On Thu Mar 27, 2025 at 8:44 PM CET, Tamir Duberstein wrote:
On Thu, Mar 27, 2025 at 10:15 AM Tamir Duberstein tamird@gmail.com wrote:
On Wed, Mar 26, 2025 at 6:15 PM Benno Lossin benno.lossin@proton.me wrote:
On Wed Mar 26, 2025 at 11:09 PM CET, Tamir Duberstein wrote:
On Wed, Mar 26, 2025 at 5:09 PM Benno Lossin benno.lossin@proton.me wrote:
On Wed Mar 26, 2025 at 8:06 PM CET, Tamir Duberstein wrote:
On Wed, Mar 26, 2025 at 1:36 PM Benno Lossin benno.lossin@proton.me wrote: > On Wed Mar 26, 2025 at 5:57 PM CET, Tamir Duberstein wrote: > > > > 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:
- transmute reference to reference.
- 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?
Not that I am aware of anything.
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'twrite 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)
Seems like your email client auto-wrapped that :(
Benno, would that work for you? Same in str.rs, of course.
For this specific case, I do have a `cast_slice_mut` function I mentioned in the other thread, but that is still stuck in the untrusted data series, I hope that it's ready tomorrow or maybe next week. I'd prefer if we use that (since its implementation also doesn't use `as` casts :). But if you can't wait, then the above is fine.
--- Cheers, Benno