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From: Alice Ryhl <aliceryhl@google.com>
Date: Thu, 18 Apr 2024 15:17:51 +0200
Message-ID: <CAH5fLgiMLxmmm0AVX_5HQF61FzzN69GCKabrr-uM_oV-rRMuHw@mail.gmail.com>
Subject: Re: [PATCH v6 3/4] rust: uaccess: add typed accessors for userspace pointers
To: Benno Lossin <benno.lossin@proton.me>
Cc: Miguel Ojeda <ojeda@kernel.org>, Matthew Wilcox <willy@infradead.org>, 
	Al Viro <viro@zeniv.linux.org.uk>, Andrew Morton <akpm@linux-foundation.org>, 
	Kees Cook <keescook@chromium.org>, Alex Gaynor <alex.gaynor@gmail.com>, 
	Wedson Almeida Filho <wedsonaf@gmail.com>, Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>, 
	=?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, 
	Andreas Hindborg <a.hindborg@samsung.com>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, 
	=?UTF-8?B?QXJ2ZSBIasO4bm5ldsOlZw==?= <arve@android.com>, 
	Todd Kjos <tkjos@android.com>, Martijn Coenen <maco@android.com>, 
	Joel Fernandes <joel@joelfernandes.org>, Carlos Llamas <cmllamas@google.com>, 
	Suren Baghdasaryan <surenb@google.com>, Arnd Bergmann <arnd@arndb.de>, Trevor Gross <tmgross@umich.edu>, 
	linux-mm@kvack.org, linux-kernel@vger.kernel.org, 
	rust-for-linux@vger.kernel.org, Christian Brauner <brauner@kernel.org>
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On Thu, Apr 18, 2024 at 3:02=E2=80=AFPM Benno Lossin <benno.lossin@proton.m=
e> 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 typ=
e `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 n=
ot okay.
> > +///
> > +/// It's okay for the type to have padding, as initializing those byte=
s has no effect.
> > +///
> > +/// # Safety
> > +///
> > +/// All bit-patterns must be valid for this type. This type must not h=
ave 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".

If T: FromBytes allows transmuting &[u8; size_of::<T>] into &T, then
it would be a problem as you could then use it to modify the original
&[u8].

Alice