On Wed Mar 26, 2025 at 11:35 AM CET, Tamir Duberstein wrote:

On Wed, Mar 26, 2025 at 6:31 AM Benno Lossin [off-list ref] wrote:

quoted

On Wed Mar 26, 2025 at 12:54 AM CET, Tamir Duberstein wrote:

quoted

On Tue, Mar 25, 2025 at 6:40 PM Benno Lossin [off-list ref] wrote:

quoted

On Tue Mar 25, 2025 at 11:33 PM CET, Tamir Duberstein wrote:

quoted

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

quoted

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.

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.

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?

[1]: https://github.com/rust-lang/rust-clippy/issues/6372

---
Cheers,
Benno