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

Tamir Duberstein
2025-03-26T16:58:01Z

On Wed, Mar 26, 2025 at 12:43 PM Benno Lossin <benno.lossin@proton.me> wrote:
>
> On Wed Mar 26, 2025 at 11:35 AM CET, Tamir Duberstein wrote:
> > On Wed, Mar 26, 2025 at 6:31 AM Benno Lossin <benno.lossin@proton.me> wrote:
> >> On Wed Mar 26, 2025 at 12:54 AM CET, Tamir Duberstein wrote:
> >> > On Tue, Mar 25, 2025 at 6:40 PM Benno Lossin <benno.lossin@proton.me> wrote:
> >> >> On Tue Mar 25, 2025 at 11:33 PM CET, Tamir Duberstein wrote:
> >> >> > On Tue, Mar 25, 2025 at 6:11 PM Benno Lossin <benno.lossin@proton.me> wrote:
> >> >> >> On Tue Mar 25, 2025 at 9:07 PM CET, Tamir Duberstein wrote:
> >> >> >> > 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.
> >> >>
> >> >> This is AFAIK one of the only places where we cannot get rid of the `as`
> >> >> cast. So:
> >> >>
> >> >> let bytes: *const [u8] = bytes;
> >> >> // CAST: `BStr` transparently wraps `[u8]`.
> >> >> let bytes = bytes as *const BStr;
> >> >> // SAFETY: `bytes` is derived from a reference.
> >> >> unsafe { &*bytes }
> >> >>
> >> >> IMO a `transmute` is worse than an `as` cast :)
> >> >
> >> > Hmm, looking at this again we can just transmute ref-to-ref and avoid
> >> > pointers entirely. We're already doing that in
> >> > `CStr::from_bytes_with_nul_unchecked`
> >> >
> >> > Why is transmute worse than an `as` cast?
> >>
> >> It's right in the docs: "`transmute` should be the absolute last
> >> resort." [1]. IIRC, Gary was a bit more lenient in its use, but I think
> >> we should avoid it as much as possible such that people copying code or
> >> taking inspiration also don't use it.
> >>
> >> So for both cases I'd prefer an `as` cast.
> >>
> >> [1]: https://doc.rust-lang.org/std/mem/fn.transmute.html
> >
> > I don't follow the logic. The trouble with `as` casts is that they are
> > very lenient in what they allow, and to do these conversions with `as`
> > casts requires ref -> pointer -> pointer -> pointer deref versus a
> > single transmute. The safety comment perfectly describes why it's OK
> > to do: the types are transparent. So why is `as` casting pointers
> > better? It's just as unchecked as transmuting, and worse, it requires
> > a raw pointer dereference.
>
> Note that you're not transmuting `[u8]` to `BStr`, but `*const [u8]` to
> `*const BStr`. Those pointers have provenance and I'm not sure if
> transmuting them preserves it.

In the current code you're looking at, yes. But in the code I have
locally I'm transmuting `[u8]` to `BStr`. See my earlier reply where I
said "Hmm, looking at this again we can just transmute ref-to-ref and
avoid pointers entirely. We're already doing that in
`CStr::from_bytes_with_nul_unchecked`".

> I tried to find some existing issues about the topic and found that
> there exists a clippy lint `transmute_ptr_to_ptr`. There is an issue
> asking for a better justification [1] and it seems like nobody provided
> one there. Maybe we should ask the opsem team what happens to provenance
> when transmuting?

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