On Thu, Mar 27, 2025 at 10:15 AM Tamir Duberstein [off-list ref] wrote:

On Wed, Mar 26, 2025 at 6:15 PM Benno Lossin [off-list ref] wrote:

quoted

On Wed Mar 26, 2025 at 11:09 PM CET, Tamir Duberstein wrote:

quoted

On Wed, Mar 26, 2025 at 5:09 PM Benno Lossin [off-list ref] wrote:

quoted

On Wed Mar 26, 2025 at 8:06 PM CET, Tamir Duberstein wrote:

quoted

On Wed, Mar 26, 2025 at 1:36 PM Benno Lossin [off-list ref] wrote:

quoted

On Wed Mar 26, 2025 at 5:57 PM CET, Tamir Duberstein wrote:

quoted

Yeah, we should do this - but again: not relevant in this discussion.

I think it's pretty relevant.

It's not relevant because we're no longer talking about transmuting
pointer to pointer. The two options are:
1. transmute reference to reference.
2. coerce reference to pointer, `as` cast pointer to pointer (triggers
`ptr_as_ptr`), reborrow pointer to reference.

If anyone can help me understand why (2) is better than (1), I'd
certainly appreciate it.

I am very confident that (2) is correct. With (1) I'm not sure (see
above), so that's why I mentioned it.

Can you help me understand why you're confident about (2) but not (1)?

My explanation from above explains why I'm not confident about (1):

    For ptr-to-int transmutes, I know that they will probably remove
    provenance, hence I am a bit cautious about using them for ptr-to-ptr or
    ref-to-ref.

The reason I'm confident about (2) is that that is the canonical way to
cast the type of a reference pointing to an `!Sized` value.

Do you have a citation, other than the transmute doc?

Turns out this appeases clippy:

diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
index 80a9782b1c6e..7a6fc78fc314 100644
--- a/rust/kernel/uaccess.rs
+++ b/rust/kernel/uaccess.rs

@@ -240,9 +240,10 @@ pub fn read_raw(&mut self, out: &mut

[MaybeUninit<u8>]) -> Result {
     /// Fails with [`EFAULT`] if the read happens on a bad address,
or if the read goes out of
     /// bounds of this [`UserSliceReader`]. This call may modify
`out` even if it returns an error.
     pub fn read_slice(&mut self, out: &mut [u8]) -> Result {
+        let out: *mut [u8] = out;
         // 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 *(out as *mut [MaybeUninit<u8>]) };
         self.read_raw(out)
     }

Benno, would that work for you? Same in str.rs, of course.