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:

quoted

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 <redacted>
Link: https://lore.kernel.org/all/D8EIXDMRXMJP.36TFCGWZBRS3Y@proton.me/ (local)
Signed-off-by: Tamir Duberstein <redacted>

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.
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).

Thus I think we should keep this patch (with Boqun's improvement).

quoted

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 :)

---
Cheers,
Benno