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 <[email protected]>
>> ---
>> 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) };
>> + }
>> }
