Re: [PATCH v8 11/12] rust: id_pool: do not round capacity up to BitmapVec::MAX_INLINE_LEN

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Calling `find_unused_id(0)` or `reserve_ids(0, ...)` on an `IdPool` initialized with a capacity of 0 triggers a kernel panic when `CONFIG_RUST_BITMAP_HARDENED` is enabled.
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commit c0cd93bbf9f36768f04bc377021b9e32ae0dce05
Author: Eliot Courtney <ecourtney@nvidia.com>

rust: id_pool: do not round capacity up to BitmapVec::MAX_INLINE_LEN

This commit modifies IdPool::with_capacity to stop rounding the capacity up
to BitmapVec::MAX_INLINE_LEN. This allows explicitly creating pools with 0
capacity, and adjusts grow_request and shrink_request to handle the new floor.

> diff --git a/rust/kernel/id_pool.rs b/rust/kernel/id_pool.rs
> index 06a4c71c4c6c9..4f329249df9d0 100644
> --- a/rust/kernel/id_pool.rs
> +++ b/rust/kernel/id_pool.rs
> @@ -112,13 +112,8 @@ pub fn new() -> Self {
>      }
>  
>      /// Constructs a new [`IdPool`] with space for a specific number of bits.
> -    ///
> -    /// A capacity below [`MAX_INLINE_LEN`] is adjusted to [`MAX_INLINE_LEN`].
> -    ///
> -    /// [`MAX_INLINE_LEN`]: BitmapVec::MAX_INLINE_LEN
>      #[inline]
>      pub fn with_capacity(num_ids: usize, flags: Flags) -> Result<Self, AllocError> {
> -        let num_ids = usize::max(num_ids, BitmapVec::MAX_INLINE_LEN);
>          let map = BitmapVec::new(num_ids, flags)?;
>          Ok(Self { map })
>      }

[Severity: High]
Does allowing a capacity of 0 here introduce a kernel panic during standard
allocations if CONFIG_RUST_BITMAP_HARDENED is enabled?

If a caller attempts to allocate an ID on a 0-capacity pool using patterns like
pool.find_unused_id(0) or pool.reserve_ids(0, ...), the call delegates directly
to the underlying bitmap without its own bounds check:

IdPool::find_unused_id() {
    Some(UnusedId {
        id: self.map.next_zero_bit(offset)?,
        pool: self,
    })
}

This passes offset=0 to Bitmap::next_zero_bit(), which contains an assertion
that start < self.len(). Since 0 < 0 is false on a 0-capacity pool, it panics
immediately instead of gracefully returning None to prompt a grow_request().

[ ... ]

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Sashiko AI review · https://sashiko.dev/#/patchset/20260827-chid-v8-0-bc74c77d0214@nvidia.com?part=11