Module kernel::utilities::mut_imut_buffer

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An enum that can contain a reference to either a mutable or an immutable buffer.

This type is intended for internal use in implementations of HILs or abstractions which need to handle both mutable and immutable buffers.

One motivating use case is keys for public key cryptography. Public keys are often distributed as constant values in the flash of a kernel image (e.g., to verify signatures), which requires they be immutable. Copying them to RAM is expensive because these keys can be very large (e.g., 512 bytes for a 4096-bit RSA key). At the same time, some clients may use dynamically generated or received keys, which are stored in mutable RAM. Requiring that keys be immutable would discard mut on this memory. An implementation can use this type to store either mutable and immutable buffers. The OTBN (OpenTitan Big Number accelerator) is one example use of MutImutBuffer.

Because this type requires dynamic runtime checks that types match, it should not be used in any HILs or standard, external APIs. It is intended only for internal use in implementations.

Author: Alistair Francis


use kernel::utilities::mut_imut_buffer::MutImutBuffer;

let mut mutable = ['a', 'b', 'c', 'd'];
let immutable = ['e', 'f', 'g', 'h'];

let shared_buf = MutImutBuffer::Mutable(&mut mutable);
let shared_buf2 = MutImutBuffer::Immutable(&immutable);


  • An enum which can hold either a mutable or an immutable buffer