kernel/

processbuffer.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.

//! Data structures for passing application memory to the kernel.
//!
//! A Tock process can pass read-write or read-only buffers into the
//! kernel for it to use. The kernel checks that read-write buffers
//! exist within a process's RAM address space, and that read-only
//! buffers exist either within its RAM or flash address space. These
//! buffers are shared with the allow_read_write() and
//! allow_read_only() system calls.
//!
//! A read-write and read-only call is mapped to the high-level Rust
//! types [`ReadWriteProcessBuffer`] and [`ReadOnlyProcessBuffer`]
//! respectively. The memory regions can be accessed through the
//! [`ReadableProcessBuffer`] and [`WriteableProcessBuffer`] traits,
//! implemented on the process buffer structs.
//!
//! Each access to the buffer structs requires a liveness check to ensure that
//! the process memory is still valid. For a more traditional interface, users
//! can convert buffers into [`ReadableProcessSlice`] or
//! [`WriteableProcessSlice`] and use these for the lifetime of their
//! operations. Users cannot hold live-lived references to these slices,
//! however.

use core::cell::Cell;
use core::marker::PhantomData;
use core::ops::{Deref, Index, Range, RangeFrom, RangeTo};

use crate::ErrorCode;
use crate::capabilities;
use crate::process::{self, ProcessId};

/// Convert a process buffer's internal pointer+length representation to a
/// [`ReadableProcessSlice`].
///
/// This function will automatically convert zero-length process buffers into
/// valid zero-sized Rust slices, regardless of the value of `ptr` (i.e., `ptr`
/// is allowed to be null for these slices).
///
/// # Safety
///
/// In the case of `len != 0`, the memory `[ptr; ptr + len)` must be assigned to
/// one or more processes, and `ptr` must be nonzero. This memory region must be
/// mapped as _readable_, and optionally _writable_. It must remain a valid,
/// readable allocation assigned to one or more processes for the entire
/// lifetime `'a`, and must not be used as backing memory for any Rust
/// allocations (apart from other process slices).
///
/// Callers must ensure that, for its lifetime `'a`, no other programs (other
/// than this Tock kernel instance) modify the memory behind a
/// [`ReadableProcessSlice`]. This includes userspace programs, which must not
/// run in parallel to the Tock kernel holding a process slice reference, or
/// other Tock kernel instances executing in parallel.
///
/// It is sound for multiple (partially) aliased [`ReadableProcessSlice`]s or
/// [`WriteableProcessSlice`]s to be in scope at the same time, as they use
/// interior mutability, and their memory is not accessed in parallel by
/// userspace or other programs running concurrently.
unsafe fn raw_processbuf_to_roprocessslice<'a>(
    ptr: *const u8,
    len: usize,
) -> &'a ReadableProcessSlice {
    let ptr: *const ReadableProcessByte = ptr.cast();

    // Transmute a slice reference over readable (read-only or read-write, and
    // potentially aliased) bytes into a `ReadableProcessSlice` reference.
    //
    // # Safety
    //
    // This is sound, as `ReadableProcessSlice` is merely a
    // `#[repr(transparent)]` wrapper around `[ReadableProcessByte]`. However,
    // we cannot build this struct safely from an intermediate
    // `[ReadableProcessByte]` slice reference, as we cannot dereference this
    // unsized type.
    unsafe {
        core::mem::transmute::<&[ReadableProcessByte], &ReadableProcessSlice>(
            // Create a slice of `ReadableProcessByte`s from the supplied
            // pointer. `ReadableProcessByte` itself permits interior mutability,
            // and hence this intermediate reference is safe to construct given the
            // safety contract of this function.
            //
            // Rust has very strict requirements on pointer validity[1] which also
            // in part apply to accesses of length 0. We allow an application to
            // supply arbitrary pointers if the buffer length is 0, but this is not
            // allowed for Rust slices. For instance, a null pointer is _never_
            // valid, not even for accesses of size zero.
            //
            // To get a pointer which does not point to valid (allocated) memory,
            // but is safe to construct for accesses of size zero, we must call
            // NonNull::dangling(). The resulting pointer is guaranteed to be
            // well-aligned and uphold the guarantees required for accesses of size
            // zero.
            //
            // [1]: https://doc.rust-lang.org/core/ptr/index.html#safety
            match len {
                0 => core::slice::from_raw_parts(
                    core::ptr::NonNull::<ReadableProcessByte>::dangling().as_ptr(),
                    0,
                ),
                _ => core::slice::from_raw_parts(ptr, len),
            },
        )
    }
}

/// Convert a process buffer's internal pointer+length representation to a
/// [`WriteableProcessSlice`].
///
/// This function will automatically convert zero-length process buffers into
/// valid zero-sized Rust slices, regardless of the value of `ptr` (i.e., `ptr`
/// is allowed to be null for these slices).
///
/// # Safety
///
/// In the case of `len != 0`, the memory `[ptr; ptr + len)` must be assigned to
/// one or more processes, and `ptr` must be nonzero. This memory region must be
/// mapped as _readable_ and _writable_. It must remain a valid, readable and
/// writeable allocation assigned to one or more processes for the entire
/// lifetime `'a`, and must not be used as backing memory for any Rust
/// allocations (apart from other process slices).
///
/// Callers must ensure that, for its lifetime `'a`, no other programs (other
/// than this Tock kernel instance) modify the memory behind a
/// [`ReadableProcessSlice`]. This includes userspace programs, which must not
/// run in parallel to the Tock kernel holding a process slice reference, or
/// other Tock kernel instances executing in parallel.
///
/// It is sound for multiple (partially) aliased [`ReadableProcessSlice`]s or
/// [`WriteableProcessSlice`]s to be in scope at the same time, as they use
/// interior mutability, and their memory is not accessed in parallel by
/// userspace or other programs running concurrently.
unsafe fn raw_processbuf_to_rwprocessslice<'a>(
    ptr: *mut u8,
    len: usize,
) -> &'a WriteableProcessSlice {
    // Transmute a slice reference over writeable and potentially aliased bytes
    // into a `WriteableProcessSlice` reference.
    //
    // # Safety
    //
    // This is sound, as `WriteableProcessSlice` is merely a
    // `#[repr(transparent)]` wrapper around `[Cell<u8>]`. However, we cannot
    // build this struct safely from an intermediate `[WriteableProcessByte]`
    // slice reference, as we cannot dereference this unsized type.
    unsafe {
        core::mem::transmute::<&[Cell<u8>], &WriteableProcessSlice>(
            // Create a slice of `Cell<u8>`s from the supplied pointer. `Cell<u8>`
            // itself permits interior mutability, and hence this intermediate
            // reference is safe to construct given the safety contract of this
            // function.
            //
            // Rust has very strict requirements on pointer validity[1] which also
            // in part apply to accesses of length 0. We allow an application to
            // supply arbitrary pointers if the buffer length is 0, but this is not
            // allowed for Rust slices. For instance, a null pointer is _never_
            // valid, not even for accesses of size zero.
            //
            // To get a pointer which does not point to valid (allocated) memory,
            // but is safe to construct for accesses of size zero, we must call
            // NonNull::dangling(). The resulting pointer is guaranteed to be
            // well-aligned and uphold the guarantees required for accesses of size
            // zero.
            //
            // [1]: https://doc.rust-lang.org/core/ptr/index.html#safety
            match len {
                0 => core::slice::from_raw_parts(
                    core::ptr::NonNull::<Cell<u8>>::dangling().as_ptr(),
                    0,
                ),
                _ => core::slice::from_raw_parts(ptr as *const Cell<u8>, len),
            },
        )
    }
}

/// A readable region of userspace process memory.
///
/// This trait can be used to gain read-only access to memory regions
/// wrapped in either a [`ReadOnlyProcessBuffer`] or a
/// [`ReadWriteProcessBuffer`] type.
///
/// # Safety
///
/// This is an `unsafe trait` as users of this trait need to trust that the
/// implementation of [`ReadableProcessBuffer::ptr`] is correct. Implementors of
/// this trait must ensure that the [`ReadableProcessBuffer::ptr`] method
/// follows the semantics and invariants described in its documentation.
pub unsafe trait ReadableProcessBuffer {
    /// Length of the memory region.
    ///
    /// If the process is no longer alive and the memory has been
    /// reclaimed, this method must return 0.
    ///
    /// # Default Process Buffer
    ///
    /// A default instance of a process buffer must return 0.
    fn len(&self) -> usize;

    /// Pointer to the first byte of the userspace-allowed memory region.
    ///
    /// If [`ReadableProcessBuffer::len`] returns a non-zero value,
    /// then this method is guaranteed to return a pointer to the
    /// start address of a memory region (of length returned by
    /// `len`), allowable by a userspace process, and allowed to the
    /// kernel for read operations. The memory region must not be
    /// written to through this pointer.
    ///
    /// If the length of the initially shared memory region
    /// (irrespective of the return value of
    /// [`len`](ReadableProcessBuffer::len)) is 0, this function
    /// returns a pointer to address `0x0`. This is because processes
    /// may allow zero-length buffer to share no memory with the
    /// kernel. Because these buffers have zero length, they may have
    /// any arbitrary pointer value. However, these "dummy addresses"
    /// should not be leaked, so this method returns 0 for zero-length
    /// slices. Care must be taken to not create a Rust (slice)
    /// reference over a null-pointer, as that is undefined behavior.
    ///
    /// Users of this pointer must not produce any mutable aliasing, such as by
    /// creating a reference from this pointer concurrently with calling
    /// [`WriteableProcessBuffer::mut_enter`].
    ///
    /// # Default Process Buffer
    ///
    /// A default instance of a process buffer must return a pointer
    /// to address `0x0`.
    fn ptr(&self) -> *const u8;

    /// Applies a function to the (read only) process slice reference
    /// pointed to by the process buffer.
    ///
    /// If the process is no longer alive and the memory has been
    /// reclaimed, this method must return
    /// `Err(process::Error::NoSuchApp)`.
    ///
    /// # Default Process Buffer
    ///
    /// A default instance of a process buffer must return
    /// `Err(process::Error::NoSuchApp)` without executing the passed
    /// closure.
    fn enter<F, R>(&self, fun: F) -> Result<R, process::Error>
    where
        F: FnOnce(&ReadableProcessSlice) -> R;
}

/// A readable and writeable region of userspace process memory.
///
/// This trait can be used to gain read-write access to memory regions
/// wrapped in a [`ReadWriteProcessBuffer`].
///
/// This is a supertrait of [`ReadableProcessBuffer`], which features
/// methods allowing mutable access.
///
/// # Safety
///
/// This is an `unsafe trait` as users of this trait need to trust that the
/// implementation of [`WriteableProcessBuffer::mut_ptr`] is
/// correct.
///
/// Implementors of this trait must ensure that the
/// [`WriteableProcessBuffer::mut_ptr`] method follows the semantics and
/// invariants described in its documentation, and that the length of the
/// [`WriteableProcessBuffer`] is identical to the value returned by the
/// [`ReadableProcessBuffer::len`] supertrait method.
///
/// Additionally, when using the default implementation of `mut_ptr` provided by
/// this trait, implementors guarantee that the readable pointer returned by
/// [`ReadableProcessBuffer::ptr`] points to the same read-write allowed shared
/// memory region as described by the [`WriteableProcessBuffer`], and that
/// writes through the pointer returned by [`ReadableProcessBuffer::ptr`] are
/// sound for [`ReadableProcessBuffer::len`] bytes, notwithstanding any aliasing
/// requirements.
pub unsafe trait WriteableProcessBuffer: ReadableProcessBuffer {
    /// Pointer to the first byte of the userspace-allowed memory region.
    ///
    /// If [`ReadableProcessBuffer::len`] returns a non-zero value,
    /// then this method is guaranteed to return a pointer to the
    /// start address of a memory region (of length returned by
    /// `len`), allowable by a userspace process, and allowed to the
    /// kernel for read or write operations.
    ///
    /// If the length of the initially shared memory region
    /// (irrespective of the return value of
    /// [`len`](ReadableProcessBuffer::len)) is 0, this function
    /// returns a pointer to address `0x0`. This is because processes
    /// may allow zero-length buffer to share no memory with the
    /// kernel. Because these buffers have zero length, they may have
    /// any arbitrary pointer value. However, these "dummy addresses"
    /// should not be leaked, so this method returns 0 for zero-length
    /// slices. Care must be taken to not create a Rust (slice)
    /// reference over a null-pointer, as that is undefined behavior.
    ///
    /// Users of this pointer must not produce any mutable aliasing, such as by
    /// creating a reference from this pointer concurrently with calling
    /// [`WriteableProcessBuffer::mut_enter`].
    ///
    /// # Default Process Buffer
    ///
    /// A default instance of a process buffer must return a pointer
    /// to address `0x0`.
    fn mut_ptr(&self) -> *mut u8 {
        ReadableProcessBuffer::ptr(self).cast_mut()
    }

    /// Applies a function to the mutable process slice reference
    /// pointed to by the [`ReadWriteProcessBuffer`].
    ///
    /// If the process is no longer alive and the memory has been
    /// reclaimed, this method must return
    /// `Err(process::Error::NoSuchApp)`.
    ///
    /// # Default Process Buffer
    ///
    /// A default instance of a process buffer must return
    /// `Err(process::Error::NoSuchApp)` without executing the passed
    /// closure.
    fn mut_enter<F, R>(&self, fun: F) -> Result<R, process::Error>
    where
        F: FnOnce(&WriteableProcessSlice) -> R;
}

/// Read-only buffer shared by a userspace process.
///
/// This struct is provided to capsules when a process `allow`s a
/// particular section of its memory to the kernel and gives the
/// kernel read access to this memory.
///
/// It can be used to obtain a [`ReadableProcessSlice`], which is
/// based around a slice of [`Cell`]s. This is because a userspace can
/// `allow` overlapping sections of memory into different
/// [`ReadableProcessSlice`]. Having at least one mutable Rust slice
/// along with read-only slices to overlapping memory in Rust violates
/// Rust's aliasing rules. A slice of [`Cell`]s avoids this issue by
/// explicitly supporting interior mutability. Still, a memory barrier
/// prior to switching to userspace is required, as the compiler is
/// free to reorder reads and writes, even through [`Cell`]s.
pub struct ReadOnlyProcessBuffer {
    ptr: *const u8,
    len: usize,
    process_id: Option<ProcessId>,
}

impl ReadOnlyProcessBuffer {
    /// Construct a new [`ReadOnlyProcessBuffer`] over a given pointer and
    /// length.
    ///
    /// # Safety requirements
    ///
    /// Refer to the safety requirements of
    /// [`ReadOnlyProcessBuffer::new_external`].
    pub(crate) unsafe fn new(ptr: *const u8, len: usize, process_id: ProcessId) -> Self {
        ReadOnlyProcessBuffer {
            ptr,
            len,
            process_id: Some(process_id),
        }
    }

    /// Construct a new [`ReadOnlyProcessBuffer`] over a given pointer
    /// and length.
    ///
    /// Publicly accessible constructor, which requires the
    /// [`capabilities::ExternalProcessCapability`] capability. This
    /// is provided to allow implementations of the
    /// [`Process`](crate::process::Process) trait outside of the
    /// `kernel` crate.
    ///
    /// # Safety requirements
    ///
    /// If the length is `0`, an arbitrary pointer may be passed into
    /// `ptr`. It does not necessarily have to point to allocated
    /// memory, nor does it have to meet [Rust's pointer validity
    /// requirements](https://doc.rust-lang.org/core/ptr/index.html#safety).
    /// [`ReadOnlyProcessBuffer`] must ensure that all Rust slices
    /// with a length of `0` must be constructed over a valid (but not
    /// necessarily allocated) base pointer.
    ///
    /// If the length is not `0`, the memory region of `[ptr; ptr +
    /// len)` must be valid memory of the process of the given
    /// [`ProcessId`]. It must be allocated and and accessible over
    /// the entire lifetime of the [`ReadOnlyProcessBuffer`]. It must
    /// not point to memory outside of the process' accessible memory
    /// range, or point (in part) to other processes or kernel
    /// memory. The `ptr` must meet [Rust's requirements for pointer
    /// validity](https://doc.rust-lang.org/core/ptr/index.html#safety),
    /// in particular it must have a minimum alignment of
    /// `core::mem::align_of::<u8>()` on the respective platform. It
    /// must point to memory mapped as _readable_ and optionally
    /// _writable_ and _executable_.
    pub unsafe fn new_external(
        ptr: *const u8,
        len: usize,
        process_id: ProcessId,
        _cap: &dyn capabilities::ExternalProcessCapability,
    ) -> Self {
        // # Safety
        //
        // See function description.
        unsafe { Self::new(ptr, len, process_id) }
    }

    /// Consumes the ReadOnlyProcessBuffer, returning its constituent
    /// pointer and size. This ensures that there cannot
    /// simultaneously be both a `ReadOnlyProcessBuffer` and a pointer
    /// to its internal data.
    ///
    /// `consume` can be used when the kernel needs to pass the
    /// underlying values across the kernel-to-user boundary (e.g., in
    /// return values to system calls).
    pub(crate) fn consume(self) -> (*const u8, usize) {
        (self.ptr, self.len)
    }
}

unsafe impl ReadableProcessBuffer for ReadOnlyProcessBuffer {
    /// Return the length of the buffer in bytes.
    fn len(&self) -> usize {
        self.process_id
            .map_or(0, |pid| pid.kernel.process_map_or(0, pid, |_| self.len))
    }

    /// Return the pointer to the start of the buffer.
    fn ptr(&self) -> *const u8 {
        if self.len == 0 {
            core::ptr::null::<u8>()
        } else {
            self.ptr
        }
    }

    /// Access the contents of the buffer in a closure.
    ///
    /// This verifies the process is still valid before accessing the underlying
    /// memory.
    fn enter<F, R>(&self, fun: F) -> Result<R, process::Error>
    where
        F: FnOnce(&ReadableProcessSlice) -> R,
    {
        match self.process_id {
            None => Err(process::Error::NoSuchApp),
            Some(pid) => pid
                .kernel
                .process_map_or(Err(process::Error::NoSuchApp), pid, |_| {
                    // Safety: `kernel.process_map_or()` validates that
                    // the process still exists and its memory is still
                    // valid. In particular, `Process` tracks the "high water
                    // mark" of memory that the process has `allow`ed to the
                    // kernel. Because `Process` does not feature an API to
                    // move the "high water mark" down again, which would be
                    // called once a `ProcessBuffer` has been passed back into
                    // the kernel, a given `Process` implementation must assume
                    // that the memory described by a once-allowed
                    // `ProcessBuffer` is still in use, and thus will not
                    // permit the process to free any memory after it has
                    // been `allow`ed to the kernel once. This guarantees
                    // that the buffer is safe to convert into a slice
                    // here. For more information, refer to the
                    // comment and subsequent discussion on tock/tock#2632:
                    // https://github.com/tock/tock/pull/2632#issuecomment-869974365
                    Ok(fun(unsafe {
                        raw_processbuf_to_roprocessslice(self.ptr, self.len)
                    }))
                }),
        }
    }
}

impl Default for ReadOnlyProcessBuffer {
    fn default() -> Self {
        ReadOnlyProcessBuffer {
            ptr: core::ptr::null_mut::<u8>(),
            len: 0,
            process_id: None,
        }
    }
}

/// Provides access to a [`ReadOnlyProcessBuffer`] with a restricted lifetime.
/// This automatically dereferences into a ReadOnlyProcessBuffer
pub struct ReadOnlyProcessBufferRef<'a> {
    buf: ReadOnlyProcessBuffer,
    _phantom: PhantomData<&'a ()>,
}

impl ReadOnlyProcessBufferRef<'_> {
    /// Construct a new [`ReadOnlyProcessBufferRef`] over a given pointer and
    /// length with a lifetime derived from the caller.
    ///
    /// # Safety requirements
    ///
    /// Refer to the safety requirements of
    /// [`ReadOnlyProcessBuffer::new_external`]. The derived lifetime can
    /// help enforce the invariant that this incoming pointer may only
    /// be access for a certain duration.
    pub(crate) unsafe fn new(ptr: *const u8, len: usize, process_id: ProcessId) -> Self {
        // # Safety
        //
        // See function description.
        unsafe {
            Self {
                buf: ReadOnlyProcessBuffer::new(ptr, len, process_id),
                _phantom: PhantomData,
            }
        }
    }
}

impl Deref for ReadOnlyProcessBufferRef<'_> {
    type Target = ReadOnlyProcessBuffer;
    fn deref(&self) -> &Self::Target {
        &self.buf
    }
}

/// Read-writable buffer shared by a userspace process.
///
/// This struct is provided to capsules when a process `allows` a
/// particular section of its memory to the kernel and gives the
/// kernel read and write access to this memory.
///
/// It can be used to obtain a [`WriteableProcessSlice`], which is
/// based around a slice of [`Cell`]s. This is because a userspace can
/// `allow` overlapping sections of memory into different
/// [`WriteableProcessSlice`]. Having at least one mutable Rust slice
/// along with read-only or other mutable slices to overlapping memory
/// in Rust violates Rust's aliasing rules. A slice of [`Cell`]s
/// avoids this issue by explicitly supporting interior
/// mutability. Still, a memory barrier prior to switching to
/// userspace is required, as the compiler is free to reorder reads
/// and writes, even through [`Cell`]s.
pub struct ReadWriteProcessBuffer {
    ptr: *mut u8,
    len: usize,
    process_id: Option<ProcessId>,
}

impl ReadWriteProcessBuffer {
    /// Construct a new [`ReadWriteProcessBuffer`] over a given
    /// pointer and length.
    ///
    /// # Safety requirements
    ///
    /// Refer to the safety requirements of
    /// [`ReadWriteProcessBuffer::new_external`].
    pub(crate) unsafe fn new(ptr: *mut u8, len: usize, process_id: ProcessId) -> Self {
        ReadWriteProcessBuffer {
            ptr,
            len,
            process_id: Some(process_id),
        }
    }

    /// Construct a new [`ReadWriteProcessBuffer`] over a given
    /// pointer and length.
    ///
    /// Publicly accessible constructor, which requires the
    /// [`capabilities::ExternalProcessCapability`] capability. This
    /// is provided to allow implementations of the
    /// [`Process`](crate::process::Process) trait outside of the
    /// `kernel` crate.
    ///
    /// # Safety requirements
    ///
    /// If the length is `0`, an arbitrary pointer may be passed into
    /// `ptr`. It does not necessarily have to point to allocated
    /// memory, nor does it have to meet [Rust's pointer validity
    /// requirements](https://doc.rust-lang.org/core/ptr/index.html#safety).
    /// [`ReadWriteProcessBuffer`] must ensure that all Rust slices
    /// with a length of `0` must be constructed over a valid (but not
    /// necessarily allocated) base pointer.
    ///
    /// If the length is not `0`, the memory region of `[ptr; ptr +
    /// len)` must be valid memory of the process of the given
    /// [`ProcessId`]. It must be allocated and and accessible over
    /// the entire lifetime of the [`ReadWriteProcessBuffer`]. It must
    /// not point to memory outside of the process' accessible memory
    /// range, or point (in part) to other processes or kernel
    /// memory. The `ptr` must meet [Rust's requirements for pointer
    /// validity](https://doc.rust-lang.org/core/ptr/index.html#safety),
    /// in particular it must have a minimum alignment of
    /// `core::mem::align_of::<u8>()` on the respective platform. It
    /// must point to memory mapped as _readable_ and optionally
    /// _writable_ and _executable_.
    pub unsafe fn new_external(
        ptr: *mut u8,
        len: usize,
        process_id: ProcessId,
        _cap: &dyn capabilities::ExternalProcessCapability,
    ) -> Self {
        // # Safety
        //
        // See function description.
        unsafe { Self::new(ptr, len, process_id) }
    }

    /// Consumes the ReadWriteProcessBuffer, returning its constituent
    /// pointer and size. This ensures that there cannot
    /// simultaneously be both a `ReadWriteProcessBuffer` and a pointer to
    /// its internal data.
    ///
    /// `consume` can be used when the kernel needs to pass the
    /// underlying values across the kernel-to-user boundary (e.g., in
    /// return values to system calls).
    pub(crate) fn consume(self) -> (*mut u8, usize) {
        (self.ptr, self.len)
    }

    /// This is a `const` version of `Default::default` with the same
    /// semantics.
    ///
    /// Having a const initializer allows initializing a fixed-size
    /// array with default values without the struct being marked
    /// `Copy` as such:
    ///
    /// ```
    /// use kernel::processbuffer::ReadWriteProcessBuffer;
    /// const DEFAULT_RWPROCBUF_VAL: ReadWriteProcessBuffer
    ///     = ReadWriteProcessBuffer::const_default();
    /// let my_array = [DEFAULT_RWPROCBUF_VAL; 12];
    /// ```
    pub const fn const_default() -> Self {
        Self {
            ptr: core::ptr::null_mut::<u8>(),
            len: 0,
            process_id: None,
        }
    }
}

unsafe impl ReadableProcessBuffer for ReadWriteProcessBuffer {
    /// Return the length of the buffer in bytes.
    fn len(&self) -> usize {
        self.process_id
            .map_or(0, |pid| pid.kernel.process_map_or(0, pid, |_| self.len))
    }

    /// Return the pointer to the start of the buffer.
    fn ptr(&self) -> *const u8 {
        if self.len == 0 {
            core::ptr::null::<u8>()
        } else {
            self.ptr
        }
    }

    /// Access the contents of the buffer in a closure.
    ///
    /// This verifies the process is still valid before accessing the underlying
    /// memory.
    fn enter<F, R>(&self, fun: F) -> Result<R, process::Error>
    where
        F: FnOnce(&ReadableProcessSlice) -> R,
    {
        match self.process_id {
            None => Err(process::Error::NoSuchApp),
            Some(pid) => pid
                .kernel
                .process_map_or(Err(process::Error::NoSuchApp), pid, |_| {
                    // Safety: `kernel.process_map_or()` validates that
                    // the process still exists and its memory is still
                    // valid. In particular, `Process` tracks the "high water
                    // mark" of memory that the process has `allow`ed to the
                    // kernel. Because `Process` does not feature an API to
                    // move the "high water mark" down again, which would be
                    // called once a `ProcessBuffer` has been passed back into
                    // the kernel, a given `Process` implementation must assume
                    // that the memory described by a once-allowed
                    // `ProcessBuffer` is still in use, and thus will not
                    // permit the process to free any memory after it has
                    // been `allow`ed to the kernel once. This guarantees
                    // that the buffer is safe to convert into a slice
                    // here. For more information, refer to the
                    // comment and subsequent discussion on tock/tock#2632:
                    // https://github.com/tock/tock/pull/2632#issuecomment-869974365
                    Ok(fun(unsafe {
                        raw_processbuf_to_roprocessslice(self.ptr, self.len)
                    }))
                }),
        }
    }
}

unsafe impl WriteableProcessBuffer for ReadWriteProcessBuffer {
    fn mut_enter<F, R>(&self, fun: F) -> Result<R, process::Error>
    where
        F: FnOnce(&WriteableProcessSlice) -> R,
    {
        match self.process_id {
            None => Err(process::Error::NoSuchApp),
            Some(pid) => pid
                .kernel
                .process_map_or(Err(process::Error::NoSuchApp), pid, |_| {
                    // Safety: `kernel.process_map_or()` validates that
                    // the process still exists and its memory is still
                    // valid. In particular, `Process` tracks the "high water
                    // mark" of memory that the process has `allow`ed to the
                    // kernel. Because `Process` does not feature an API to
                    // move the "high water mark" down again, which would be
                    // called once a `ProcessBuffer` has been passed back into
                    // the kernel, a given `Process` implementation must assume
                    // that the memory described by a once-allowed
                    // `ProcessBuffer` is still in use, and thus will not
                    // permit the process to free any memory after it has
                    // been `allow`ed to the kernel once. This guarantees
                    // that the buffer is safe to convert into a slice
                    // here. For more information, refer to the
                    // comment and subsequent discussion on tock/tock#2632:
                    // https://github.com/tock/tock/pull/2632#issuecomment-869974365
                    Ok(fun(unsafe {
                        raw_processbuf_to_rwprocessslice(self.ptr, self.len)
                    }))
                }),
        }
    }
}

impl Default for ReadWriteProcessBuffer {
    fn default() -> Self {
        Self::const_default()
    }
}

/// Provides access to a [`ReadWriteProcessBuffer`] with a restricted lifetime.
/// This automatically dereferences into a ReadWriteProcessBuffer
pub struct ReadWriteProcessBufferRef<'a> {
    buf: ReadWriteProcessBuffer,
    _phantom: PhantomData<&'a ()>,
}

impl ReadWriteProcessBufferRef<'_> {
    /// Construct a new [`ReadWriteProcessBufferRef`] over a given pointer and
    /// length with a lifetime derived from the caller.
    ///
    /// # Safety requirements
    ///
    /// Refer to the safety requirements of
    /// [`ReadWriteProcessBuffer::new_external`]. The derived lifetime can
    /// help enforce the invariant that this incoming pointer may only
    /// be access for a certain duration.
    pub(crate) unsafe fn new(ptr: *mut u8, len: usize, process_id: ProcessId) -> Self {
        // # Safety
        //
        // See function description.
        unsafe {
            Self {
                buf: ReadWriteProcessBuffer::new(ptr, len, process_id),
                _phantom: PhantomData,
            }
        }
    }
}

impl Deref for ReadWriteProcessBufferRef<'_> {
    type Target = ReadWriteProcessBuffer;
    fn deref(&self) -> &Self::Target {
        &self.buf
    }
}

/// A shareable region of userspace memory.
///
/// This trait can be used to gain read-write access to memory regions
/// wrapped in a ProcessBuffer type.
// We currently don't need any special functionality in the kernel for this
// type so we alias it as `ReadWriteProcessBuffer`.
pub type UserspaceReadableProcessBuffer = ReadWriteProcessBuffer;

/// Equivalent of the Rust core library's
/// [`SliceIndex`](core::slice::SliceIndex) type for process slices.
///
/// This helper trait is used to abstract over indexing operators into
/// process slices, and is used to "overload" the `.get()` methods
/// such that it can be called with multiple different indexing
/// operators.
///
/// While we can use the core library's `SliceIndex` trait, parameterized over
/// our own `ProcessSlice` types, this trait includes mandatory methods that are
/// undesirable for the process buffer infrastructure, such as unchecked or
/// mutable index operations. Furthermore, implementing it requires the
/// `slice_index_methods` nightly feature. Thus we vendor our own, small variant
/// of this trait.
pub trait ProcessSliceIndex<PB: ?Sized>: private_process_slice_index::Sealed {
    type Output: ?Sized;
    fn get(self, slice: &PB) -> Option<&Self::Output>;
    fn index(self, slice: &PB) -> &Self::Output;
}

// Analog to `private_slice_index` from
// https://github.com/rust-lang/rust/blob/a1eceec00b2684f947481696ae2322e20d59db60/library/core/src/slice/index.rs#L149
mod private_process_slice_index {
    use core::ops::{Range, RangeFrom, RangeTo};

    pub trait Sealed {}

    impl Sealed for usize {}
    impl Sealed for Range<usize> {}
    impl Sealed for RangeFrom<usize> {}
    impl Sealed for RangeTo<usize> {}
}

/// Read-only wrapper around a [`Cell`]
///
/// This type is used in providing the [`ReadableProcessSlice`]. The
/// memory over which a [`ReadableProcessSlice`] exists must never be
/// written to by the kernel. However, it may either exist in flash
/// (read-only memory) or RAM (read-writeable memory). Consequently, a
/// process may `allow` memory overlapping with a
/// [`ReadOnlyProcessBuffer`] also simultaneously through a
/// [`ReadWriteProcessBuffer`]. Hence, the kernel can have two
/// references to the same memory, where one can lead to mutation of
/// the memory contents. Therefore, the kernel must use [`Cell`]s
/// around the bytes shared with userspace, to avoid violating Rust's
/// aliasing rules.
///
/// This read-only wrapper around a [`Cell`] only exposes methods
/// which are safe to call on a process-shared read-only `allow`
/// memory.
#[repr(transparent)]
pub struct ReadableProcessByte {
    cell: Cell<u8>,
}

impl ReadableProcessByte {
    #[inline]
    pub fn get(&self) -> u8 {
        self.cell.get()
    }
}

/// Readable and accessible slice of memory of a process buffer.
///
///
/// The only way to obtain this struct is through a
/// [`ReadWriteProcessBuffer`] or [`ReadOnlyProcessBuffer`].
///
/// Slices provide a more convenient, traditional interface to process
/// memory. These slices are transient, as the underlying buffer must
/// be checked each time a slice is created. This is usually enforced
/// by the anonymous lifetime defined by the creation of the slice.
#[repr(transparent)]
pub struct ReadableProcessSlice {
    slice: [ReadableProcessByte],
}

fn cast_byte_slice_to_process_slice(byte_slice: &[ReadableProcessByte]) -> &ReadableProcessSlice {
    // As ReadableProcessSlice is a transparent wrapper around its inner type,
    // [ReadableProcessByte], we can safely transmute a reference to the inner
    // type as a reference to the outer type with the same lifetime.
    unsafe { core::mem::transmute::<&[ReadableProcessByte], &ReadableProcessSlice>(byte_slice) }
}

// Allow a u8 slice to be viewed as a ReadableProcessSlice to allow client code
// to be authored once and accept either [u8] or ReadableProcessSlice.
impl<'a> From<&'a [u8]> for &'a ReadableProcessSlice {
    fn from(val: &'a [u8]) -> Self {
        // # Safety
        //
        // The layout of a [u8] and ReadableProcessSlice are guaranteed to be
        // the same. This also extends the lifetime of the buffer, so aliasing
        // rules are thus maintained properly.
        unsafe { core::mem::transmute(val) }
    }
}

// Allow a mutable u8 slice to be viewed as a ReadableProcessSlice to allow
// client code to be authored once and accept either [u8] or
// ReadableProcessSlice.
impl<'a> From<&'a mut [u8]> for &'a ReadableProcessSlice {
    fn from(val: &'a mut [u8]) -> Self {
        // # Safety
        //
        // The layout of a [u8] and ReadableProcessSlice are guaranteed to be
        // the same. This also extends the mutable lifetime of the buffer, so
        // aliasing rules are thus maintained properly.
        unsafe { core::mem::transmute(val) }
    }
}

impl ReadableProcessSlice {
    /// Copy the contents of a [`ReadableProcessSlice`] into a mutable
    /// slice reference.
    ///
    /// The length of `self` must be the same as `dest`. Subslicing
    /// can be used to obtain a slice of matching length.
    ///
    /// # Panics
    ///
    /// This function will panic if `self.len() != dest.len()`.
    pub fn copy_to_slice(&self, dest: &mut [u8]) {
        // The panic code path was put into a cold function to not
        // bloat the call site.
        #[inline(never)]
        #[cold]
        #[track_caller]
        fn len_mismatch_fail(dst_len: usize, src_len: usize) -> ! {
            panic!(
                "source slice length ({}) does not match destination slice length ({})",
                src_len, dst_len,
            );
        }

        if self.copy_to_slice_or_err(dest).is_err() {
            len_mismatch_fail(dest.len(), self.len());
        }
    }

    /// Copy the contents of a [`ReadableProcessSlice`] into a mutable
    /// slice reference.
    ///
    /// The length of `self` must be the same as `dest`. Subslicing
    /// can be used to obtain a slice of matching length.
    pub fn copy_to_slice_or_err(&self, dest: &mut [u8]) -> Result<(), ErrorCode> {
        // Method implemetation adopted from the
        // core::slice::copy_from_slice method implementation:
        // https://doc.rust-lang.org/src/core/slice/mod.rs.html#3034-3036

        if self.len() != dest.len() {
            Err(ErrorCode::SIZE)
        } else {
            // _If_ this turns out to not be efficiently optimized, it
            // should be possible to use a ptr::copy_nonoverlapping here
            // given we have exclusive mutable access to the destination
            // slice which will never be in process memory, and the layout
            // of &[ReadableProcessByte] is guaranteed to be compatible to
            // &[u8].
            for (i, b) in self.slice.iter().enumerate() {
                dest[i] = b.get();
            }
            Ok(())
        }
    }

    /// Return the length of the slice in bytes.
    pub fn len(&self) -> usize {
        self.slice.len()
    }

    /// Return an iterator over the bytes of the slice.
    pub fn iter(&self) -> core::slice::Iter<'_, ReadableProcessByte> {
        self.slice.iter()
    }

    /// Iterate the slice in chunks.
    pub fn chunks(
        &self,
        chunk_size: usize,
    ) -> impl core::iter::Iterator<Item = &ReadableProcessSlice> {
        self.slice
            .chunks(chunk_size)
            .map(cast_byte_slice_to_process_slice)
    }

    /// Access a portion of the slice with bounds checking. If the access is not
    /// within the slice then `None` is returned.
    pub fn get<I: ProcessSliceIndex<Self>>(
        &self,
        index: I,
    ) -> Option<&<I as ProcessSliceIndex<Self>>::Output> {
        index.get(self)
    }

    /// Access a portion of the slice with bounds checking. If the access is not
    /// within the slice then `None` is returned.
    #[deprecated = "Use ReadableProcessSlice::get instead"]
    pub fn get_from(&self, range: RangeFrom<usize>) -> Option<&ReadableProcessSlice> {
        range.get(self)
    }

    /// Access a portion of the slice with bounds checking. If the access is not
    /// within the slice then `None` is returned.
    #[deprecated = "Use ReadableProcessSlice::get instead"]
    pub fn get_to(&self, range: RangeTo<usize>) -> Option<&ReadableProcessSlice> {
        range.get(self)
    }
}

impl ProcessSliceIndex<ReadableProcessSlice> for usize {
    type Output = ReadableProcessByte;

    fn get(self, slice: &ReadableProcessSlice) -> Option<&Self::Output> {
        slice.slice.get(self)
    }

    fn index(self, slice: &ReadableProcessSlice) -> &Self::Output {
        &slice.slice[self]
    }
}

impl ProcessSliceIndex<ReadableProcessSlice> for Range<usize> {
    type Output = ReadableProcessSlice;

    fn get(self, slice: &ReadableProcessSlice) -> Option<&Self::Output> {
        slice.slice.get(self).map(cast_byte_slice_to_process_slice)
    }

    fn index(self, slice: &ReadableProcessSlice) -> &Self::Output {
        cast_byte_slice_to_process_slice(&slice.slice[self])
    }
}

impl ProcessSliceIndex<ReadableProcessSlice> for RangeFrom<usize> {
    type Output = ReadableProcessSlice;

    fn get(self, slice: &ReadableProcessSlice) -> Option<&Self::Output> {
        slice.slice.get(self).map(cast_byte_slice_to_process_slice)
    }

    fn index(self, slice: &ReadableProcessSlice) -> &Self::Output {
        cast_byte_slice_to_process_slice(&slice.slice[self])
    }
}

impl ProcessSliceIndex<ReadableProcessSlice> for RangeTo<usize> {
    type Output = ReadableProcessSlice;

    fn get(self, slice: &ReadableProcessSlice) -> Option<&Self::Output> {
        slice.slice.get(self).map(cast_byte_slice_to_process_slice)
    }

    fn index(self, slice: &ReadableProcessSlice) -> &Self::Output {
        cast_byte_slice_to_process_slice(&slice.slice[self])
    }
}

impl<I: ProcessSliceIndex<Self>> Index<I> for ReadableProcessSlice {
    type Output = I::Output;

    fn index(&self, index: I) -> &Self::Output {
        index.index(self)
    }
}

/// Read-writeable and accessible slice of memory of a process buffer
///
/// The only way to obtain this struct is through a
/// [`ReadWriteProcessBuffer`].
///
/// Slices provide a more convenient, traditional interface to process
/// memory. These slices are transient, as the underlying buffer must
/// be checked each time a slice is created. This is usually enforced
/// by the anonymous lifetime defined by the creation of the slice.
#[repr(transparent)]
pub struct WriteableProcessSlice {
    slice: [Cell<u8>],
}

fn cast_cell_slice_to_process_slice(cell_slice: &[Cell<u8>]) -> &WriteableProcessSlice {
    // # Safety
    //
    // As WriteableProcessSlice is a transparent wrapper around its inner type,
    // [Cell<u8>], we can safely transmute a reference to the inner type as the
    // outer type with the same lifetime.
    unsafe { core::mem::transmute(cell_slice) }
}

// Allow a mutable u8 slice to be viewed as a WritableProcessSlice to allow
// client code to be authored once and accept either [u8] or
// WriteableProcessSlice.
impl<'a> From<&'a mut [u8]> for &'a WriteableProcessSlice {
    fn from(val: &'a mut [u8]) -> Self {
        // # Safety
        //
        // The layout of a [u8] and WriteableProcessSlice are guaranteed to be
        // the same. This also extends the mutable lifetime of the buffer, so
        // aliasing rules are thus maintained properly.
        unsafe { core::mem::transmute(val) }
    }
}

impl WriteableProcessSlice {
    /// Copy the contents of a [`WriteableProcessSlice`] into a mutable
    /// slice reference.
    ///
    /// The length of `self` must be the same as `dest`. Subslicing
    /// can be used to obtain a slice of matching length.
    ///
    /// # Panics
    ///
    /// This function will panic if `self.len() != dest.len()`.
    pub fn copy_to_slice(&self, dest: &mut [u8]) {
        // The panic code path was put into a cold function to not
        // bloat the call site.
        #[inline(never)]
        #[cold]
        #[track_caller]
        fn len_mismatch_fail(dst_len: usize, src_len: usize) -> ! {
            panic!(
                "source slice length ({}) does not match destination slice length ({})",
                src_len, dst_len,
            );
        }

        if self.copy_to_slice_or_err(dest).is_err() {
            len_mismatch_fail(dest.len(), self.len());
        }
    }

    /// Copy the contents of a [`WriteableProcessSlice`] into a mutable
    /// slice reference.
    ///
    /// The length of `self` must be the same as `dest`. Subslicing
    /// can be used to obtain a slice of matching length.
    pub fn copy_to_slice_or_err(&self, dest: &mut [u8]) -> Result<(), ErrorCode> {
        // Method implemetation adopted from the
        // core::slice::copy_from_slice method implementation:
        // https://doc.rust-lang.org/src/core/slice/mod.rs.html#3034-3036

        if self.len() != dest.len() {
            Err(ErrorCode::SIZE)
        } else {
            // _If_ this turns out to not be efficiently optimized, it
            // should be possible to use a ptr::copy_nonoverlapping here
            // given we have exclusive mutable access to the destination
            // slice which will never be in process memory, and the layout
            // of &[Cell<u8>] is guaranteed to be compatible to &[u8].
            self.slice
                .iter()
                .zip(dest.iter_mut())
                .for_each(|(src, dst)| *dst = src.get());
            Ok(())
        }
    }

    /// Copy the contents of a slice of bytes into a [`WriteableProcessSlice`].
    ///
    /// The length of `src` must be the same as `self`. Subslicing can
    /// be used to obtain a slice of matching length.
    ///
    /// # Panics
    ///
    /// This function will panic if `src.len() != self.len()`.
    pub fn copy_from_slice(&self, src: &[u8]) {
        // Method implemetation adopted from the
        // core::slice::copy_from_slice method implementation:
        // https://doc.rust-lang.org/src/core/slice/mod.rs.html#3034-3036

        // The panic code path was put into a cold function to not
        // bloat the call site.
        #[inline(never)]
        #[cold]
        #[track_caller]
        fn len_mismatch_fail(dst_len: usize, src_len: usize) -> ! {
            panic!(
                "src slice len ({}) != dest slice len ({})",
                src_len, dst_len,
            );
        }

        if self.copy_from_slice_or_err(src).is_err() {
            len_mismatch_fail(self.len(), src.len());
        }
    }

    /// Copy the contents of a slice of bytes into a [`WriteableProcessSlice`].
    ///
    /// The length of `src` must be the same as `self`. Subslicing can
    /// be used to obtain a slice of matching length.
    pub fn copy_from_slice_or_err(&self, src: &[u8]) -> Result<(), ErrorCode> {
        // Method implemetation adopted from the
        // core::slice::copy_from_slice method implementation:
        // https://doc.rust-lang.org/src/core/slice/mod.rs.html#3034-3036

        if self.len() != src.len() {
            Err(ErrorCode::SIZE)
        } else {
            // _If_ this turns out to not be efficiently optimized, it
            // should be possible to use a ptr::copy_nonoverlapping here
            // given we have exclusive mutable access to the destination
            // slice which will never be in process memory, and the layout
            // of &[Cell<u8>] is guaranteed to be compatible to &[u8].
            src.iter()
                .zip(self.slice.iter())
                .for_each(|(src, dst)| dst.set(*src));
            Ok(())
        }
    }

    /// Return the length of the slice in bytes.
    pub fn len(&self) -> usize {
        self.slice.len()
    }

    /// Return an iterator over the slice.
    pub fn iter(&self) -> core::slice::Iter<'_, Cell<u8>> {
        self.slice.iter()
    }

    /// Iterate over the slice in chunks.
    pub fn chunks(
        &self,
        chunk_size: usize,
    ) -> impl core::iter::Iterator<Item = &WriteableProcessSlice> {
        self.slice
            .chunks(chunk_size)
            .map(cast_cell_slice_to_process_slice)
    }

    /// Access a portion of the slice with bounds checking. If the access is not
    /// within the slice then `None` is returned.
    pub fn get<I: ProcessSliceIndex<Self>>(
        &self,
        index: I,
    ) -> Option<&<I as ProcessSliceIndex<Self>>::Output> {
        index.get(self)
    }

    /// Access a portion of the slice with bounds checking. If the access is not
    /// within the slice then `None` is returned.
    #[deprecated = "Use WriteableProcessSlice::get instead"]
    pub fn get_from(&self, range: RangeFrom<usize>) -> Option<&WriteableProcessSlice> {
        range.get(self)
    }

    /// Access a portion of the slice with bounds checking. If the access is not
    /// within the slice then `None` is returned.
    #[deprecated = "Use WriteableProcessSlice::get instead"]
    pub fn get_to(&self, range: RangeTo<usize>) -> Option<&WriteableProcessSlice> {
        range.get(self)
    }
}

impl ProcessSliceIndex<WriteableProcessSlice> for usize {
    type Output = Cell<u8>;

    fn get(self, slice: &WriteableProcessSlice) -> Option<&Self::Output> {
        slice.slice.get(self)
    }

    fn index(self, slice: &WriteableProcessSlice) -> &Self::Output {
        &slice.slice[self]
    }
}

impl ProcessSliceIndex<WriteableProcessSlice> for Range<usize> {
    type Output = WriteableProcessSlice;

    fn get(self, slice: &WriteableProcessSlice) -> Option<&Self::Output> {
        slice.slice.get(self).map(cast_cell_slice_to_process_slice)
    }

    fn index(self, slice: &WriteableProcessSlice) -> &Self::Output {
        cast_cell_slice_to_process_slice(&slice.slice[self])
    }
}

impl ProcessSliceIndex<WriteableProcessSlice> for RangeFrom<usize> {
    type Output = WriteableProcessSlice;

    fn get(self, slice: &WriteableProcessSlice) -> Option<&Self::Output> {
        slice.slice.get(self).map(cast_cell_slice_to_process_slice)
    }

    fn index(self, slice: &WriteableProcessSlice) -> &Self::Output {
        cast_cell_slice_to_process_slice(&slice.slice[self])
    }
}

impl ProcessSliceIndex<WriteableProcessSlice> for RangeTo<usize> {
    type Output = WriteableProcessSlice;

    fn get(self, slice: &WriteableProcessSlice) -> Option<&Self::Output> {
        slice.slice.get(self).map(cast_cell_slice_to_process_slice)
    }

    fn index(self, slice: &WriteableProcessSlice) -> &Self::Output {
        cast_cell_slice_to_process_slice(&slice.slice[self])
    }
}

impl<I: ProcessSliceIndex<Self>> Index<I> for WriteableProcessSlice {
    type Output = I::Output;

    fn index(&self, index: I) -> &Self::Output {
        index.index(self)
    }
}

#[cfg(test)]
mod miri_tests {
    use super::*;
    use core::cell::UnsafeCell;

    // Helper to get a raw mutable pointer to the backing memory we use to
    // create process slices over. This backing memory, though allocated by
    // Rust, contains only `UnsafeCell`s and thus is suitable for creating
    // process slice references over.
    fn get_backing_memory_ptr<const N: usize>(mem: &[UnsafeCell<u8>; N]) -> *mut u8 {
        mem as *const _ as *mut u8
    }

    #[test]
    fn test_basic_read_write() {
        let memory = [const { UnsafeCell::new(0u8) }; 16];
        let ptr = get_backing_memory_ptr(&memory);
        let slice = unsafe { raw_processbuf_to_rwprocessslice(ptr, memory.len()) };

        // Test writing via the slice
        slice[0].set(42);
        slice[5].set(100);

        // Test reading back
        assert_eq!(slice[0].get(), 42);
        assert_eq!(slice[5].get(), 100);

        // Verify backing memory was actually updated
        assert_eq!(unsafe { *memory[0].get() }, 42);
    }

    #[test]
    fn test_concurrent_rw_rw_aliasing() {
        // Ensure multiple mutable slices to the same memory do not violate tree
        // borrows. This works because WriteableProcessSlice uses Cell
        // internally.
        let memory = [const { UnsafeCell::new(0u8) }; 16];
        let ptr = get_backing_memory_ptr(&memory);

        // Create two overlapping slices
        let slice1 = unsafe { raw_processbuf_to_rwprocessslice(ptr, memory.len()) };
        let slice2 = unsafe { raw_processbuf_to_rwprocessslice(ptr, memory.len()) };

        slice1[0].set(10);
        assert_eq!(slice2[0].get(), 10);

        slice2[0].set(20);
        assert_eq!(slice1[0].get(), 20);

        // Test interleaved access
        let sub1 = slice1.get(0..4).unwrap();
        let sub2 = slice2.get(2..6).unwrap();

        // sub1: [0, 1, 2, 3]
        // sub2:       [2, 3, 4, 5]
        // Intersection at indices 2 and 3 of the original buffer

        sub1[2].set(55); // Index 2 of backing
        assert_eq!(sub2[0].get(), 55); // Idx 0 of sub2 is idx 2 of backing
    }

    #[test]
    fn test_concurrent_ro_rw_aliasing() {
        // Ensure multiple mutable slices to the same memory do not violate tree
        // borrows. This works because ReadnableProcessSlice and
        // WriteableProcessSlice both use Cell internally.
        let memory = [const { UnsafeCell::new(0u8) }; 16];
        let ptr = get_backing_memory_ptr(&memory);

        // Create two overlapping slices
        let slice1 = unsafe { raw_processbuf_to_roprocessslice(ptr, memory.len()) };
        let slice2 = unsafe { raw_processbuf_to_rwprocessslice(ptr, memory.len()) };

        slice2[0].set(20);
        assert_eq!(slice1[0].get(), 20);

        // Test interleaved access
        let sub1 = slice1.get(0..4).unwrap();
        let sub2 = slice2.get(2..6).unwrap();

        // sub1: [0, 1, 2, 3]
        // sub2:       [2, 3, 4, 5]
        // Intersection at indices 2 and 3 of the original buffer

        sub2[0].set(55); // Index 0 of sub2 is index 2 of backing
        assert_eq!(sub1[2].get(), 55); // Index 2 of backing
    }

    #[test]
    fn test_zero_length_null_ptr_ro() {
        // Should be safe to create a 0-len slice from a null pointer
        let slice = unsafe { raw_processbuf_to_roprocessslice(core::ptr::null_mut(), 0) };
        assert_eq!(slice.len(), 0);
        assert!(slice.get(0).is_none());

        // Iteration should simply yield nothing
        let mut count = 0;
        for _ in slice.iter() {
            count += 1;
        }
        assert_eq!(count, 0);

        // Slice should be created over a non-null pointer
        // (NonNull::dangling()):
        assert_eq!(
            slice as *const ReadableProcessSlice as *const u8,
            core::ptr::NonNull::<u8>::dangling().as_ptr(),
        );
    }

    #[test]
    fn test_zero_length_non_null_ptr_ro() {
        // Should be safe to create a 0-len slice from any arbitrary
        // non-null pointer:
        let slice = unsafe {
            raw_processbuf_to_roprocessslice(
                // Under strict provenance, we cannot simply cast an arbitrary
                // integer into a pointer. However, with a zero-length process
                // slice, the pointer passed to this function must never be
                // dereferencable anyways. Thus we simply start from a
                // null-pointer, and derive another pointer from it (and its
                // provenance) at an offset.
                core::ptr::null_mut::<u8>().wrapping_byte_add(42),
                0,
            )
        };
        assert_eq!(slice.len(), 0);
        assert!(slice.get(0).is_none());

        // Iteration should simply yield nothing
        let mut count = 0;
        for _ in slice.iter() {
            count += 1;
        }
        assert_eq!(count, 0);

        // Slice should not retain its pointer, and return a non-null
        // (dangling) pointer instead:
        assert_eq!(
            slice as *const ReadableProcessSlice as *const u8,
            core::ptr::NonNull::<u8>::dangling().as_ptr()
        );
    }

    #[test]
    fn test_zero_length_null_ptr_rw() {
        // Should be safe to create a 0-len slice from a null pointer
        let slice = unsafe { raw_processbuf_to_rwprocessslice(core::ptr::null_mut(), 0) };
        assert_eq!(slice.len(), 0);
        assert!(slice.get(0).is_none());

        // Iteration should simply yield nothing
        let mut count = 0;
        for _ in slice.iter() {
            count += 1;
        }
        assert_eq!(count, 0);

        // Slice should be created over a non-null pointer
        // (NonNull::dangling()):
        assert_eq!(
            slice as *const WriteableProcessSlice as *const u8,
            core::ptr::NonNull::<u8>::dangling().as_ptr(),
        );
    }

    #[test]
    fn test_zero_length_non_null_ptr_rw() {
        // Should be safe to create a 0-len slice from any arbitrary
        // non-null pointer:
        let slice = unsafe {
            raw_processbuf_to_rwprocessslice(
                // Under strict provenance, we cannot simply cast an arbitrary
                // integer into a pointer. However, with a zero-length process
                // slice, the pointer passed to this function must never be
                // dereferencable anyways. Thus we simply start from a
                // null-pointer, and derive another pointer from it (and its
                // provenance) at an offset.
                core::ptr::null_mut::<u8>().wrapping_byte_add(42),
                0,
            )
        };
        assert_eq!(slice.len(), 0);
        assert!(slice.get(0).is_none());

        // Iteration should simply yield nothing
        let mut count = 0;
        for _ in slice.iter() {
            count += 1;
        }
        assert_eq!(count, 0);

        // Slice should not retain its pointer, and return a non-null
        // (dangling) pointer instead:
        assert_eq!(
            slice as *const WriteableProcessSlice as *const u8,
            core::ptr::NonNull::<u8>::dangling().as_ptr()
        );
    }

    #[test]
    fn test_out_of_bounds_ro() {
        let memory = [const { UnsafeCell::new(0u8) }; 4];
        let ptr = get_backing_memory_ptr(&memory);
        let slice = unsafe { raw_processbuf_to_roprocessslice(ptr, 4) };

        assert!(slice.get(3).is_some());
        assert!(slice.get(4).is_none());
        assert!(slice.get(100).is_none());

        // Range OOB
        assert!(slice.get(2..5).is_none());
    }

    #[test]
    #[should_panic(expected = "index out of bounds: the len is 4 but the index is 4")]
    fn test_out_of_bounds_panic_ro() {
        let memory = [const { UnsafeCell::new(0u8) }; 4];
        let ptr = get_backing_memory_ptr(&memory);
        let slice = unsafe { raw_processbuf_to_roprocessslice(ptr, 4) };

        assert_eq!(slice[3].get(), 0);

        // This is out of bounds and will panic:
        assert_eq!(slice[4].get(), 0);
    }

    #[test]
    fn test_out_of_bounds_rw() {
        let memory = [const { UnsafeCell::new(0u8) }; 4];
        let ptr = get_backing_memory_ptr(&memory);
        let slice = unsafe { raw_processbuf_to_rwprocessslice(ptr, 4) };

        assert!(slice.get(3).is_some());
        assert!(slice.get(4).is_none());
        assert!(slice.get(100).is_none());

        // Range OOB
        assert!(slice.get(2..5).is_none());
    }

    #[test]
    #[should_panic(expected = "index out of bounds: the len is 4 but the index is 4")]
    fn test_out_of_bounds_panic_rw() {
        let memory = [const { UnsafeCell::new(0u8) }; 4];
        let ptr = get_backing_memory_ptr(&memory);
        let slice = unsafe { raw_processbuf_to_rwprocessslice(ptr, 4) };

        assert_eq!(slice[3].get(), 0);

        // This is out of bounds and will panic:
        assert_eq!(slice[4].get(), 0);
    }

    #[test]
    fn test_copy_logic() {
        let memory = [const { UnsafeCell::new(0u8) }; 4];
        let ptr = get_backing_memory_ptr(&memory);
        let src_data = [10, 20, 30, 40];
        let mut dst_data = [0u8; 4];

        let slice = unsafe { raw_processbuf_to_rwprocessslice(ptr, 4) };

        // Copy into slice
        slice.copy_from_slice(&src_data);
        assert_eq!(slice[0].get(), 10);
        assert_eq!(slice[3].get(), 40);

        // Copy out of slice
        slice.copy_to_slice(&mut dst_data);
        assert_eq!(dst_data, src_data);
    }

    #[test]
    #[should_panic(
        expected = "source slice length (4) does not match destination slice length (2)"
    )]
    fn test_copy_panic_len_mismatch() {
        let memory = [const { UnsafeCell::new(0u8) }; 4];
        let ptr = get_backing_memory_ptr(&memory);
        let mut small_dst = [0u8; 2];

        let slice = unsafe { raw_processbuf_to_rwprocessslice(ptr, 4) };
        slice.copy_to_slice(&mut small_dst);
    }

    #[test]
    fn test_transmute_from_immutable_slice() {
        // This test exercises the `From<&[u8]>` implementation for
        // ReadableProcessSlice.
        //
        // We take a standard, immutable Rust slice (`&[u8]`). This creates a
        // shared, read-only borrow of the stack memory.  We then convert it
        // into a `&ReadableProcessSlice`. This struct wraps
        // `ReadableProcessByte`, which wraps `Cell<u8>`.
        //
        // This is problematic under stacked-borrows, as we are transmuting
        // `&[u8]` (immutable, noalias) to `&[Cell<u8>]` (shared, interior
        // mutability).
        //
        // Therefore, we expect the following results:
        //
        // - Stacked Borrows (Default Miri as of Jan 2026): FAIL.
        //
        //   Stacked Borrows forbids "upgrading" a SharedReadOnly reference to
        //   one that claims it can mutate (SharedReadWrite), even if we don't
        //   actually write.
        //
        // - Tree Borrows (`-Zmiri-tree-borrows`): PASS.
        //
        //   Tree Borrows is experimental and handles "retagging" differently.
        //   It tolerates this transmute as long as we do not actually perform a
        //   write operation through the Cell while the original data is frozen.
        //
        let data = [10u8, 20, 30, 40];
        let slice: &[u8] = &data;

        // 1. Convert &u8 to &ReadableProcessSlice (which wraps Cell<u8>)
        let proc_slice: &ReadableProcessSlice = slice.into();

        // 2. Read from it.
        //
        // Even though we only read, the type of `proc_slice` implies the
        // *capability* to mutate, which contradicts the provenance of `slice`.
        assert_eq!(proc_slice[0].get(), 10);
        assert_eq!(proc_slice[3].get(), 40);
    }
}