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

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Date: Thu, 18 Apr 2024 13:52:12 -0400
From: Trevor Gross <tmgross@...ch.edu>
To: Benno Lossin <benno.lossin@...ton.me>, Boqun Feng <boqun.feng@...il.com>
Cc: Alice Ryhl <aliceryhl@...gle.com>, Miguel Ojeda <ojeda@...nel.org>, 
	Matthew Wilcox <willy@...radead.org>, Al Viro <viro@...iv.linux.org.uk>, 
	Andrew Morton <akpm@...ux-foundation.org>, Kees Cook <keescook@...omium.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Andreas Hindborg <a.hindborg@...sung.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Arve Hjønnevåg <arve@...roid.com>, 
	Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>, 
	Joel Fernandes <joel@...lfernandes.org>, Carlos Llamas <cmllamas@...gle.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, Arnd Bergmann <arnd@...db.de>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, 
	Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH v6 3/4] rust: uaccess: add typed accessors for userspace pointers

On Thu, Apr 18, 2024 at 9:03 AM Benno Lossin <benno.lossin@...ton.me> 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.
> For example it is fine to convert `u8` to `UnsafeCell<u8>`. Niches also
> should not be a problem, since eg `Option<UnsafeCell<NonNull<u8>>>`
> already fails the "All bit-patterns must be valid for this type".
>
> --
> Cheers,
> Benno
>
> > +pub unsafe trait FromBytes {}

See also the reasoning in zerocopy [1] and bytemuck [2]

[1]: https://docs.rs/zerocopy/latest/zerocopy/derive.FromBytes.html#analysis
[2]: https://docs.rs/bytemuck/latest/bytemuck/trait.AnyBitPattern.html#safety