On Tue, Mar 25, 2025 at 6:11 PM Benno Lossin [off-list ref] wrote:

On Tue Mar 25, 2025 at 9:07 PM CET, Tamir Duberstein wrote:

quoted

diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
index 40034f77fc2f..6233af50bab7 100644
--- a/rust/kernel/str.rs
+++ b/rust/kernel/str.rs

@@ -29,7 +29,7 @@ pub const fn is_empty(&self) -> bool {
     #[inline]
     pub const fn from_bytes(bytes: &[u8]) -> &Self {
         // SAFETY: `BStr` is transparent to `[u8]`.
-        unsafe { &*(bytes as *const [u8] as *const BStr) }
+        unsafe { &*(core::mem::transmute::<*const [u8], *const Self>(bytes)) }

Hmm I'm not sure about using `transmute` here. Yes the types are
transparent, but I don't think that we should use it here.

What's your suggestion? I initially tried

let bytes: *const [u8] = bytes;
unsafe { &*bytes.cast() }

but that doesn't compile because of the implicit Sized bound on pointer::cast.

quoted

     }

     /// Strip a prefix from `self`. Delegates to [`slice::strip_prefix`].

@@ -290,7 +290,7 @@ pub const fn from_bytes_with_nul(bytes: &[u8]) -> Result<&Self, CStrConvertError
     #[inline]
     pub unsafe fn from_bytes_with_nul_unchecked_mut(bytes: &mut [u8]) -> &mut CStr {
         // SAFETY: Properties of `bytes` guaranteed by the safety precondition.
-        unsafe { &mut *(bytes as *mut [u8] as *mut CStr) }
+        unsafe { &mut *(core::mem::transmute::<*mut [u8], *mut Self>(bytes)) }
     }

     /// Returns a C pointer to the string.

diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
index 80a9782b1c6e..c042b1fe499e 100644
--- a/rust/kernel/uaccess.rs
+++ b/rust/kernel/uaccess.rs

@@ -242,7 +242,7 @@ pub fn read_raw(&mut self, out: &mut [MaybeUninit<u8>]) -> Result {
     pub fn read_slice(&mut self, out: &mut [u8]) -> Result {
         // SAFETY: The types are compatible and `read_raw` doesn't write uninitialized bytes to
         // `out`.
-        let out = unsafe { &mut *(out as *mut [u8] as *mut [MaybeUninit<u8>]) };
+        let out = unsafe { &mut *(core::mem::transmute::<*mut [u8], *mut [MaybeUninit<u8>]>(out)) };

I have a patch that adds a `cast_slice_mut` method that could be used
here, so I can fix it in that series. But let's not use `transmute` here
either.

See above - I don't know what else I could write here.