1 files changed, 112 insertions, 2 deletions

diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
index 1c7d43771a37..cfc37d81adf2 100644
--- a/rust/kernel/transmute.rs
+++ b/rust/kernel/transmute.rs
@@ -2,6 +2,8 @@
 
 //! Traits for transmuting types.
 
+use core::mem::size_of;
+
 /// Types for which any bit pattern is valid.
 ///
 /// Not all types are valid for all values. For example, a `bool` must be either zero or one, so
@@ -9,10 +11,93 @@
 ///
 /// It's okay for the type to have padding, as initializing those bytes has no effect.
 ///
+/// # Examples
+///
+/// ```
+/// use kernel::transmute::FromBytes;
+///
+/// # fn test() -> Option<()> {
+/// let raw = [1, 2, 3, 4];
+///
+/// let result = u32::from_bytes(&raw)?;
+///
+/// #[cfg(target_endian = "little")]
+/// assert_eq!(*result, 0x4030201);
+///
+/// #[cfg(target_endian = "big")]
+/// assert_eq!(*result, 0x1020304);
+///
+/// # Some(()) }
+/// # test().ok_or(EINVAL)?;
+/// # Ok::<(), Error>(())
+/// ```
+///
 /// # Safety
 ///
 /// All bit-patterns must be valid for this type. This type must not have interior mutability.
-pub unsafe trait FromBytes {}
+pub unsafe trait FromBytes {
+    /// Converts a slice of bytes to a reference to `Self`.
+    ///
+    /// Succeeds if the reference is properly aligned, and the size of `bytes` is equal to that of
+    /// `T` and different from zero.
+    ///
+    /// Otherwise, returns [`None`].
+    fn from_bytes(bytes: &[u8]) -> Option<&Self>
+    where
+        Self: Sized,
+    {
+        let slice_ptr = bytes.as_ptr().cast::<Self>();
+        let size = size_of::<Self>();
+
+        #[allow(clippy::incompatible_msrv)]
+        if bytes.len() == size && slice_ptr.is_aligned() {
+            // SAFETY: Size and alignment were just checked.
+            unsafe { Some(&*slice_ptr) }
+        } else {
+            None
+        }
+    }
+
+    /// Converts a mutable slice of bytes to a reference to `Self`.
+    ///
+    /// Succeeds if the reference is properly aligned, and the size of `bytes` is equal to that of
+    /// `T` and different from zero.
+    ///
+    /// Otherwise, returns [`None`].
+    fn from_bytes_mut(bytes: &mut [u8]) -> Option<&mut Self>
+    where
+        Self: AsBytes + Sized,
+    {
+        let slice_ptr = bytes.as_mut_ptr().cast::<Self>();
+        let size = size_of::<Self>();
+
+        #[allow(clippy::incompatible_msrv)]
+        if bytes.len() == size && slice_ptr.is_aligned() {
+            // SAFETY: Size and alignment were just checked.
+            unsafe { Some(&mut *slice_ptr) }
+        } else {
+            None
+        }
+    }
+
+    /// Creates an owned instance of `Self` by copying `bytes`.
+    ///
+    /// Unlike [`FromBytes::from_bytes`], which requires aligned input, this method can be used on
+    /// non-aligned data at the cost of a copy.
+    fn from_bytes_copy(bytes: &[u8]) -> Option<Self>
+    where
+        Self: Sized,
+    {
+        if bytes.len() == size_of::<Self>() {
+            // SAFETY: we just verified that `bytes` has the same size as `Self`, and per the
+            // invariants of `FromBytes`, any byte sequence of the correct length is a valid value
+            // for `Self`.
+            Some(unsafe { core::ptr::read_unaligned(bytes.as_ptr().cast::<Self>()) })
+        } else {
+            None
+        }
+    }
+}
 
 macro_rules! impl_frombytes {
     ($($({$($generics:tt)*})? $t:ty, )*) => {
@@ -47,7 +132,32 @@ impl_frombytes! {
 ///
 /// Values of this type may not contain any uninitialized bytes. This type must not have interior
 /// mutability.
-pub unsafe trait AsBytes {}
+pub unsafe trait AsBytes {
+    /// Returns `self` as a slice of bytes.
+    fn as_bytes(&self) -> &[u8] {
+        // CAST: `Self` implements `AsBytes` thus all bytes of `self` are initialized.
+        let data = core::ptr::from_ref(self).cast::<u8>();
+        let len = core::mem::size_of_val(self);
+
+        // SAFETY: `data` is non-null and valid for reads of `len * sizeof::<u8>()` bytes.
+        unsafe { core::slice::from_raw_parts(data, len) }
+    }
+
+    /// Returns `self` as a mutable slice of bytes.
+    fn as_bytes_mut(&mut self) -> &mut [u8]
+    where
+        Self: FromBytes,
+    {
+        // CAST: `Self` implements both `AsBytes` and `FromBytes` thus making `Self`
+        // bi-directionally transmutable to `[u8; size_of_val(self)]`.
+        let data = core::ptr::from_mut(self).cast::<u8>();
+        let len = core::mem::size_of_val(self);
+
+        // SAFETY: `data` is non-null and valid for read and writes of `len * sizeof::<u8>()`
+        // bytes.
+        unsafe { core::slice::from_raw_parts_mut(data, len) }
+    }
+}
 
 macro_rules! impl_asbytes {
     ($($({$($generics:tt)*})? $t:ty, )*) => {