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

[Eliot Courtney]

On Thu Aug 13, 2026 at 5:31 AM JST, Yury Norov wrote:
> 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.

NonZero sounds good to me for `nbits`. For `align`, it's already
guaranteed to be at least 1. For `align_offset`, passing 0 is normal and
valid (and we need to for implementing `next_zero_area` just below)

>
>> +        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

Yeah, NonZero sounds good to me here too. Thanks!

>
>> +
>> +    /// 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) };
>> +    }
>>  }