Re: [PATCH v3 2/4] rust: bitmap: add contiguous area operations

[Yury Norov]

On Wed, Jul 29, 2026 at 03:54:13PM +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.
> 
> The C code uses signed integers for some parameters, for example the
> length for `__bitmap_set`, so bounds check against i32::MAX. We can't
> rely on `BitmapVec::MAX_LEN` because `Bitmap` may not necessarily be
> backed by `BitmapVec`.
> 
> Add tests demonstrating the edge cases.
> 
> Signed-off-by: Eliot Courtney <ecourtney@xxxxxxxxxx>
> ---
>  rust/kernel/bitmap.rs | 194 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 194 insertions(+)
> 
> diff --git a/rust/kernel/bitmap.rs b/rust/kernel/bitmap.rs
> index a43bfe0ec3dc..f4b0b8ae39d8 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.

Some comments use indicative form in the file, but the imperative
'represent' is a more standard way. Can you please use it instead?

> @@ -497,6 +498,116 @@ pub fn next_zero_bit(&self, start: usize) -> Option<usize> {
>              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`. Alignments where `self.len() + align - 1`
> +    /// overflows a `usize` can hang the underlying C code.
> +    ///
> +    /// # 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> {

Please create the rust wrapper next_zero_area_off() around
bitmap_find_next_zero_area_off(), then in rust create the
next_zero_area(), if you need it.

> +        bitmap_assert!(
> +            start < self.len(),
> +            "`start` must be < {}, was {}",
> +            self.len(),
> +            start
> +        );
> +
> +        let nr = u32::try_from(nbits).ok()?;
> +
> +        // SAFETY: `bitmap_find_next_zero_area_off` is safe to use with an out of bounds `start`
> +        // value and 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.as_usize() - 1,
> +                0,
> +            )
> +        };
> +
> +        // In case of overflow, we may get back a range outside of what we requested.

No, we can't. We've got the test_bitmap_find_next_zero_area_off() for
it (in next). If you think the test is incomplete, please extend it.

If you believe that bitmap_find_next_zero_area_off() may return something
like that, it means the function is buggy, and you shouldn't trust it at
all.

> +        let end = index.checked_add(nbits)?;
> +        if index < start || index >= self.len() || end > self.len() {
> +            None
> +        } else {
> +            Some(index)
> +        }

So, this should be a simple:

        (i < len).then_some(i)

> +    }
> +
> +    /// 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() && end <= i32::MAX as usize),
> +            "Area `start..start + nbits` ({}..{}) must be within bounds {}",
> +            start,
> +            start.saturating_add(nbits),
> +            self.len()
> +        );
> +        // SAFETY: The area `start..start + nbits` is within bounds.

Not sure I understand. In the above assertion block you check for it,
now you say it's always true...

I think, your language should be similar to the
find_next_zero_area_off() case: it's safe to call the function with
the out-of-bounds start and nbits.

> +        unsafe { bindings::__bitmap_set(self.as_mut_ptr(), start as u32, nbits as i32) };
> +    }
> +
> +    /// 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() && end <= i32::MAX as usize),
> +            "Area `start..start + nbits` ({}..{}) must be within bounds {}",
> +            start,
> +            start.saturating_add(nbits),
> +            self.len()
> +        );
> +        // SAFETY: The area `start..start + nbits` is within bounds.
> +        unsafe { bindings::__bitmap_clear(self.as_mut_ptr(), start as u32, nbits as i32) };
> +    }
>  }
>  
>  #[cfg(CONFIG_RUST_BITMAP_KUNIT_TEST)]
> @@ -614,4 +725,87 @@ fn bitmap_copy_and_extend() -> Result<(), AllocError> {
>          assert_eq!(Some(17), long_bitmap.last_bit());
>          Ok(())
>      }
> +
> +    #[test]
> +    fn bitmap_area_set_clear_find() -> Result<(), AllocError> {
> +        let mut b = BitmapVec::new(128, GFP_KERNEL)?;
> +        let unaligned = Alignment::new::<1>();
> +
> +        assert_eq!(Some(0), b.next_zero_area(0, 5, unaligned));
> +        b.set(0, 5); // Now contains {[0, 5)}.
> +
> +        assert_eq!(Some(0), b.next_bit(0));
> +        assert_eq!(Some(4), b.next_bit(4));
> +        assert_eq!(Some(5), b.next_zero_bit(0));
> +        assert_eq!(Some(5), b.next_zero_area(0, 5, unaligned));
> +        assert_eq!(Some(8), b.next_zero_area(0, 5, Alignment::new::<8>()));
> +
> +        b.set(8, 8); // Now contains {[0, 5), [8, 16)}.
> +        assert_eq!(Some(16), b.next_zero_area(0, 4, Alignment::new::<16>()));
> +        assert_eq!(Some(16), b.next_zero_area(0, 4, unaligned));
> +
> +        b.clear(0, 5); // Now contains {[8, 16)}.
> +        assert_eq!(Some(0), b.next_zero_area(0, 5, unaligned));
> +        assert_eq!(Some(8), b.next_bit(0));
> +        assert_eq!(Some(15), b.last_bit());
> +
> +        b.clear(16, 0); // Zero-length in-bounds clears are no-ops.
> +        assert_eq!(Some(8), b.next_bit(0));
> +        assert_eq!(Some(15), b.last_bit());
> +
> +        // A zero-length request returns the first aligned position at or
> +        // after the next zero bit, even if that position's own bit is set.
> +        assert_eq!(Some(1), b.next_zero_area(1, 0, unaligned));
> +        assert_eq!(Some(8), b.next_zero_area(1, 0, Alignment::new::<8>()));
> +
> +        b.set(60, 10); // Now contains {[8, 16), [60, 70)}.
> +        assert_eq!(Some(60), b.next_bit(16));
> +        assert_eq!(Some(69), b.last_bit());
> +        assert_eq!(Some(16), b.next_zero_area(9, 40, unaligned));
> +        assert_eq!(Some(70), b.next_zero_area(0, 45, unaligned));
> +
> +        b.clear(62, 6); // Now contains {[8, 16), [60, 62), [68, 70)}.
> +        assert_eq!(Some(62), b.next_zero_area(60, 6, unaligned));
> +        assert_eq!(Some(61), b.next_bit(61));
> +        assert_eq!(Some(69), b.last_bit());
> +
> +        b.set(64, 0); // Zero-length in-bounds sets are no-ops.
> +        assert_eq!(Some(62), b.next_zero_bit(62));
> +        Ok(())
> +    }
> +
> +    #[test]
> +    fn bitmap_area_exhaustion() -> Result<(), AllocError> {
> +        let mut b = BitmapVec::new(64, GFP_KERNEL)?;
> +        let unaligned = Alignment::new::<1>();
> +
> +        assert_eq!(None, b.next_zero_area(0, 65, unaligned));
> +        assert_eq!(None, b.next_zero_area(0, usize::MAX, unaligned));
> +        assert_eq!(None, b.next_zero_area(1, usize::MAX, unaligned));
> +
> +        b.set_bit(0); // Now contains {[0, 1)}.
> +        assert_eq!(None, b.next_zero_area(0, usize::MAX, unaligned));
> +
> +        b.set(0, 61); // Now contains {[0, 61)}.
> +        assert_eq!(None, b.next_zero_area(0, 4, unaligned));
> +        assert_eq!(Some(61), b.next_zero_area(0, 3, unaligned));
> +        assert_eq!(None, b.next_zero_area(0, 1, Alignment::new::<64>()));
> +        Ok(())
> +    }
> +
> +    #[test]
> +    #[cfg(not(CONFIG_RUST_BITMAP_HARDENED))]
> +    fn owned_bitmap_area_out_of_bounds() -> Result<(), AllocError> {
> +        let mut b = BitmapVec::new(64, GFP_KERNEL)?;
> +
> +        // Should be ignored since out of bounds.
> +        b.set(64, 4);
> +        b.set(62, 8);
> +        b.set(usize::MAX, 0);
> +        b.clear(usize::MAX, 0);
> +        b.clear(2048, 8);
> +        assert_eq!(None, b.next_bit(0));
> +        assert_eq!(None, b.next_zero_area(64, 1, Alignment::new::<1>()));
> +        Ok(())
> +    }
>  }
> 
> -- 
> 2.55.0