rust: kernel: move `FromBytes` and `AsBytes` traits to a new `transmute` module

Refactor the `FromBytes` and `AsBytes` traits from `types.rs` into a new
`transmute.rs` module:

 - Add `rust/kernel/transmute.rs` with the definitions of `FromBytes`
   and `AsBytes`.

 - Remove the same trait definitions from `rust/kernel/types.rs`.

 - Update `rust/kernel/uaccess.rs` to import `AsBytes` and `FromBytes`
   from `transmute.rs`.

The traits and their implementations remain unchanged.

Suggested-by: Benno Lossin <benno.lossin@proton.me>
Link: https://github.com/Rust-for-Linux/linux/issues/1117
Signed-off-by: Aliet Exposito Garcia <aliet.exposito@gmail.com>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20240918225115.2309224-2-aliet.exposito@gmail.com
[ Rebased on top of the lints series and slightly reworded. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 71 insertions, 0 deletions

diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
new file mode 100644
index 000000000000..1c7d43771a37
--- /dev/null
+++ b/rust/kernel/transmute.rs
@@ -0,0 +1,71 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Traits for transmuting types.
+
+/// 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
+/// reading arbitrary bytes into something that contains a `bool` is not okay.
+///
+/// It's okay for the type to have padding, as initializing those bytes has no effect.
+///
+/// # Safety
+///
+/// All bit-patterns must be valid for this type. This type must not have interior mutability.
+pub unsafe trait FromBytes {}
+
+macro_rules! impl_frombytes {
+    ($($({$($generics:tt)*})? $t:ty, )*) => {
+        // SAFETY: Safety comments written in the macro invocation.
+        $(unsafe impl$($($generics)*)? FromBytes for $t {})*
+    };
+}
+
+impl_frombytes! {
+    // SAFETY: All bit patterns are acceptable values of the types below.
+    u8, u16, u32, u64, usize,
+    i8, i16, i32, i64, isize,
+
+    // SAFETY: If all bit patterns are acceptable for individual values in an array, then all bit
+    // patterns are also acceptable for arrays of that type.
+    {<T: FromBytes>} [T],
+    {<T: FromBytes, const N: usize>} [T; N],
+}
+
+/// Types that can be viewed as an immutable slice of initialized bytes.
+///
+/// If a struct implements this trait, then it is okay to copy it byte-for-byte to userspace. This
+/// means that it should not have any padding, as padding bytes are uninitialized. Reading
+/// uninitialized memory is not just undefined behavior, it may even lead to leaking sensitive
+/// information on the stack to userspace.
+///
+/// The struct should also not hold kernel pointers, as kernel pointer addresses are also considered
+/// sensitive. However, leaking kernel pointers is not considered undefined behavior by Rust, so
+/// this is a correctness requirement, but not a safety requirement.
+///
+/// # Safety
+///
+/// Values of this type may not contain any uninitialized bytes. This type must not have interior
+/// mutability.
+pub unsafe trait AsBytes {}
+
+macro_rules! impl_asbytes {
+    ($($({$($generics:tt)*})? $t:ty, )*) => {
+        // SAFETY: Safety comments written in the macro invocation.
+        $(unsafe impl$($($generics)*)? AsBytes for $t {})*
+    };
+}
+
+impl_asbytes! {
+    // SAFETY: Instances of the following types have no uninitialized portions.
+    u8, u16, u32, u64, usize,
+    i8, i16, i32, i64, isize,
+    bool,
+    char,
+    str,
+
+    // SAFETY: If individual values in an array have no uninitialized portions, then the array
+    // itself does not have any uninitialized portions either.
+    {<T: AsBytes>} [T],
+    {<T: AsBytes, const N: usize>} [T; N],
+}