Re: [PATCH v6 3/4] rust: uaccess: add typed accessors for userspace pointers

[Boqun Feng]

On Thu, Apr 18, 2024 at 09:35:53PM +0200, Alice Ryhl wrote:
> On Thu, Apr 18, 2024 at 7:27 PM Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
> >
> > On Thu, Apr 18, 2024 at 04:23:06PM +0000, Benno Lossin wrote:
> > > On 18.04.24 15:17, Alice Ryhl wrote:
> > > > On Thu, Apr 18, 2024 at 3:02 PM Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
> > > >>
> > > >> On 18.04.24 10:59, Alice Ryhl wrote:
> > > >>> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> > > >>> index 8fad61268465..9c57c6c75553 100644
> > > >>> --- a/rust/kernel/types.rs
> > > >>> +++ b/rust/kernel/types.rs
> > > >>> @@ -409,3 +409,67 @@ pub enum Either<L, R> {
> > > >>>       /// Constructs an instance of [`Either`] containing a value of type `R`.
> > > >>>       Right(R),
> > > >>>   }
> > > >>> +
> > > >>> +/// Types for which any bit pattern is valid.
> > > >>> +///
> > > >>> +/// Not all types are valid for all values. For example, a `bool` must be either zero or one, so
> > > >>> +/// reading arbitrary bytes into something that contains a `bool` is not okay.
> > > >>> +///
> > > >>> +/// It's okay for the type to have padding, as initializing those bytes has no effect.
> > > >>> +///
> > > >>> +/// # Safety
> > > >>> +///
> > > >>> +/// All bit-patterns must be valid for this type. This type must not have interior mutability.
> > > >>
> > > >> What is the reason for disallowing interior mutability here? I agree
> > > >> that it is necessary for `AsBytes`, but I don't think we need it here.
> >
> > Hmm.. technically, if the interior mutability behaves in a way that each
> > byte is still initialized during the modification, then it should be
> > fine for `AsBytes`, for example and `AtomicI32` (implemented by asm
> > blocks)? Not making any change suggestion, just checking my understand.
> 
> No, that's UB. When the type is not interior mutable, then any two
> loads from the same immutable reference may be assumed to return the
> same value. Changing it with an atomic would violate that since the
> value changes.
> 

Oh, that makes sense! Thanks!

Regards,
Boqun

> Alice