On Wed Mar 19, 2025 at 1:21 PM CET, Alice Ryhl wrote: > 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. I disagree, it's only going to get more painful in the long run to change this. >> 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. I see this as a double edged sword, yes `addr as *mut c_void` might seem more easily digestible on the first encounter, but that might also lead them to never look up what it exactly does. And I don't think that we should optimize these functions for C readers. They aren't used commonly (or supposed to IMO) and there are several other functions that are similarly confusing if not more already in our codebase. > 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. For userspace pointers, see below. >> 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. We should make bindgen use that type in those interfaces already. --- Cheers, Benno
