linux-kernel - Re: [PATCH v2 4/4] rust: add abstraction for `struct page`

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Date: Mon, 12 Feb 2024 10:36:48 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Martin Rodriguez Reboredo <yakoyoku@...il.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Wedson Almeida Filho <wedsonaf@...il.com>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...sung.com>, 
	Kees Cook <keescook@...omium.org>, Al Viro <viro@...iv.linux.org.uk>, 
	Andrew Morton <akpm@...ux-foundation.org>, 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 v2 4/4] rust: add abstraction for `struct page`

On Sat, Feb 10, 2024 at 5:23 AM Martin Rodriguez Reboredo
<yakoyoku@...il.com> wrote:
>
> On 2/8/24 12:47, Alice Ryhl wrote:
> > [...]
> > +    /// Maps the page and reads from it into the given buffer.
> > +    ///
> > +    /// This method will perform bounds checks on the page offset. If `offset ..
> > +    /// offset+len` goes outside ot the page, then this call returns `EINVAL`.
> > +    ///
> > +    /// # Safety
> > +    ///
> > +    /// * Callers must ensure that `dst` is valid for writing `len` bytes.
> > +    /// * Callers must ensure that this call does not race with a write to the
> > +    ///   same page that overlaps with this read.
>
> This safety section says that a call mustn't race with a page that
> overlaps this read, hmmmmm.

Is there a question here?

> > +    pub unsafe fn read_raw(&self, dst: *mut u8, offset: usize, len: usize) -> Result {
> > +        self.with_pointer_into_page(offset, len, move |src| {
> > +            // SAFETY: If `with_pointer_into_page` calls into this closure, then
> > +            // it has performed a bounds check and guarantees that `src` is
> > +            // valid for `len` bytes.
> > +            //
> > +            // There caller guarantees that there is no data race.
> > +            unsafe { ptr::copy(src, dst, len) };
>
> If `src` and `dst` overlap then wouldn't that be a bad idea? If so then
> how about mentioning that callers have to ensure that `dst` does not
> overlap with the page that's being read and use
> `core::ptr::copy_nonoverlapping` instead, otherwise the doc comment
> could mention that `dst` can overlap.

I'll use copy_nonoverlapping. Thanks for the suggestion.

Alice