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Date: Thu, 18 Apr 2024 13:01:20 +0000
To: Alice Ryhl <aliceryhl@google.com>, 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>
From: Benno Lossin <benno.lossin@proton.me>
Cc: 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?Q?Arve_Hj=C3=B8nnev=C3=A5g?= <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>
Subject: Re: [PATCH v6 3/4] rust: uaccess: add typed accessors for userspace pointers
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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 b=
e 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 hav=
e 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".

--=20
Cheers,
Benno

> +pub unsafe trait FromBytes {}
> +
> +// SAFETY: All bit patterns are acceptable values of the types below.
> +unsafe impl FromBytes for u8 {}
> +unsafe impl FromBytes for u16 {}
> +unsafe impl FromBytes for u32 {}
> +unsafe impl FromBytes for u64 {}
> +unsafe impl FromBytes for usize {}
> +unsafe impl FromBytes for i8 {}
> +unsafe impl FromBytes for i16 {}
> +unsafe impl FromBytes for i32 {}
> +unsafe impl FromBytes for i64 {}
> +unsafe impl FromBytes for isize {}