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

[Alice Ryhl]

On Wed, Mar 19, 2025 at 12:23:44AM +0000, Benno Lossin wrote:
> On Tue Mar 18, 2025 at 1:29 PM CET, Alice Ryhl 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@xxxxxxxxx>
> >> Link: https://lore.kernel.org/all/D8EIXDMRXMJP.36TFCGWZBRS3Y@xxxxxxxxx/
> >> Signed-off-by: Tamir Duberstein <tamird@xxxxxxxxx>
> >
> > 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.
> 
> 1) I think we should eliminate all possible `as` conversions. They are
>    non-descriptive (since they can do may *very* different things) and
>    ptr2int conversions are part of that.
> 2) At some point we will have to move to the provenance API, since
>    that's what Rust chose to do. I don't think that doing it at a later
>    point is doing anyone a favor.

We don't *have* to do anything. Sure, most `as` conversions can be
removed now that we have fixed the integer type mappings, but I'm still
not convinced by this case.

Like, sure, use it for that one case in `kernel::str` where it uses
integers for pointers for some reason. But most other cases, provenance
isn't useful.

> 3) I don't understand the argument that this is confusing to C devs.
>    They are just normal functions that are well-documented (and if
>    that's not the case, we can just improve them upstream). And
>    functions are much easier to learn about than `as` casts (those are
>    IMO much more difficult to figure out than then strict provenance
>    functions).

I really don't think that's true, no matter how good the docs are. If
you see `addr as *mut c_void` as a C dev, you are going to immediately
understand what that means. If you see with_exposed_provenance(addr),
you're not going to understand what that means from the name - you have
to interrupt your reading and look up the function with the weird name.

And those docs probably spend a long time talking about stuff that
doesn't matter for your pointer, since it's probably a userspace pointer
or similar.

> Thus I think we should keep this patch (with Boqun's improvement).
> 
> >> 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.
> 
> I agree for this case, but I think we shouldn't be using raw pointers
> for this to begin with. I'd think that a newtype wrapping `usize` is a
> much better fit. It can then also back the `IoRaw` type. AFAIU user
> space pointers don't have provenance, right? (if they do, then we should
> use this API :)

We're doing that to the fullest extent possible already. We only convert
them to pointers when calling C FFI functions that take user pointers as
a raw pointer.

Alice