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 <20240124-alice-mm-v1-3-d1abcec83c44@google.com> <ZbMA1yiM6Bqv9Sqg@boqun-archlinux>
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From: Alice Ryhl <aliceryhl@google.com>
Date: Fri, 26 Jan 2024 13:33:46 +0100
Message-ID: <CAH5fLgiNphSebaG82XkQHGFPFp1Mf1egyaiX6MFzsU2X3-Ni8w@mail.gmail.com>
Subject: Re: [PATCH 3/3] rust: add abstraction for `struct page`
To: Boqun Feng <boqun.feng@gmail.com>
Cc: Miguel Ojeda <ojeda@kernel.org>, Alex Gaynor <alex.gaynor@gmail.com>, 
	Wedson Almeida Filho <wedsonaf@gmail.com>, Gary Guo <gary@garyguo.net>, 
	=?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, 
	Benno Lossin <benno.lossin@proton.me>, Andreas Hindborg <a.hindborg@samsung.com>, 
	Kees Cook <keescook@chromium.org>, Al Viro <viro@zeniv.linux.org.uk>, 
	Andrew Morton <akpm@linux-foundation.org>, 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>, 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 Fri, Jan 26, 2024 at 1:47=E2=80=AFAM Boqun Feng <boqun.feng@gmail.com> w=
rote:
>
> On Wed, Jan 24, 2024 at 11:20:23AM +0000, Alice Ryhl wrote:
> > +    /// Maps the page and reads from it into the given buffer.
> > +    ///
> > +    /// # Safety
> > +    ///
> > +    /// Callers must ensure that `dest` is valid for writing `len` byt=
es.
> > +    pub unsafe fn read(&self, dest: *mut u8, offset: usize, len: usize=
) -> Result {
> > +        self.with_pointer_into_page(offset, len, move |from_ptr| {
> > +            // SAFETY: If `with_pointer_into_page` calls into this clo=
sure, then
> > +            // it has performed a bounds check and guarantees that `fr=
om_ptr` is
> > +            // valid for `len` bytes.
> > +            unsafe { ptr::copy(from_ptr, dest, len) };
> > +            Ok(())
> > +        })
> > +    }
> > +
> > +    /// Maps the page and writes into it from the given buffer.
> > +    ///
> > +    /// # Safety
> > +    ///
> > +    /// Callers must ensure that `src` is valid for reading `len` byte=
s.
> > +    pub unsafe fn write(&self, src: *const u8, offset: usize, len: usi=
ze) -> Result {
>
> Use a slice like type as `src` maybe? Then the function can be safe:
>
>         pub fn write<S: AsRef<[u8]>>(&self, src: S, offset: usize) -> Res=
ult
>
> Besides, since `Page` impl `Sync`, shouldn't this `write` and the
> `fill_zero` be a `&mut self` function? Or make them both `unsafe`
> because of potential race and add some safety requirement?

Ideally, we don't want data races with these methods to be UB. They
could be mapped into the address space of a userspace process.

Alice