Re: [PATCH v5 3/5] rust: bitmap: add contiguous area operations

[Yury Norov]

On Wed, Aug 12, 2026 at 05:51:23PM +0900, Eliot Courtney wrote:
> Add bindings for area operations on bitmaps. Each one is
> made safe by adding some extra checks compared to the underlying C code
> (for example, checking bounds) and with additional checks to catch
> likely erroneous usage if `CONFIG_RUST_BITMAP_HARDENED` is on.
> 
> Add tests demonstrating the edge cases.
> 
> Signed-off-by: Eliot Courtney <ecourtney@xxxxxxxxxx>
> ---
>  rust/kernel/bitmap.rs | 236 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 236 insertions(+)
> 
> diff --git a/rust/kernel/bitmap.rs b/rust/kernel/bitmap.rs
> index fdcfc0409773..74c92cc452c9 100644
> --- a/rust/kernel/bitmap.rs
> +++ b/rust/kernel/bitmap.rs
> @@ -10,6 +10,7 @@
>  use crate::bindings;
>  #[cfg(not(CONFIG_RUST_BITMAP_HARDENED))]
>  use crate::pr_err;
> +use crate::ptr::Alignment;
>  use core::ptr::NonNull;
>  
>  /// Represents a C bitmap. Wraps underlying C bitmap API.
> @@ -523,6 +524,139 @@ pub fn next_zero_bit(&self, start: usize) -> Option<usize> {
>              Some(index)
>          }
>      }
> +
> +    /// Finds a contiguous area of `nbits` zero bits at or after `start`, where the area plus
> +    /// `align_offset` is aligned to `align`.
> +    ///
> +    /// Returns the bit index of the start of the area, or [`None`] if no such area fitting in
> +    /// the bitmap exists.
> +    ///
> +    /// The returned index plus `align_offset` is a multiple of `align`.
> +    ///
> +    /// # Panics
> +    ///
> +    /// Panics if CONFIG_RUST_BITMAP_HARDENED is enabled and `start` is out of bounds.
> +    #[inline]
> +    pub fn next_zero_area_off(
> +        &self,
> +        start: usize,
> +        nbits: usize,
> +        align: Alignment,
> +        align_offset: usize,
> +    ) -> Option<usize> {
> +        bitmap_assert!(
> +            start < self.len(),
> +            "`start` must be < {}, was {}",
> +            self.len(),
> +            start
> +        );
> +
> +        let nr = u32::try_from(nbits).ok()?;

What about nbits == 0? In C, this is a undef, and thus in the current
rust implementation. Maybe make it NonZero?

The same question about align and align_offset.

> +        let align_mask = align.as_usize() - 1;
> +
> +        // The C alignment and end arithmetic must not overflow, or it can read out of bounds.
> +        // Overflow is only possible on 32-bit.
> +        #[cfg(not(CONFIG_64BIT))]
> +        align_mask.checked_add(self.len())?.checked_add(nbits)?;
> +
> +        // SAFETY: `bitmap_find_next_zero_area_off` is safe to use with an out of bounds `start`
> +        // value and, given the overflow check above, never reads beyond `self.len()` bits.
> +        let index = unsafe {
> +            bindings::bitmap_find_next_zero_area_off(
> +                self.as_ptr().cast_mut(),
> +                self.len(),
> +                start,
> +                nr,
> +                align_mask,
> +                align_offset,
> +            )
> +        };
> +
> +        (index < self.len()).then_some(index)
> +    }
> +
> +    /// Finds a contiguous area of `nbits` zero bits at or after `start`, aligned to `align`.
> +    ///
> +    /// Returns the bit index of the start of the area, or [`None`] if no such area fitting in
> +    /// the bitmap exists.
> +    ///
> +    /// The returned index is a multiple of `align`.
> +    ///
> +    /// # Panics
> +    ///
> +    /// Panics if CONFIG_RUST_BITMAP_HARDENED is enabled and `start` is out of bounds.
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// use kernel::alloc::{AllocError, flags::GFP_KERNEL};
> +    /// use kernel::bitmap::BitmapVec;
> +    /// use kernel::ptr::Alignment;
> +    ///
> +    /// let mut b = BitmapVec::new(64, GFP_KERNEL)?;
> +    /// let unaligned = Alignment::new::<1>();
> +    ///
> +    /// assert_eq!(Some(0), b.next_zero_area(0, 8, unaligned));
> +    /// b.set(0, 5);
> +    /// assert_eq!(Some(5), b.next_zero_area(0, 8, unaligned));
> +    /// assert_eq!(Some(8), b.next_zero_area(0, 8, Alignment::new::<8>()));
> +    /// assert_eq!(None, b.next_zero_area(0, 65, unaligned));
> +    /// # Ok::<(), AllocError>(())
> +    /// ```
> +    #[inline]
> +    pub fn next_zero_area(&self, start: usize, nbits: usize, align: Alignment) -> Option<usize> {
> +        self.next_zero_area_off(start, nbits, align, 0)
> +    }
> +
> +    /// Sets a contiguous area of `nbits` bits starting at `start`.
> +    ///
> +    /// If CONFIG_RUST_BITMAP_HARDENED is not enabled and the area `start..start + nbits` is out of
> +    /// bounds, does nothing.
> +    ///
> +    /// # Panics
> +    ///
> +    /// Panics if CONFIG_RUST_BITMAP_HARDENED is enabled and the area `start..start + nbits` is out
> +    /// of bounds.
> +    #[inline]
> +    pub fn set(&mut self, start: usize, nbits: usize) {
> +        bitmap_assert_return!(
> +            start
> +                .checked_add(nbits)
> +                .is_some_and(|end| end <= self.len()),
> +            "Area `start..start + nbits` ({}..{}) must be within bounds {}",
> +            start,
> +            start.saturating_add(nbits),
> +            self.len()
> +        );
> +        // SAFETY: The area `start..start + nbits` is within bounds and a `Bitmap` is at most
> +        // `i32::MAX` bits, so the casts are lossless.
> +        unsafe { bindings::__bitmap_set(self.as_mut_ptr(), start as u32, nbits as i32) };
> +    }

In the case of bitmap_set/clear(), nbits == 0 makes it a no-op, and
guarantees that the pointer is not dereferenced. So, no undefined
behavior. But in rust case, I believe, it should be a stronger policy.

I'd add an assertion, at least, or better make it NonZero.

Thanks,
Yury

> +
> +    /// Clears a contiguous area of `nbits` bits starting at `start`.
> +    ///
> +    /// If CONFIG_RUST_BITMAP_HARDENED is not enabled and the area `start..start + nbits` is out of
> +    /// bounds, does nothing.
> +    ///
> +    /// # Panics
> +    ///
> +    /// Panics if CONFIG_RUST_BITMAP_HARDENED is enabled and the area `start..start + nbits` is out
> +    /// of bounds.
> +    #[inline]
> +    pub fn clear(&mut self, start: usize, nbits: usize) {
> +        bitmap_assert_return!(
> +            start
> +                .checked_add(nbits)
> +                .is_some_and(|end| end <= self.len()),
> +            "Area `start..start + nbits` ({}..{}) must be within bounds {}",
> +            start,
> +            start.saturating_add(nbits),
> +            self.len()
> +        );
> +        // SAFETY: The area `start..start + nbits` is within bounds and a `Bitmap` is at most
> +        // `i32::MAX` bits, so the casts are lossless.
> +        unsafe { bindings::__bitmap_clear(self.as_mut_ptr(), start as u32, nbits as i32) };
> +    }
>  }