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

Benno Lossin
2025-03-19T14:42:12Z

On Wed Mar 19, 2025 at 3:14 PM CET, Tamir Duberstein wrote:
> On Wed, Mar 19, 2025 at 8:21 AM Alice Ryhl <aliceryhl@google.com> 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:
>> > >> @@ -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
>
> Personally, I agree with Benno that `as` conversions are a misfeature
> in the language.
>
> I think this patch and the ensuing discussion is making perfect the
> enemy of good, so I'd prefer to drop it and revisit when the
> ergonomics have improved.

I don't think that we need to rush on the rest of the patch series.
Boqun's suggestion is very good and I'm not sure which ergonomics need
to be improved here.

---
Cheers,
Benno