Re: [PATCH v5 6/6] rust: use strict provenance APIs

Tamir Duberstein
2025-03-18T14:09:15Z

On Tue, Mar 18, 2025 at 8:29 AM Alice Ryhl <aliceryhl@google.com> wrote:
>
> On Mon, Mar 17, 2025 at 10:23:56AM -0400, Tamir Duberstein wrote:
> > Throughout the tree, use the strict provenance APIs stabilized in Rust
> > 1.84.0[1]. Retain backwards-compatibility by introducing forwarding
> > functions at the `kernel` crate root along with polyfills for rustc <
> > 1.84.0.
> >
> > Use `#[allow(clippy::incompatible_msrv)]` to avoid warnings on rustc <
> > 1.84.0 as our MSRV is 1.78.0.
> >
> > In the `kernel` crate, enable the strict provenance lints on rustc >=
> > 1.84.0; do this in `lib.rs` rather than `Makefile` to avoid introducing
> > compiler flags that are dependent on the rustc version in use.
> >
> > Link: https://blog.rust-lang.org/2025/01/09/Rust-1.84.0.html#strict-provenance-apis [1]
> > Suggested-by: Benno Lossin <benno.lossin@proton.me>
> > Link: https://lore.kernel.org/all/D8EIXDMRXMJP.36TFCGWZBRS3Y@proton.me/
> > Signed-off-by: Tamir Duberstein <tamird@gmail.com>
>
> I'm not convinced that the pros of this change outweigh the cons. I
> think this is going to be too confusing for the C developers who look at
> this code.
>
> > diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> > index 719b0a48ff55..96393bcf6bd7 100644
> > --- a/rust/kernel/uaccess.rs
> > +++ b/rust/kernel/uaccess.rs
> > @@ -226,7 +226,9 @@ pub fn read_raw(&mut self, out: &mut [MaybeUninit<u8>]) -> Result {
> > }
> > // SAFETY: `out_ptr` points into a mutable slice of length `len`, so we may write
> > // that many bytes to it.
> > - let res = unsafe { bindings::copy_from_user(out_ptr, self.ptr as *const c_void, len) };
> > + let res = unsafe {
> > + bindings::copy_from_user(out_ptr, crate::with_exposed_provenance(self.ptr), len)
> > + };
> > if res != 0 {
> > return Err(EFAULT);
> > }
> > @@ -264,7 +266,7 @@ pub fn read<T: FromBytes>(&mut self) -> Result<T> {
> > let res = unsafe {
> > bindings::_copy_from_user(
> > out.as_mut_ptr().cast::<c_void>(),
> > - self.ptr as *const c_void,
> > + crate::with_exposed_provenance(self.ptr),
> > len,
> > )
> > };
>
> That's especially true for cases like this. These are userspace pointers
> that are never dereferenced. It's not useful to care about provenance
> here.
>
> Alice

Let's just drop this last patch. It can be revisited later or not at
all. Perhaps in the future I need to be more willing to say no to
scope creep.