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-api... [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.
- 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.
- 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.
- 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