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Date: Fri, 26 Jan 2024 10:28:34 -0800
From: Boqun Feng <boqun.feng@gmail.com>
To: Alice Ryhl <aliceryhl@google.com>
Cc: Miguel Ojeda <ojeda@kernel.org>, Alex Gaynor <alex.gaynor@gmail.com>,
	Wedson Almeida Filho <wedsonaf@gmail.com>,
	Gary Guo <gary@garyguo.net>,
	=?iso-8859-1?Q?Bj=F6rn?= 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>,
	Arve =?iso-8859-1?B?SGr4bm5lduVn?= <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>
Subject: Re: [PATCH 3/3] rust: add abstraction for `struct page`
Message-ID: <ZbP50nEIMqULfVuj@Boquns-Mac-mini.home>
References: <20240124-alice-mm-v1-0-d1abcec83c44@google.com>
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On Fri, Jan 26, 2024 at 01:33:46PM +0100, Alice Ryhl wrote:
> On Fri, Jan 26, 2024 at 1:47 AM Boqun Feng <boqun.feng@gmail.com> wrote:
> >
> > 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` bytes.
> > > +    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 closure, then
> > > +            // it has performed a bounds check and guarantees that `from_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` bytes.
> > > +    pub unsafe fn write(&self, src: *const u8, offset: usize, len: usize) -> 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) -> Result
> >
> > 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

I understand that, but in the current code, you can write:

	CPU 0			CPU 1
	=====			=====

	page.write(src1, 0, 8);
				page.write(src2, 0, 8);

and it's a data race at kernel end. So my question is more how we can
prevent the UB ;-)

Regards,
Boqun

> could be mapped into the address space of a userspace process.
> 
> Alice