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

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Date: Thu, 18 Apr 2024 13:01:17 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Benno Lossin <benno.lossin@...ton.me>, 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>, Trevor Gross <tmgross@...ch.edu>,
	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 09:35:53PM +0200, Alice Ryhl wrote:
> On Thu, Apr 18, 2024 at 7:27 PM Boqun Feng <boqun.feng@...il.com> 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@...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.
> >
> > 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