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

[Yury Norov]

On Mon, Aug 03, 2026 at 09:41:42PM +0900, Eliot Courtney wrote:
> On Thu Jul 30, 2026 at 1:56 PM JST, Yury Norov wrote:
> > 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?
> 
> I think in rust, indicative is the standard even in the kernel - e.g.
> see Documentation/rust/coding-guidelines.rst around line 208-ish, and
> that's also what I see generally in code. But please let me know if
> you'd like me to use it in this file regardless.

The documentation you've mentioned doesn't say: use indicative. This
is just a one example.
 
This is what my AI machine says:

  Among the 1,266 verb-led function comments, that is:

  - 67.1% indicative
  - 32.9% imperative

So, unless there's a strong (and not aligning with the rest of the
kernel) rule, please use imperative form in bitmaps.

...

> >> +        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.
> 
> TL;DR: Included some tests below that demonstrate overflow/OOB issues on
> 32-bit (with increased vmalloc) in some extreme cases. To keep the rust
> code completely safe we need to check for these, or update the C code,
> but not sure if the perf tradeoff is worth it. Please let me know.
> 
> Ok it seems I was looking at the code previous to df81d444dc74 ("lib:
> bitmap: optimize bitmap_find_next_zero_area_off()"), but overflows can
> still cause wrong behaviour after this commit too:
> 
> [1] On 32-bit, suppose we have an empty bitmap with size==64, start==32,
> nr==2^32-1, and align_mask==0. Then, computing `end` overflows to 31.
> Computing `end - off` then underflows (31 - 32) which can cause OOB
> reads. So actually we need a check before calling
> `bitmap_find_next_zero_area_off` to avoid this case.
> 
> [2] On 32-bit, suppose we have a bitmap with size==2^31+2 and all bits
> set except the 0th and 2^31+1st bit, and start==1, nr==1,
> align_mask==2^31-1. We'll compute start==2^31+1+2^31-1 which overflows
> to 0. Then we'll end up returning 0 which is below start. So we need the
> `index < start` check.

Both examples overflow int32::MAX. It is not supported in rust.
See the BitmapVec code. Your case is just 2048 bits, so it's not
a limitation for you.

On the C side, there's a historical mess - some functions work with
unsigned longs, some with unsigned ints, and so on. I'm aware of it,
and there's a process of unification the API toward the unsigned
longs. That wouldn't help 32-bit architectures because they are all
ILP32, but there's no real use case for them that would overflow the
32 bit.
 
...

> >> +    /// 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.
> 
> The assertion block still checks, even if CONFIG_RUST_BITMAP_HARDENED is
> off (just prints an error returns in this case), so at this point we
> know that the assert predicate is true. In this case, per the file
> convention we take start and nbits in usize, but the C function doesn't,
> so we need to make sure we don't truncate when converting to u32 and
> i32. Also, it's not safe to call __bitmap_set with start and nbits such
> that start+nbits is greater than self.len(), or we can write out of
> bounds, so we can't use the same language + we need the check.

OK, it's bitmap_assert_return, not bitmap_assert. Scratch that.

Thanks,
Yury