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

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Date: Thu, 18 Apr 2024 10:19:28 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Benno Lossin <benno.lossin@...ton.me>
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>, 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 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.

Regards,
Boqun

> >> 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].
> 
> Ahh that makes a lot of sense.
> 
> -- 
> Cheers,
> Benno
> 
>