linux-hardening - Re: [PATCH v9 3/5] rust: add bitmap API.

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Message-ID: <5dNbxz3h9v8q8Bt9mp5T8cWrypATF6eDqo7rp4wSQ0l0BqsPb6nvV7XLdfxC3Ex0wb9KyuTkSDXH27jaxwUaQUeHCKcZ8ImWvIpSKzRkQ9Q=@protonmail.com>
Date: Thu, 29 May 2025 20:15:05 +0000
From: Pekka Ristola <pekkarr@...tonmail.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Burak Emir <bqe@...gle.com>, Yury Norov <yury.norov@...il.com>, Kees Cook <kees@...nel.org>, Rasmus Villemoes <linux@...musvillemoes.dk>, Viresh Kumar <viresh.kumar@...aro.org>, Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, "Gustavo A . R . Silva" <gustavoars@...nel.org>, rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v9 3/5] rust: add bitmap API.

On Thursday, May 29th, 2025 at 23.07, Boqun Feng <boqun.feng@...il.com> wrote:

> On Thu, May 29, 2025 at 07:42:09PM +0000, Pekka Ristola wrote:
> [...]
> 
> > > + }
> > > +
> > > + /// Copy `src` into this [`Bitmap`] and set any remaining bits to zero.
> > > + ///
> > > + /// # Examples
> > > + ///
> > > + /// `+ /// use kernel::alloc::{AllocError, flags::GFP_KERNEL}; + /// use kernel::bitmap::Bitmap; + /// + /// let mut long_bitmap = Bitmap::new(256, GFP_KERNEL)?; + // + /// assert_eq!(None, long_bitmap.last_bit()); + // + /// let mut short_bitmap = Bitmap::new(16, GFP_KERNEL)?; + // + /// short_bitmap.set_bit(7); + /// long_bitmap.copy_and_extend(&short_bitmap); + /// assert_eq!(Some(7), long_bitmap.last_bit()); + /// + /// # Ok::<(), AllocError>(()) + ///`
> > > + #[inline]
> > > + pub fn copy_and_extend(&mut self, src: &Bitmap) {
> > > + let len = core::cmp::min(src.nbits, self.len());
> > > + // SAFETY: access to `self` and `src` is within bounds.
> > > + unsafe {
> > > + bindings::bitmap_copy_and_extend(
> > > + self.as_mut_ptr(),
> > > + src.as_ptr(),
> > > + len as u32,
> > > + self.len() as u32,
> > > + )
> > 
> > Would this cause a data race if `src` is concurrently (atomically)
> > modified? The C function seems to use a plain `memcpy` which is not atomic.
> 
> 
> We need some better documentation on the effect of kernel C's
> `memcpy()`-like functions regarding data races, but in general a kernel
> C's `memcpy` can be treated as a volatile one (or per-byte atomic), so
> it won't cause data race.

Ah, thanks for the clarification.

Pekka