Re: [PATCH v7 7/7] rust: enable `clippy::ref_as_ptr` lint

[Benno Lossin]

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@xxxxxxxxx> wrote:
>> On Wed, Mar 26, 2025 at 6:15 PM Benno Lossin <benno.lossin@xxxxxxxxx> 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@xxxxxxxxx> 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@xxxxxxxxx> 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:
>> > >> > 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?

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'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)
>      }

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