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Date: Sat, 10 Feb 2024 01:23:38 -0300
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Subject: Re: [PATCH v2 4/4] rust: add abstraction for `struct page`
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To: Alice Ryhl <aliceryhl@google.com>, Miguel Ojeda <ojeda@kernel.org>,
 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>,
 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>
Cc: 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>, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org,
 Christian Brauner <brauner@kernel.org>
References: <20240208-alice-mm-v2-0-d821250204a6@google.com>
 <20240208-alice-mm-v2-4-d821250204a6@google.com>
From: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
In-Reply-To: <20240208-alice-mm-v2-4-d821250204a6@google.com>
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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.

> +    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.

> +            Ok(())
> +        })
> +    }
> +
> +    /// Maps the page and writes into it from 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 `src` is valid for reading `len` bytes.
> +    /// * Callers must ensure that this call does not race with a read or write
> +    ///   to the same page that overlaps with this write.
> +    pub unsafe fn write_raw(&self, src: *const u8, offset: usize, len: usize) -> Result {
> +        self.with_pointer_into_page(offset, len, move |dst| {
> +            // SAFETY: If `with_pointer_into_page` calls into this closure, then
> +            // it has performed a bounds check and guarantees that `dst` is
> +            // valid for `len` bytes.
> +            //
> +            // There caller guarantees that there is no data race.
> +            unsafe { ptr::copy(src, dst, len) };

Same as above

> +            Ok(())
> +        })
> +    }
> [...]