = mem::align_of::<Task>();

    // Memory offset to make room for this process's control block.
    const PROCESS_STRUCT_SIZE: usize = mem::size_of::<ProcessStandard<C, D>>();
    /// Alignment requirement for `ProcessStandard`.
    const PROCESS_STRUCT_ALIGNMENT: usize = mem::align_of::<ProcessStandard<C, D>>();

    /// Create a `ProcessStandard` object based on the found `ProcessBinary`.
    pub(crate) unsafe fn create(
        kernel: &'static Kernel,
        chip: &'static C,
        pb: ProcessBinary,
        remaining_memory: *mut [u8],
        fault_policy: &'static dyn ProcessFaultPolicy,
        storage_permissions_policy: &'static dyn ProcessStandardStoragePermissionsPolicy<C, D>,
        app_id: ShortId,
        index: usize,
    ) -> Result<(Option<&'static dyn Process>, *mut [u8]), (ProcessLoadError, *mut [u8])> {
        let process_name = pb.header.get_package_name();
        let process_ram_requested_size = pb.header.get_minimum_app_ram_size() as usize;

        // Initialize MPU region configuration.
        let mut mpu_config = match chip.mpu().new_config() {
            Some(mpu_config) => mpu_config,
            None => return Err((ProcessLoadError::MpuConfigurationError, remaining_memory)),
        };

        // Allocate MPU region for flash.
        if chip
            .mpu()
            .allocate_region(
                pb.flash.as_ptr(),
                pb.flash.len(),
                pb.flash.len(),
                mpu::Permissions::ReadExecuteOnly,
                &mut mpu_config,
            )
            .is_none()
        {
            if config::CONFIG.debug_load_processes {
                debug!(
                    "[!] flash={:#010X}-{:#010X} process={:?} - couldn't allocate MPU region for flash",
                    pb.flash.as_ptr() as usize,
                    pb.flash.as_ptr() as usize + pb.flash.len() - 1,
                    process_name
                );
            }
            return Err((ProcessLoadError::MpuInvalidFlashLength, remaining_memory));
        }

        // Determine how much space we need in the application's memory space
        // just for kernel and grant state. We need to make sure we allocate
        // enough memory just for that.

        // Calculate how many bytes we need for grant pointers.
        let grant_ptr_size = mem::size_of::<GrantPointerEntry>();
        let grant_ptrs_num = kernel.get_grant_count_and_finalize();
        let grant_ptrs_size = grant_ptrs_num * grant_ptr_size;

        // Initial size of the kernel-owned part of process memory can be
        // calculated directly based on the initial size of all kernel-owned
        // data structures.
        //
        // Add the alignment size of each data structure to handle the case
        // where the memory region starts at the worst possible address and we
        // need to add padding. Note: the maximum shift will only ever be one
        // less than the alignment size, but to avoid unusual addresses we just
        // use the full alignment.
        let initial_kernel_memory_size = grant_ptrs_size
            + Self::GRANT_POINTERS_ALIGNMENT
            + Self::CALLBACKS_SIZE
            + Self::CALLBACKS_ALIGNMENT
            + Self::PROCESS_STRUCT_SIZE
            + Self::PROCESS_STRUCT_ALIGNMENT;

        // By default we start with the initial size of process-accessible
        // memory set to 0. This maximizes the flexibility that processes have
        // to allocate their memory as they see fit. If a process needs more
        // accessible memory it must use the `brk` memop syscalls to request
        // more memory.
        //
        // We must take into account any process-accessible memory required by
        // the context switching implementation and allocate at least that much
        // memory so that we can successfully switch to the process. This is
        // architecture and implementation specific, so we query that now.
        let min_process_memory_size = chip
            .userspace_kernel_boundary()
            .initial_process_app_brk_size();

        // We have to ensure that we at least ask the MPU for
        // `min_process_memory_size` so that we can be sure that `app_brk` is
        // not set inside the kernel-owned memory region. Now, in practice,
        // processes should not request 0 (or very few) bytes of memory in their
        // TBF header (i.e. `process_ram_requested_size` will almost always be
        // much larger than `min_process_memory_size`), as they are unlikely to
        // work with essentially no available memory. But, we still must protect
        // for that case.
        let min_process_ram_size = cmp::max(process_ram_requested_size, min_process_memory_size);

        // Minimum memory size for the process.
        let min_total_memory_size = min_process_ram_size + initial_kernel_memory_size;

        // Check if this process requires a fixed memory start address. If so,
        // try to adjust the memory region to work for this process.
        //
        // Right now, we only support skipping some RAM and leaving a chunk
        // unused so that the memory region starts where the process needs it
        // to.
        let remaining_memory = if let Some(fixed_memory_start) = pb
            .header
            .get_fixed_address_ram()
            .map(|addr: u32| remaining_memory.cast::<u8>().with_addr(addr as usize))
        {
            // The process does have a fixed address.
            if fixed_memory_start == remaining_memory.cast() {
                // Address already matches.
                remaining_memory
            } else if fixed_memory_start > remaining_memory.cast() {
                // Process wants a memory address farther in memory. Try to
                // advance the memory region to make the address match.
                let diff = fixed_memory_start.addr() - remaining_memory.addr();
                if diff > remaining_memory.len() {
                    // We ran out of memory.
                    let actual_address = (remaining_memory.cast::<u8>())
                        .wrapping_byte_add(remaining_memory.len())
                        .wrapping_byte_sub(1);
                    let expected_address = fixed_memory_start;
                    return Err((
                        ProcessLoadError::MemoryAddressMismatch {
                            actual_address,
                            expected_address,
                        },
                        remaining_memory,
                    ));
                } else {
                    // Change the memory range to start where the process requested it.
                    // Because of the if statement above we know this should work. Doing
                    // it more cleanly would be good but was a bit beyond my borrow ken;
                    // calling get_mut has a mutable borrow.-pal
                    //
                    // # Safety
                    //
                    // `diff` must be within the `remaining_memory` slice. Because we
                    // check that `diff` is less than the length of `remaining_memory`
                    // we know diff will be within  `remaining_memory`.
                    let (_, sliced) = unsafe { raw_slice_split_at_mut(remaining_memory, diff) };
                    sliced
                }
            } else {
                // Address is earlier in memory, nothing we can do.
                let actual_address = remaining_memory.cast();
                let expected_address = fixed_memory_start;
                return Err((
                    ProcessLoadError::MemoryAddressMismatch {
                        actual_address,
                        expected_address,
                    },
                    remaining_memory,
                ));
            }
        } else {
            remaining_memory
        };

        // Determine where process memory will go and allocate an MPU region.
        //
        // `[allocation_start, allocation_size)` will cover both
        //
        // - the app-owned `min_process_memory_size`-long part of memory (at
        //   some offset within `remaining_memory`), as well as
        //
        // - the kernel-owned allocation growing downward starting at the end
        //   of this allocation, `initial_kernel_memory_size` bytes long.
        //
        let (allocation_start, allocation_size) = match chip.mpu().allocate_app_memory_region(
            remaining_memory.cast(),
            remaining_memory.len(),
            min_total_memory_size,
            min_process_memory_size,
            initial_kernel_memory_size,
            mpu::Permissions::ReadWriteOnly,
            &mut mpu_config,
        ) {
            Some((memory_start, memory_size)) => (memory_start, memory_size),
            None => {
                // Failed to load process. Insufficient memory.
                if config::CONFIG.debug_load_processes {
                    debug!(
                        "[!] flash={:#010X}-{:#010X} process={:?} - couldn't allocate memory region of size >= {:#X}",
                        pb.flash.as_ptr() as usize,
                        pb.flash.as_ptr() as usize + pb.flash.len() - 1,
                        process_name,
                        min_total_memory_size
                    );
                }
                return Err((ProcessLoadError::NotEnoughMemory, remaining_memory));
            }
        };

        // Determine the offset of the app-owned part of the above memory
        // allocation. An MPU may not place it at the very start of
        // `remaining_memory` for internal alignment constraints. This can only
        // overflow if the MPU implementation is incorrect; a compliant
        // implementation must return a memory allocation within the
        // `remaining_memory` slice.
        let app_memory_start_offset = allocation_start.addr() - remaining_memory.addr();

        // Check if the memory region is valid for the process. If a process
        // included a fixed address for the start of RAM in its TBF header (this
        // field is optional, processes that are position independent do not
        // need a fixed address) then we check that we used the same address
        // when we allocated it in RAM.
        if let Some(fixed_memory_start) = pb
            .header
            .get_fixed_address_ram()
            .map(|addr: u32| remaining_memory.cast::<u8>().with_addr(addr as usize))
        {
            let actual_address = remaining_memory
                .cast::<u8>()
                .wrapping_byte_add(app_memory_start_offset);
            let expected_address = fixed_memory_start;
            if actual_address != expected_address {
                return Err((
                    ProcessLoadError::MemoryAddressMismatch {
                        actual_address,
                        expected_address,
                    },
                    remaining_memory,
                ));
            }
        }

        // With our MPU allocation, we can begin to divide up the
        // `remaining_memory` slice into individual regions for the process and
        // kernel, as follows:
        //
        //
        //  ┌───────────────────────────────────────────────────────────────────────────────────────
        //  │ remaining_memory
        //  └───────────────────────────────────────────────────────────────────────────────────────
        //  ┆                                                                        ┆
        //  ┌────────────────────────────────────────────────────────────────────────┬──────────────
        //  │ allocated_padded_memory                                                │ unused_memory
        //  └────────────────────────────────────────────────────────────────────────┴──────────────
        //      ┆                                                                    ┆
        //      ┌────────────────────────────────────────────────────────────────────┐
        //  ┆←─→│ allocated_memory                                                   │
        //    ↑ └────────────────────────────────────────────────────────────────────┘
        //    └┄┄┄app_memory_start_offset                                            ┆
        //      ┌─────────────────────────┬──────────────────────────────────────────┐
        //      │ app_accessible_memory   │ allocated_kernel_memory                  │
        //      └─────────────────────────┴──────────────────────────────────────────┘
        //      ┆←min_process_memory_size→┆←────→┆←────initial_kernel_memory_size───→┆
        //                                ↑  ↑   ┆                                   ┆
        //              initial_app_brk┄┄┄┘  ┆   ┆ ┌┄┄┄kernel_memory_break           ┆
        //           initially unallocated┄┄┄┘   ┆ ↓                                 ┆
        //                                       ┆ ┌───────┬──────────┬──────────────┐
        //                                       ┆↔│process│upcall_buf│grant_pointers│
        //                                       ┆ └───────┴──────────┴──────────────┘
        //                                        ↑       ↑          ↑              ↑
        //             possible alignment padding┄┴┄┄┄┄┄┄┄┴┄┄┄┄┄┄┄┄┄┄┴┄┄┄┄┄┄┄┄┄┄┄┄┄┄┘
        //
        //
        // First split the `remaining_memory` into two slices:
        //
        // 1. `allocated_padded_memory`: the allocated memory region, containing:
        //
        //    1. optional padding at the start of the memory region of
        //       `app_memory_start_offset` bytes,
        //    2. the app accessible memory region of `min_process_memory_size`,
        //    3. optional unallocated memory, and
        //    4. kernel-reserved memory, growing downward starting at the end of
        //       `allocated_memory`.
        //
        // 2. `unused_memory`: the rest of the `remaining_memory`, not assigned
        //    to this app.
        //
        // # Safety
        //
        // `app_memory_start_offset + allocation_size` must be within `remaining_memory`.
        let (allocated_padded_memory, unused_memory) = unsafe {
            raw_slice_split_at_mut(remaining_memory, app_memory_start_offset + allocation_size)
        };

        // Now, slice off the (optional) padding at the start:
        //
        // # Safety
        //
        // `app_memory_start_offset` must be within `allocated_padded_memory`.
        let (_padding, allocated_memory) =
            unsafe { raw_slice_split_at_mut(allocated_padded_memory, app_memory_start_offset) };

        // We continue to sub-slice the `allocated_memory` into
        // process-accessible and kernel-owned memory. Prior to that, store the
        // start and length ofthe overall allocation:
        let allocated_memory_start = allocated_memory.cast();
        let allocated_memory_len = allocated_memory.len();

        // Slice off the process-accessible memory:
        //
        // # Safety
        //
        // `min_process_memory_size` must be within `allocated_memory`.
        let (app_accessible_memory, allocated_kernel_memory) =
            unsafe { raw_slice_split_at_mut(allocated_memory, min_process_memory_size) };

        // Initialize (zero) the initial process-accessible memory region. This
        // serves two purposes:
        //
        // 1. It prevents a process from accessing any information still
        //    contained in this memory from prior kernel instances or processes.
        //
        // 2. It satisfies Rust's requirements that all dereferencable memory be
        //    properly initialized. This is important, as we'll be creating
        //    references into this process-accessible memory region through the
        //    process buffer infrastructure.
        let app_accessible_memory_bytes: *mut u8 = app_accessible_memory.cast();
        // # Safety
        //
        // `app_accessible_memory_bytes` is from a slice, and we use that
        // slice's length, so we know that there is enough memory and that the
        // pointer is aligned.
        unsafe {
            core::ptr::write_bytes(
                app_accessible_memory_bytes,
                // Set the entire app-accessible memory region to `0`:
                0_u8,
                app_accessible_memory.len(),
            );
        }

        // Set the initial process-accessible memory.
        //
        // # Safety
        //
        // By using the slice `app_accessible_memory` and getting a pointer to
        // the byte after the slice, we are ensured that the memory between the
        // start of the allocation and the new pointer (at the end of the slice)
        // is valid because of the existing slice.
        let initial_app_brk = unsafe {
            app_accessible_memory
                .cast::<u8>()
                .add(app_accessible_memory.len())
        };

        // Set the initial allow high water mark to the start of process memory
        // since no `allow` calls have been made yet.
        let initial_allow_high_water_mark = app_accessible_memory.cast();

        // Set up initial grant region.
        //
        // `kernel_memory_break` is set to the end of kernel-accessible memory
        // and grows downward.
        let mut kernel_memory_break: *mut u8 = allocated_kernel_memory
            .cast::<u8>()
            .wrapping_add(allocated_kernel_memory.len());

        ////////////////////////
        // Grant Region Pointers
        ////////////////////////

        // Now that we know we have the space we can setup the grant pointers.
        //
        // First, move `kernel_memory_break` up to make room for the grant pointers.
        kernel_memory_break = kernel_memory_break.wrapping_sub(grant_ptrs_size);
        // Next, use the extra alignment padding we factored in to the total
        // calculation of `initial_kernel_memory_size` to ensure that
        // `kernel_memory_break` is aligned to a `GrantPointerEntry`.
        kernel_memory_break = kernel_memory_break
            .wrapping_sub(kernel_memory_break as usize % Self::GRANT_POINTERS_ALIGNMENT);

        // Create the `GrantPointerEntry`s in the kernel region, then initialize
        // them.
        //
        // Where we calculated the updated kernel_memory_break is where the
        // `GrantPointerEntry`s start.
        let grant_pointers_memory_location: *mut MaybeUninit<GrantPointerEntry> =
            kernel_memory_break.cast();
        // Get a reference to the slice of `GrantPointerEntry`s.
        //
        // # Safety
        //
        // This is safe, as `grant_pointers_memory_location` is aligned to a
        // `GrantPointerEntry`, and we ensured there is space for
        // `grant_ptrs_num` of `GrantPointerEntry`s allocated.
        let grant_pointers_uninit: &mut [MaybeUninit<GrantPointerEntry>] =
            unsafe { slice::from_raw_parts_mut(grant_pointers_memory_location, grant_ptrs_num) };
        // Set all grant pointers to null.
        for grant_entry in grant_pointers_uninit.iter_mut() {
            grant_entry.write(GrantPointerEntry {
                driver_num: 0,
                grant_ptr: core::ptr::null_mut(),
            });
        }
        // # Safety
        //
        // All values in this slice have been properly initialized.
        let grant_pointers = unsafe { maybe_uninit_slice_assume_init_mut(grant_pointers_uninit) };

        ////////////////////////
        // Upcall Queue
        ////////////////////////

        // Now that we know we have the space we can setup the memory for the
        // upcalls.
        //
        // Move our `kernel_memory_break` up the size of the callback slice.
        kernel_memory_break = kernel_memory_break.wrapping_sub(Self::CALLBACKS_SIZE);
        // Use the space we allocated for alignment in case the pointer is not aligned.
        kernel_memory_break = kernel_memory_break
            .wrapping_sub(kernel_memory_break as usize % Self::CALLBACKS_ALIGNMENT);

        // Set up ring buffer for upcalls to the process. The memory is uninitialized here,
        // so we cast to MaybeUninit<Task> which accurately represents that state.
        let upcall_buf: *mut core::mem::MaybeUninit<Task> = kernel_memory_break.cast();

        // Get a reference `&mut [Task; Self:CALLBACK_LEN]`
        //
        // # Safety
        //
        // This needs to be aligned and have allocated space for
        // `Self::CALLBACK_LEN` instances of `Task`. We ensured there is enough
        // space when we allocated `allocated_kernel_memory` and we moved
        // kernel_memory_break up to fit the callbacks. We ensured this is
        // aligned by moving `kernel_memory_break` up if needed, and we
        // accounted for that potential increase in size when we allocated
        // `allocated_kernel_memory`.
        let upcall_buf = unsafe { slice::from_raw_parts_mut(upcall_buf, Self::CALLBACK_LEN) };
        // Actually setup the ring buffer.
        let tasks = RingBuffer::new(upcall_buf);

        ////////////////////////
        // ProcessStandard
        ////////////////////////

        // Last thing in the kernel region of process RAM is the process struct.
        //
        // Move `kernel_memory_break` to make room for the `ProcessStandard` struct.
        kernel_memory_break = kernel_memory_break.wrapping_sub(Self::PROCESS_STRUCT_SIZE);
        // Use the space we allocated for alignment in case the pointer is not aligned.
        kernel_memory_break = kernel_memory_break
            .wrapping_sub(kernel_memory_break as usize % Self::PROCESS_STRUCT_ALIGNMENT);

        // Create a pointer to where the `ProcessStandard` struct will go in the
        // app grant region.
        let process_struct_memory_location: *mut MaybeUninit<ProcessStandard<'static, C, D>> =
            kernel_memory_break.cast();
        // Get a reference to the uninitialized `ProcessStandard` object.
        //
        // Note, this requires every field be explicitly initialized, as we are
        // just transforming a pointer into a structure. Because the
        // `ProcessStandard` is not initialized we mark it with `MaybeUninit`.
        //
        // # Safety
        //
        // This must have sufficient allocated space and proper alignment. When
        // we sized `allocated_kernel_memory` we ensured there was room for the
        // entire `ProcessStandard` struct. We also allocated space for
        // potential alignment issues, and ensured `kernel_memory_break` was
        // properly aligned for `ProcessStandard`.
        let process_uninit: &mut MaybeUninit<ProcessStandard<C, D>> =
            unsafe { &mut *process_struct_memory_location };

        // Initialize ALL fields of `ProcessStandard`.
        //
        // # Safety
        //
        // These need to be valid and aligned writes. When we create the `MaybeUnint`
        // ProcessStandard struct we ensure there is valid memory for the object and
        // that it is aligned.
        unsafe {
            init_uninit_struct!(process_uninit => ProcessStandard<C, D> {
                process_id: Cell::new(ProcessId::new(
                    kernel,
                    kernel.create_process_identifier(),
                    index,
                )),
                app_id: app_id,
                kernel: kernel,
                chip: chip,
                allow_high_water_mark: Cell::new(initial_allow_high_water_mark),
                memory_start: allocated_memory_start,
                memory_len: allocated_memory_len,
                header: pb.header,
                kernel_memory_break: Cell::new(kernel_memory_break),
                app_break: Cell::new(initial_app_brk),
                grant_pointers: MapCell::new(grant_pointers),

                credential: pb.credential.get(),
                footers: pb.footers,
                flash: pb.flash,

                stored_state: MapCell::new(Default::default()),
                state: Cell::new(State::Yielded),
                fault_policy: fault_policy,
                restart_count: Cell::new(0),
                completion_code: OptionalCell::empty(),

                storage_permissions: Cell::new(StoragePermissions::new_null()),

                mpu_config: MapCell::new(mpu_config),
                mpu_regions: [
                    Cell::new(None),
                    Cell::new(None),
                    Cell::new(None),
                    Cell::new(None),
                    Cell::new(None),
                    Cell::new(None),
                ],
                tasks: MapCell::new(tasks),
                is_yield_wait_for_ready: Cell::new(false),

                debug: D::default(),
            });
        }

        // Convert the originally uninitialized `ProcessStandard` to a proper
        // `ProcessStandard` struct reference.
        //
        // # Safety
        //
        // All fields in `ProcessStandard` must be initialized. We guaranteed
        // this by using the `init_uninit_struct!()` macro, which causes a
        // compiler error if there is a missing field.
        let process = unsafe { process_uninit.assume_init_mut() };

        // Save copies of these in case the app was compiled for fixed addresses
        // for later debugging.
        let fixed_address_flash = process.header.get_fixed_address_flash();
        let fixed_address_ram = process.header.get_fixed_address_ram();
        if let Some(fix_addr_flash) = fixed_address_flash {
            process.debug.set_fixed_address_flash(fix_addr_flash);
        }
        if let Some(fix_addr_ram) = fixed_address_ram {
            process.debug.set_fixed_address_ram(fix_addr_ram);
        }

        // Handle any architecture-specific requirements for a new process.
        match process.stored_state.map(|stored_state| {
            // # Safety
            //
            // NOTE! We have to ensure that the start of process-accessible memory
            // (`app_memory_start`) is word-aligned. Since we currently start
            // process-accessible memory at the beginning of the allocated memory
            // region, we trust the MPU to give us a word-aligned starting address.
            //
            // TODO: https://github.com/tock/tock/issues/1739
            unsafe {
                chip.userspace_kernel_boundary().initialize_process(
                    app_accessible_memory.cast(),
                    initial_app_brk,
                    stored_state,
                )
            }
        }) {
            Some(Ok(())) => {}
            _ => {
                if config::CONFIG.debug_load_processes {
                    debug!(
                        "[!] flash={:#010X}-{:#010X} process={:?} - couldn't initialize process",
                        pb.flash.as_ptr() as usize,
                        pb.flash.as_ptr() as usize + pb.flash.len() - 1,
                        process_name
                    );
                }
                // Note that since remaining_memory was split by split_at_mut into
                // application memory and unused_memory, a failure here will leak
                // the application memory. Not leaking it requires being able to
                // reconstitute the original memory slice.
                return Err((ProcessLoadError::InternalError, unused_memory));
            }
        }

        let flash_start = process.flash.as_ptr();
        let app_start =
            flash_start.wrapping_add(process.header.get_app_start_offset() as usize) as usize;
        let init_addr =
            flash_start.wrapping_add(process.header.get_init_function_offset() as usize) as usize;
        let fn_base = flash_start as usize;
        let fn_len = process.flash.len();

        // We need to construct a capability with sufficient authority to cover
        // all of a user's code, with permissions to execute it. The entirety of
        // flash is sufficient.
        //
        // # Safety
        //
        // TODO? I don't understand the `new_with_authority()` safety block as
        // it doesn't define what the caller must do.
        let init_fn = unsafe {
            CapabilityPtr::new_with_authority(
                init_addr as *const (),
                fn_base,
                fn_len,
                CapabilityPtrPermissions::Execute,
            )
        };

        process.tasks.map(|tasks| {
            tasks.enqueue(Task::FunctionCall(FunctionCall {
                source: FunctionCallSource::Kernel,
                pc: init_fn,
                argument0: app_start,
                argument1: process.memory_start as usize,
                argument2: process.memory_len,
                argument3: (process.app_break.get() as usize).into(),
            }));
        });

        // Set storage permissions. Put this at the end so that `process` is
        // completely formed before using it to determine the storage
        // permissions.
        process
            .storage_permissions
            .set(storage_permissions_policy.get_permissions(process));

        // Return the process object and a remaining memory for processes slice.
        Ok((Some(process), unused_memory))
    }

    /// Reset the process, resetting all of its state and re-initializing it so
    /// it can start running. Assumes the process is not running but is still in
    /// flash and still has its memory region allocated to it.
    fn reset(&self) -> Result<(), ErrorCode> {
        // We need a new process identifier for this process since the restarted
        // version is in effect a new process. This is also necessary to
        // invalidate any stored `ProcessId`s that point to the old version of
        // the process. However, the process has not moved locations in the
        // processes array, so we copy the existing index.
        let old_index = self.process_id.get().index;
        let new_identifier = self.kernel.create_process_identifier();
        self.process_id
            .set(ProcessId::new(self.kernel, new_identifier, old_index));

        // Reset debug information that is per-execution and not per-process.
        self.debug.reset_last_syscall();
        self.debug.reset_syscall_count();
        self.debug.reset_dropped_upcall_count();
        self.debug.reset_timeslice_expiration_count();

        // Reset MPU region configuration.
        //
        // TODO: ideally, this would be moved into a helper function used by
        // both create() and reset(), but process load debugging complicates
        // this. We just want to create new config with only flash and memory
        // regions.
        //
        // We must have a previous MPU configuration stored, fault the
        // process if this invariant is violated. We avoid allocating
        // a new MPU configuration, as this may eventually exhaust the
        // number of available MPU configurations.
        let mut mpu_config = self.mpu_config.take().ok_or(ErrorCode::FAIL)?;
        self.chip.mpu().reset_config(&mut mpu_config);

        // Allocate MPU region for flash.
        let app_mpu_flash = self.chip.mpu().allocate_region(
            self.flash.as_ptr(),
            self.flash.len(),
            self.flash.len(),
            mpu::Permissions::ReadExecuteOnly,
            &mut mpu_config,
        );
        if app_mpu_flash.is_none() {
            // We were unable to allocate an MPU region for flash. This is very
            // unexpected since we previously ran this process. However, we
            // return now and leave the process faulted and it will not be
            // scheduled.
            return Err(ErrorCode::FAIL);
        }

        // RAM

        // Re-determine the minimum amount of RAM the kernel must allocate to
        // the process based on the specific requirements of the syscall
        // implementation.
        let min_process_memory_size = self
            .chip
            .userspace_kernel_boundary()
            .initial_process_app_brk_size();

        // Recalculate initial_kernel_memory_size as was done in create()
        let grant_ptr_size = mem::size_of::<GrantPointerEntry>();
        let grant_ptrs_num = self.kernel.get_grant_count_and_finalize();
        let grant_ptrs_size = grant_ptrs_num * grant_ptr_size;

        let initial_kernel_memory_size = grant_ptrs_size
            + Self::GRANT_POINTERS_ALIGNMENT
            + Self::CALLBACKS_SIZE
            + Self::CALLBACKS_ALIGNMENT
            + Self::PROCESS_STRUCT_SIZE
            + Self::PROCESS_STRUCT_ALIGNMENT;

        let app_mpu_mem = self.chip.mpu().allocate_app_memory_region(
            self.mem_start(),
            self.memory_len,
            self.memory_len, //we want exactly as much as we had before restart
            min_process_memory_size,
            initial_kernel_memory_size,
            mpu::Permissions::ReadWriteOnly,
            &mut mpu_config,
        );
        let (app_mpu_mem_start, app_mpu_mem_len) = match app_mpu_mem {
            Some((start, len)) => (start, len),
            None => {
                // We couldn't configure the MPU for the process. This shouldn't
                // happen since we were able to start the process before, but at
                // this point it is better to leave the app faulted and not
                // schedule it.
                return Err(ErrorCode::NOMEM);
            }
        };

        // Reset memory pointers now that we know the layout of the process
        // memory and know that we can configure the MPU.

        // app_brk is set based on minimum syscall size above the start of
        // memory.
        let app_brk = app_mpu_mem_start.wrapping_add(min_process_memory_size);
        self.app_break.set(app_brk);
        // kernel_brk is calculated backwards from the end of memory the size of
        // the initial kernel data structures.
        let kernel_brk = app_mpu_mem_start
            .wrapping_add(app_mpu_mem_len)
            .wrapping_sub(initial_kernel_memory_size);
        self.kernel_memory_break.set(kernel_brk);
        // High water mark for `allow`ed memory is reset to the start of the
        // process's memory region.
        self.allow_high_water_mark.set(app_mpu_mem_start);

        // Store the adjusted MPU configuration:
        self.mpu_config.replace(mpu_config);

        // Handle any architecture-specific requirements for a process when it
        // first starts (as it would when it is new).
        let ukb_init_process = self.stored_state.map_or(Err(()), |stored_state| unsafe {
            self.chip.userspace_kernel_boundary().initialize_process(
                app_mpu_mem_start,
                app_brk,
                stored_state,
            )
        });
        match ukb_init_process {
            Ok(()) => {}
            Err(()) => {
                // We couldn't initialize the architecture-specific state for
                // this process. This shouldn't happen since the app was able to
                // be started before, but at this point the app is no longer
                // valid. The best thing we can do now is leave the app as still
                // faulted and not schedule it.
                return Err(ErrorCode::RESERVE);
            }
        }

        self.restart_count.increment();

        // Mark the state as `Yielded` for the scheduler.
        self.state.set(State::Yielded);

        // And queue up this app to be restarted.
        let flash_start = self.flash_start();
        let app_start =
            flash_start.wrapping_add(self.header.get_app_start_offset() as usize) as usize;
        let init_addr =
            flash_start.wrapping_add(self.header.get_init_function_offset() as usize) as usize;

        // We need to construct a capability with sufficient authority to cover all of a user's
        // code, with permissions to execute it. The entirety of flash is sufficient.

        let init_fn = unsafe {
            CapabilityPtr::new_with_authority(
                init_addr as *const (),
                flash_start as usize,
                (self.flash_end() as usize) - (flash_start as usize),
                CapabilityPtrPermissions::Execute,
            )
        };

        self.enqueue_task(Task::FunctionCall(FunctionCall {
            source: FunctionCallSource::Kernel,
            pc: init_fn,
            argument0: app_start,
            argument1: self.memory_start as usize,
            argument2: self.memory_len,
            argument3: (self.app_break.get() as usize).into(),
        }))
    }

    /// Checks if the buffer represented by the passed in base pointer and size
    /// is within the RAM bounds currently exposed to the processes (i.e. ending
    /// at `app_break`). If this method returns `true`, the buffer is guaranteed
    /// to be accessible to the process and to not overlap with the grant
    /// region.
    fn in_app_owned_memory(&self, buf_start_addr: *const u8, size: usize) -> bool {
        // TODO: On some platforms, CapabilityPtr has sufficient authority that we
        // could skip this check.
        // CapabilityPtr needs to make it slightly further, and we need to add
        // interfaces that tell us how much assurance it gives on the current
        // platform.
        let buf_end_addr = buf_start_addr.wrapping_add(size);

        buf_end_addr >= buf_start_addr
            && buf_start_addr >= self.mem_start()
            && buf_end_addr <= self.app_break.get()
    }

    /// Checks if the buffer represented by the passed in base pointer and size
    /// are within the readable region of an application's flash memory.  If
    /// this method returns true, the buffer is guaranteed to be readable to the
    /// process.
    fn in_app_flash_memory(&self, buf_start_addr: *const u8, size: usize) -> bool {
        // TODO: On some platforms, CapabilityPtr has sufficient authority that we
        // could skip this check.
        // CapabilityPtr needs to make it slightly further, and we need to add
        // interfaces that tell us how much assurance it gives on the current
        // platform.
        let buf_end_addr = buf_start_addr.wrapping_add(size);

        buf_end_addr >= buf_start_addr
            && buf_start_addr >= self.flash_non_protected_start()
            && buf_end_addr <= self.flash_end()
    }

    /// Reset all `grant_ptr`s to NULL.
    unsafe fn grant_ptrs_reset(&self) {
        self.grant_pointers.map(|grant_pointers| {
            for grant_entry in grant_pointers.iter_mut() {
                grant_entry.driver_num = 0;
                grant_entry.grant_ptr = ptr::null_mut();
            }
        });
    }

    /// Allocate memory in a process's grant region.
    ///
    /// Ensures that the allocation is of `size` bytes and aligned to `align`
    /// bytes.
    ///
    /// If there is not enough memory, or the MPU cannot isolate the process
    /// accessible region from the new kernel memory break after doing the
    /// allocation, then this will return `None`.
    fn allocate_in_grant_region_internal(&self, size: usize, align: usize) -> Option<NonNull<u8>> {
        self.mpu_config.and_then(|config| {
            // First, compute the candidate new pointer. Note that at this point
            // we have not yet checked whether there is space for this
            // allocation or that it meets alignment requirements.
            let new_break_unaligned = self.kernel_memory_break.get().wrapping_sub(size);

            // Our minimum alignment requirement is two bytes, so that the
            // lowest bit of the address will always be zero and we can use it
            // as a flag. It doesn't hurt to increase the alignment (except for
            // potentially a wasted byte) so we make sure `align` is at least
            // two.
            let align = cmp::max(align, 2);

            // The alignment must be a power of two, 2^a. The expression
            // `!(align - 1)` then returns a mask with leading ones, followed by
            // `a` trailing zeros.
            let alignment_mask = !(align - 1);
            let new_break = (new_break_unaligned as usize & alignment_mask) as *const u8;

            // Verify there is space for this allocation
            if new_break < self.app_break.get() {
                None
                // Verify it didn't wrap around
            } else if new_break > self.kernel_memory_break.get() {
                None
                // Verify this is compatible with the MPU.
            } else if let Err(()) = self.chip.mpu().update_app_memory_region(
                self.app_break.get(),
                new_break,
                mpu::Permissions::ReadWriteOnly,
                config,
            ) {
                None
            } else {
                // Allocation is valid.

                // We always allocate down, so we must lower the
                // kernel_memory_break.
                self.kernel_memory_break.set(new_break);

                // We need `grant_ptr` as a mutable pointer.
                let grant_ptr: *mut u8 = new_break.cast_mut();

                // ### Safety
                //
                // Here we are guaranteeing that `grant_ptr` is not null. We can
                // ensure this because we just created `grant_ptr` based on the
                // process's allocated memory, and we know it cannot be null.
                unsafe { Some(NonNull::new_unchecked(grant_ptr)) }
            }
        })
    }

    /// Create the identifier for a custom grant that grant.rs uses to access
    /// the custom grant.
    ///
    /// We create this identifier by calculating the number of bytes between
    /// where the custom grant starts and the end of the process memory.
    fn create_custom_grant_identifier(&self, ptr: NonNull<u8>) -> ProcessCustomGrantIdentifier {
        let custom_grant_address = ptr.as_ptr() as usize;
        let process_memory_end = self.mem_end() as usize;

        ProcessCustomGrantIdentifier {
            offset: process_memory_end - custom_grant_address,
        }
    }

    /// Use a `ProcessCustomGrantIdentifier` to find the address of the
    /// custom grant.
    ///
    /// This reverses `create_custom_grant_identifier()`.
    fn get_custom_grant_address(&self, identifier: ProcessCustomGrantIdentifier) -> usize {
        let process_memory_end = self.mem_end() as usize;

        // Subtract the offset in the identifier from the end of the process
        // memory to get the address of the custom grant.
        process_memory_end - identifier.offset
    }

    /// Return the app's read and modify storage permissions from the TBF header
    /// if it exists.
    ///
    /// If the header does not exist then return `None`. If the header does
    /// exist, this returns a 5-tuple with:
    ///
    /// - `write_allowed`: bool. If this process should have write permissions.
    /// - `read_count`: usize. How many read IDs are valid.
    /// - `read_ids`: [u32]. The read IDs.
    /// - `modify_count`: usze. How many modify IDs are valid.
    /// - `modify_ids`: [u32]. The modify IDs.
    pub fn get_tbf_storage_permissions(&self) -> Option<(bool, usize, [u32; 8], usize, [u32; 8])> {
        let read_perms = self.header.get_storage_read_ids();
        let modify_perms = self.header.get_storage_modify_ids();

        match (read_perms, modify_perms) {
            (Some((read_count, read_ids)), Some((modify_count, modify_ids))) => Some((
                self.header.get_storage_write_id().is_some(),
                read_count,
                read_ids,
                modify_count,
                modify_ids,
            )),
            _ => None,
        }
    }

    /// The start address of allocated RAM for this process.
    fn mem_start(&self) -> *const u8 {
        self.memory_start
    }

    /// The first address after the end of the allocated RAM for this process.
    fn mem_end(&self) -> *const u8 {
        self.memory_start.wrapping_add(self.memory_len)
    }

    /// The start address of the flash region allocated for this process.
    fn flash_start(&self) -> *const u8 {
        self.flash.as_ptr()
    }

    /// Get the first address of process's flash that isn't protected by the
    /// kernel. The protected range of flash contains the TBF header and
    /// potentially other state the kernel is storing on behalf of the process,
    /// and cannot be edited by the process.
    fn flash_non_protected_start(&self) -> *const u8 {
        ((self.flash.as_ptr() as usize) + self.header.get_protected_size() as usize) as *const u8
    }

    /// The first address after the end of the flash region allocated for this
    /// process.
    fn flash_end(&self) -> *const u8 {
        self.flash.as_ptr().wrapping_add(self.flash.len())
    }

    /// The lowest address of the grant region for the process.
    fn kernel_memory_break(&self) -> *const u8 {
        self.kernel_memory_break.get()
    }

    /// Return the highest address the process has access to, or the current
    /// process memory brk.
    fn app_memory_break(&self) -> *const u8 {
        self.app_break.get()
    }
}