Message-ID: <CAH5fLgi1GBqDHvMh30fbxVUjnTETLXthpK6eGLJA0Vh_TZRnsQ@mail.gmail.com>
Date: Wed, 11 Jun 2025 23:58:16 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Burak Emir <bqe@...gle.com>
Cc: Yury Norov <yury.norov@...il.com>, Kees Cook <kees@...nel.org>, 
	Rasmus Villemoes <linux@...musvillemoes.dk>, Viresh Kumar <viresh.kumar@...aro.org>, 
	Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, "Gustavo A . R . Silva" <gustavoars@...nel.org>, 
	Carlos LLama <cmllamas@...gle.com>, Pekka Ristola <pekkarr@...tonmail.com>, 
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH v12 3/5] rust: add bitmap API.

On Wed, Jun 11, 2025 at 9:48 PM Burak Emir <bqe@...gle.com> wrote:
>
> Provides an abstraction for C bitmap API and bitops operations.
>
> This commit enables a Rust implementation of an Android Binder
> data structure from commit 15d9da3f818c ("binder: use bitmap for faster
> descriptor lookup"), which can be found in drivers/android/dbitmap.h.
> It is a step towards upstreaming the Rust port of Android Binder driver.
>
> We follow the C Bitmap API closely in naming and semantics, with
> a few differences that take advantage of Rust language facilities
> and idioms:
>
>   * We leverage Rust type system guarantees as follows:
>
>     * all (non-atomic) mutating operations require a &mut reference which
>       amounts to exclusive access.
>
>     * the Bitmap type implements Send. This enables transferring
>       ownership between threads and is needed for Binder.
>
>     * the Bitmap type implements Sync, which enables passing shared
>       references &Bitmap between threads. Atomic operations can be
>       used to safely modify from multiple threads (interior
>       mutability), though without ordering guarantees.
>
>   * The Rust API uses `{set,clear}_bit` vs `{set,clear}_bit_atomic` as
>     names, which differs from the C naming convention which uses
>     set_bit for atomic vs __set_bit for non-atomic.
>
>   * we include enough operations for the API to be useful, but not all
>     operations are exposed yet in order to avoid dead code. The missing
>     ones can be added later.
>
>   * We follow the C API closely with a fine-grained approach to safety:
>
>     * Low-level bit-ops get a safe API with bounds checks. Calling with
>       an out-of-bounds arguments to {set,clear}_bit becomes a no-op and
>       get logged as errors.
>
>     * We introduce a RUST_BITMAP_HARDENED config, which
>       causes invocations with out-of-bounds arguments to panic.
>
>     * methods correspond to find_* C methods tolerate out-of-bounds
>       since the C implementation does. Also here, we log out-of-bounds
>       arguments as errors and panic in RUST_BITMAP_HARDENED mode.
>
>     * We add a way to "borrow" bitmaps from C in Rust, to make C bitmaps
>       that were allocated in C directly usable in Rust code (`CBitmap`).
>
>   * the Rust API is optimized to represent the bitmap inline if it would
>     fit into a pointer. This saves allocations which is
>     relevant in the Binder use case.
>
> The underlying C bitmap is *not* exposed, and must never be exposed
> (except in tests). Exposing the representation of the owned bitmap would
> lose static guarantees.
>
> An alternative route of vendoring an existing Rust bitmap package was
> considered but suboptimal overall. Reusing the C implementation is
> preferable for a basic data structure like bitmaps. It enables Rust
> code to be a lot more similar and predictable with respect to C code
> that uses the same data structures and enables the use of code that
> has been tried-and-tested in the kernel, with the same performance
> characteristics whenever possible.
>
> We use the `usize` type for sizes and indices into the bitmap,
> because Rust generally always uses that type for indices and lengths
> and it will be more convenient if the API accepts that type. This means
> that we need to perform some casts to/from u32 and usize, since the C
> headers use unsigned int instead of size_t/unsigned long for these
> numbers in some places.
>
> Adds new MAINTAINERS section BITMAP API [RUST].
>
> Suggested-by: Alice Ryhl <aliceryhl@...gle.com>
> Suggested-by: Yury Norov <yury.norov@...il.com>
> Signed-off-by: Burak Emir <bqe@...gle.com>
> ---
>  MAINTAINERS                |   7 +
>  rust/kernel/bitmap.rs      | 582 +++++++++++++++++++++++++++++++++++++
>  rust/kernel/lib.rs         |   1 +
>  security/Kconfig.hardening |  10 +
>  4 files changed, 600 insertions(+)
>  create mode 100644 rust/kernel/bitmap.rs
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 04d6727e944c..565eaa015d9e 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -4127,6 +4127,13 @@ S:       Maintained
>  F:     rust/helpers/bitmap.c
>  F:     rust/helpers/cpumask.c
>
> +BITMAP API [RUST]
> +M:     Alice Ryhl <aliceryhl@...gle.com>
> +M:     Burak Emir <bqe@...gle.com>
> +R:     Yury Norov <yury.norov@...il.com>
> +S:     Maintained
> +F:     rust/kernel/bitmap.rs
> +
>  BITOPS API
>  M:     Yury Norov <yury.norov@...il.com>
>  R:     Rasmus Villemoes <linux@...musvillemoes.dk>
> diff --git a/rust/kernel/bitmap.rs b/rust/kernel/bitmap.rs
> new file mode 100644
> index 000000000000..1fe72ca980ac
> --- /dev/null
> +++ b/rust/kernel/bitmap.rs
> @@ -0,0 +1,582 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +// Copyright (C) 2025 Google LLC.
> +
> +//! Rust API for bitmap.
> +//!
> +//! C headers: [`include/linux/bitmap.h`](srctree/include/linux/bitmap.h).
> +
> +use crate::alloc::{AllocError, Flags};
> +use crate::bindings;
> +#[cfg(not(CONFIG_RUST_BITMAP_HARDENED))]
> +use crate::pr_err;
> +use core::ptr::NonNull;
> +
> +/// Represents a C bitmap. Wraps underlying C bitmap API.
> +///
> +/// # Invariants
> +///
> +/// Must reference a `[c_ulong]` long enough to fit `data.len()` bits.
> +#[cfg_attr(CONFIG_64BIT, repr(align(8)))]
> +#[cfg_attr(not(CONFIG_64BIT), repr(align(4)))]
> +pub struct CBitmap {
> +    data: [()],
> +}

I wonder if we should just call this type Bitmap?

> +
> +/// SAFETY: All methods that take immutable references are either atomic or read-only.
> +unsafe impl Sync for CBitmap {}

You don't have any fields wrapping C types anymore, so this has no effect.

> +
> +impl CBitmap {
> +    /// Borrows a C bitmap.
> +    ///
> +    /// # Safety
> +    ///
> +    /// * `ptr` holds a non-null address of an initialized array of `unsigned long`
> +    ///   that is large enough to hold `nbits` bits.
> +    /// * the array must not be freed for the lifetime of this [`CBitmap`]
> +    /// * concurrent access only happens through atomic operations
> +    pub unsafe fn from_raw<'a>(ptr: *const usize, nbits: usize) -> &'a CBitmap {
> +        let data: *const [()] = core::ptr::slice_from_raw_parts(ptr.cast(), nbits);
> +        // INVARIANT: `data` references an initialized array that can hold `nbits` bits.
> +        // SAFETY:
> +        // The caller guarantees that `data` (derived from `ptr` and `nbits`)
> +        // points to a valid, initialized, and appropriately sized memory region
> +        // that will not be freed for the lifetime 'a.
> +        // We are casting `*const [()]` to `*const CBitmap`. The `CBitmap`
> +        // struct is a ZST with a `data: [()]` field. This means its layout
> +        // is compatible with a slice of `()`, and effectively it's a "thin pointer"
> +        // (its size is 0 and alignment is 1). The `slice_from_raw_parts`
> +        // function correctly encodes the length (number of bits, not elements)
> +        // into the metadata of the fat pointer. Therefore, dereferencing this
> +        // pointer as `&CBitmap` is safe given the caller's guarantees.
> +        unsafe { &*(data as *const CBitmap) }
> +    }
> +
> +    /// Borrows a C bitmap exclusively.
> +    ///
> +    /// # Safety
> +    ///
> +    /// * `ptr` holds a non-null address of an initialized array of `unsigned long`
> +    ///   that is large enough to hold `nbits` bits.
> +    /// * the array must not be freed for the lifetime of this [`CBitmap`]
> +    /// * no concurrent access may happen.
> +    pub unsafe fn from_raw_mut<'a>(ptr: *mut usize, nbits: usize) -> &'a mut CBitmap {
> +        let data: *mut [()] = core::ptr::slice_from_raw_parts_mut(ptr.cast(), nbits);
> +        // INVARIANT: `data` references an initialized array that can hold `nbits` bits.
> +        // SAFETY:
> +        // The caller guarantees that `data` (derived from `ptr` and `nbits`)
> +        // points to a valid, initialized, and appropriately sized memory region
> +        // that will not be freed for the lifetime 'a.
> +        // Furthermore, the caller guarantees no concurrent access will happen,
> +        // which upholds the exclusivity requirement for a mutable reference.
> +        // Similar to `from_raw`, casting `*mut [()]` to `*mut CBitmap` is
> +        // safe because `CBitmap` is a ZST with a `data: [()]` field,
> +        // making its layout compatible with a slice of `()`.
> +        unsafe { &mut *(data as *mut CBitmap) }
> +    }
> +
> +    /// Returns a raw pointer to the backing [`Bitmap`].
> +    pub fn as_ptr(&self) -> *const usize {
> +        self as *const CBitmap as *const usize
> +    }
> +
> +    /// Returns a mutable raw pointer to the backing [`Bitmap`].
> +    pub fn as_mut_ptr(&mut self) -> *mut usize {
> +        self as *mut CBitmap as *mut usize
> +    }
> +
> +    /// Returns length of this [`CBitmap`].
> +    #[expect(clippy::len_without_is_empty)]
> +    pub fn len(&self) -> usize {
> +        self.data.len()
> +    }
> +}
> +
> +/// Holds either a pointer to array of `unsigned long` or a small bitmap.
> +#[repr(C)]
> +union BitmapRepr {
> +    bitmap: usize,
> +    ptr: NonNull<usize>,
> +}
> +
> +macro_rules! bitmap_assert {
> +    ($cond:expr, $($arg:tt)+) => {
> +        #[cfg(CONFIG_RUST_BITMAP_HARDENED)]
> +        assert!($cond, $($arg)*);
> +    }
> +}
> +
> +macro_rules! bitmap_assert_return {
> +    ($cond:expr, $($arg:tt)+) => {
> +        #[cfg(CONFIG_RUST_BITMAP_HARDENED)]
> +        assert!($cond, $($arg)*);
> +
> +        #[cfg(not(CONFIG_RUST_BITMAP_HARDENED))]
> +        if !($cond) {
> +            pr_err!($($arg)*);
> +            return
> +        }
> +    }
> +}
> +
> +/// Represents an owned bitmap.
> +///
> +/// Wraps underlying C bitmap API. See [`CBitmap`] for available
> +/// methods.
> +///
> +/// # Examples
> +///
> +/// Basic usage
> +///
> +/// ```
> +/// use kernel::alloc::flags::GFP_KERNEL;
> +/// use kernel::bitmap::Bitmap;
> +///
> +/// let mut b = Bitmap::new(16, GFP_KERNEL)?;
> +///
> +/// assert_eq!(16, b.len());
> +/// for i in 0..16 {
> +///     if i % 4 == 0 {
> +///       b.set_bit(i);
> +///     }
> +/// }
> +/// assert_eq!(Some(0), b.next_bit(0));
> +/// assert_eq!(Some(1), b.next_zero_bit(0));
> +/// assert_eq!(Some(4), b.next_bit(1));
> +/// assert_eq!(Some(5), b.next_zero_bit(4));
> +/// assert_eq!(Some(12), b.last_bit());
> +/// # Ok::<(), Error>(())
> +/// ```
> +///
> +/// # Invariants
> +///
> +/// * `nbits` is `<= i32::MAX` and never changes.
> +/// * if `nbits <= bindings::BITS_PER_LONG`, then `repr` is a `usize`.
> +/// * otherwise, `repr` holds a non-null pointer to an initialized
> +///   array of `unsigned long` that is large enough to hold `nbits` bits.
> +pub struct Bitmap {
> +    /// Representation of bitmap.
> +    repr: BitmapRepr,
> +    /// Length of this bitmap. Must be `<= i32::MAX`.
> +    nbits: usize,
> +}
> +
> +impl core::ops::Deref for Bitmap {
> +    type Target = CBitmap;
> +
> +    fn deref(&self) -> &CBitmap {
> +        let ptr = if self.nbits <= bindings::BITS_PER_LONG as _ {

You can define a local constant with the right size to avoid these casts:
const BITS_PER_LONG: usize = bindings::BITS_PER_LONG as usize;

> +            // SAFETY: Bitmap is represented inline.
> +            unsafe { core::ptr::addr_of!(self.repr.bitmap) }
> +        } else {
> +            // SAFETY: Bitmap is represented as array of `unsigned long`.
> +            unsafe { self.repr.ptr.as_ptr() }
> +        };
> +
> +        // SAFETY: We got the right pointer and invariants of [`Bitmap`] hold.
> +        // An inline bitmap is treated like an array with single element.
> +        unsafe { CBitmap::from_raw(ptr, self.nbits) }
> +    }
> +}
> +
> +impl core::ops::DerefMut for Bitmap {
> +    fn deref_mut(&mut self) -> &mut CBitmap {
> +        let ptr = if self.nbits <= bindings::BITS_PER_LONG as _ {
> +            // SAFETY: Bitmap is represented inline.
> +            unsafe { core::ptr::addr_of_mut!(self.repr.bitmap) }
> +        } else {
> +            // SAFETY: Bitmap is represented as array of `unsigned long`.
> +            unsafe { self.repr.ptr.as_mut() }
> +        };
> +
> +        // SAFETY: We got the right pointer and invariants of [`Bitmap`] hold.
> +        // An inline bitmap is treated like an array with single element.
> +        unsafe { CBitmap::from_raw_mut(ptr, self.nbits) }
> +    }
> +}
> +
> +/// Enable ownership transfer to other threads.
> +///
> +/// SAFETY: We own the underlying bitmap representation.
> +unsafe impl Send for Bitmap {}
> +
> +/// Enable unsynchronized concurrent access to [`Bitmap`] through shared references.
> +///
> +/// SAFETY: `deref()` will return a reference to a [`CBitmap`] which is Sync. Its methods
> +/// that take immutable references are either atomic or read-only.
> +unsafe impl Sync for Bitmap {}
> +
> +impl Drop for Bitmap {
> +    fn drop(&mut self) {
> +        if self.nbits <= bindings::BITS_PER_LONG as _ {
> +            return;
> +        }
> +        // SAFETY: `self.ptr` was returned by the C `bitmap_zalloc`.
> +        //
> +        // INVARIANT: there is no other use of the `self.ptr` after this
> +        // call and the value is being dropped so the broken invariant is
> +        // not observable on function exit.
> +        unsafe { bindings::bitmap_free(self.repr.ptr.as_ptr()) };
> +    }
> +}
> +
> +impl Bitmap {
> +    /// Constructs a new [`Bitmap`].
> +    ///
> +    /// Fails with [`AllocError`] when the [`Bitmap`] could not be allocated. This
> +    /// includes the case when `nbits` is greater than `i32::MAX`.
> +    #[inline]
> +    pub fn new(nbits: usize, flags: Flags) -> Result<Self, AllocError> {
> +        if nbits <= bindings::BITS_PER_LONG as _ {
> +            return Ok(Bitmap {
> +                repr: BitmapRepr { bitmap: 0 },
> +                nbits,
> +            });
> +        }
> +        if nbits > i32::MAX.try_into().unwrap() {
> +            return Err(AllocError);
> +        }
> +        let nbits_u32 = u32::try_from(nbits).unwrap();
> +        // SAFETY: `bindings::BITS_PER_LONG < nbits` and `nbits <= i32::MAX`.
> +        let ptr = unsafe { bindings::bitmap_zalloc(nbits_u32, flags.as_raw()) };
> +        let ptr = NonNull::new(ptr).ok_or(AllocError)?;
> +        // INVARIANT: `ptr` returned by C `bitmap_zalloc` and `nbits` checked.
> +        Ok(Bitmap {
> +            repr: BitmapRepr { ptr },
> +            nbits,
> +        })
> +    }
> +
> +    /// Returns length of this [`Bitmap`].
> +    #[allow(clippy::len_without_is_empty)]
> +    #[inline]
> +    pub fn len(&self) -> usize {
> +        self.nbits
> +    }
> +}
> +
> +impl CBitmap {
> +    /// Set bit with index `index`.
> +    ///
> +    /// ATTENTION: `set_bit` is non-atomic, which differs from the naming
> +    /// convention in C code. The corresponding C function is `__set_bit`.
> +    ///
> +    /// If CONFIG_RUST_BITMAP_HARDENED is not enabled and `index` is greater than
> +    /// or equal to `self.nbits`, does nothing.
> +    ///
> +    /// # Panics
> +    ///
> +    /// Panics if CONFIG_RUST_BITMAP_HARDENED is enabled and `index` is greater than
> +    /// or equal to `self.nbits`.
> +    #[inline]
> +    pub fn set_bit(&mut self, index: usize) {
> +        bitmap_assert_return!(
> +            index < self.len(),
> +            "Bit `index` must be < {}, was {}",
> +            self.len(),
> +            index
> +        );
> +        // SAFETY: Bit `index` is within bounds.
> +        unsafe { bindings::__set_bit(index, self.as_mut_ptr()) };
> +    }
> +
> +    /// Set bit with index `index`, atomically.
> +    ///
> +    /// This is a relaxed atomic operation (no implied memory barriers).
> +    ///
> +    /// ATTENTION: The naming convention differs from C, where the corresponding
> +    /// function is called `set_bit`.
> +    ///
> +    /// If CONFIG_RUST_BITMAP_HARDENED is not enabled and `index` is greater than
> +    /// or equal to `self.len()`, does nothing.
> +    ///
> +    /// # Panics
> +    ///
> +    /// Panics if CONFIG_RUST_BITMAP_HARDENED is enabled and `index` is greater than
> +    /// or equal to `self.len()`.
> +    #[inline]
> +    pub fn set_bit_atomic(&self, index: usize) {
> +        bitmap_assert_return!(
> +            index < self.len(),
> +            "Bit `index` must be < {}, was {}",
> +            self.len(),
> +            index
> +        );
> +        // SAFETY: `index` is within bounds and the caller has ensured that
> +        // there is no mix of non-atomic and atomic operations.
> +        unsafe { bindings::set_bit(index, self.as_ptr() as *mut usize) };
> +    }
> +
> +    /// Clear `index` bit.
> +    ///
> +    /// ATTENTION: `clear_bit` is non-atomic, which differs from the naming
> +    /// convention in C code. The corresponding C function is `__clear_bit`.
> +    ///
> +    /// If CONFIG_RUST_BITMAP_HARDENED is not enabled and `index` is greater than
> +    /// or equal to `self.len()`, does nothing.
> +    ///
> +    /// # Panics
> +    ///
> +    /// Panics if CONFIG_RUST_BITMAP_HARDENED is enabled and `index` is greater than
> +    /// or equal to `self.len()`.
> +    #[inline]
> +    pub fn clear_bit(&mut self, index: usize) {
> +        bitmap_assert_return!(
> +            index < self.len(),
> +            "Bit `index` must be < {}, was {}",
> +            self.len(),
> +            index
> +        );
> +        // SAFETY: `index` is within bounds.
> +        unsafe { bindings::__clear_bit(index, self.as_mut_ptr()) };
> +    }
> +
> +    /// Clear `index` bit, atomically.
> +    ///
> +    /// This is a relaxed atomic operation (no implied memory barriers).
> +    ///
> +    /// ATTENTION: The naming convention differs from C, where the corresponding
> +    /// function is called `clear_bit`.
> +    ///
> +    /// If CONFIG_RUST_BITMAP_HARDENED is not enabled and `index` is greater than
> +    /// or equal to `self.len()`, does nothing.
> +    ///
> +    /// # Panics
> +    ///
> +    /// Panics if CONFIG_RUST_BITMAP_HARDENED is enabled and `index` is greater than
> +    /// or equal to `self.len()`.
> +    #[inline]
> +    pub fn clear_bit_atomic(&self, index: usize) {
> +        bitmap_assert_return!(
> +            index < self.len(),
> +            "Bit `index` must be < {}, was {}",
> +            self.len(),
> +            index
> +        );
> +        // SAFETY: `index` is within bounds and the caller has ensured that
> +        // there is no mix of non-atomic and atomic operations.
> +        unsafe { bindings::clear_bit(index, self.as_ptr() as *mut usize) };
> +    }
> +
> +    /// Copy `src` into this [`Bitmap`] and set any remaining bits to zero.
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// use kernel::alloc::{AllocError, flags::GFP_KERNEL};
> +    /// use kernel::bitmap::Bitmap;
> +    ///
> +    /// let mut long_bitmap = Bitmap::new(256, GFP_KERNEL)?;
> +    //
> +    /// assert_eq!(None, long_bitmap.last_bit());
> +    //
> +    /// let mut short_bitmap = Bitmap::new(16, GFP_KERNEL)?;
> +    //
> +    /// short_bitmap.set_bit(7);
> +    /// long_bitmap.copy_and_extend(&short_bitmap);
> +    /// assert_eq!(Some(7), long_bitmap.last_bit());
> +    ///
> +    /// # Ok::<(), AllocError>(())
> +    /// ```
> +    #[inline]
> +    pub fn copy_and_extend(&mut self, src: &Bitmap) {
> +        let len = core::cmp::min(src.nbits, self.len());
> +        // SAFETY: access to `self` and `src` is within bounds.
> +        unsafe {
> +            bindings::bitmap_copy_and_extend(
> +                self.as_mut_ptr(),
> +                src.as_ptr(),
> +                len as u32,
> +                self.len() as u32,
> +            )
> +        };
> +    }
> +
> +    /// Finds last set bit.
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// use kernel::alloc::{AllocError, flags::GFP_KERNEL};
> +    /// use kernel::bitmap::Bitmap;
> +    ///
> +    /// let bitmap = Bitmap::new(64, GFP_KERNEL)?;
> +    ///
> +    /// match bitmap.last_bit() {
> +    ///     Some(idx) => {
> +    ///         pr_info!("The last bit has index {idx}.\n");
> +    ///     }
> +    ///     None => {
> +    ///         pr_info!("All bits in this bitmap are 0.\n");
> +    ///     }
> +    /// }
> +    /// # Ok::<(), AllocError>(())
> +    /// ```
> +    #[inline]
> +    pub fn last_bit(&self) -> Option<usize> {
> +        // SAFETY: `_find_next_bit` access is within bounds due to invariant.
> +        let index = unsafe { bindings::_find_last_bit(self.as_ptr(), self.len()) };
> +        if index >= self.len() {
> +            None
> +        } else {
> +            Some(index)
> +        }
> +    }
> +
> +    /// Finds next set bit, starting from `start`.
> +    /// Returns `None` if `start` is greater of equal than `self.nbits`.
> +    #[inline]

The html docs look better if you include a newline:

/// Finds next set bit, starting from `start`.
///
/// Returns `None` if `start` is greater of equal than `self.nbits`.

> +    pub fn next_bit(&self, start: usize) -> Option<usize> {
> +        bitmap_assert!(
> +            start < self.len(),
> +            "`start` must be < {} was {}",
> +            self.len(),
> +            start
> +        );
> +        // SAFETY: `_find_next_bit` tolerates out-of-bounds arguments and returns a
> +        // value larger than or equal to `self.len()` in that case.
> +        let index = unsafe { bindings::_find_next_bit(self.as_ptr(), self.len(), start) };
> +        if index >= self.len() {
> +            None
> +        } else {
> +            Some(index)
> +        }
> +    }
> +
> +    /// Finds next zero bit, starting from `start`.
> +    /// Returns `None` if `start` is greater than or equal to `self.len()`.
> +    #[inline]
> +    pub fn next_zero_bit(&self, start: usize) -> Option<usize> {
> +        bitmap_assert!(
> +            start < self.len(),
> +            "`start` must be < {} was {}",
> +            self.len(),
> +            start
> +        );
> +        // SAFETY: `_find_next_zero_bit` tolerates out-of-bounds arguments and returns a
> +        // value larger than or equal to `self.len()` in that case.
> +        let index = unsafe { bindings::_find_next_zero_bit(self.as_ptr(), self.len(), start) };
> +        if index >= self.len() {
> +            None
> +        } else {
> +            Some(index)
> +        }
> +    }
> +}
> +
> +use macros::kunit_tests;
> +
> +#[kunit_tests(rust_kernel_bitmap)]
> +mod tests {
> +    use super::*;
> +    use kernel::alloc::flags::GFP_KERNEL;
> +
> +    #[test]
> +    fn cbitmap_borrow() {
> +        let fake_c_bitmap: [usize; 2] = [0, 0];
> +        // SAFETY: `fake_c_bitmap` is an array of expected length.
> +        let b = unsafe {
> +            CBitmap::from_raw(
> +                core::ptr::addr_of!(fake_c_bitmap) as *const usize,

You can just do fake_c_bitmap.as_ptr()

> +                2 * bindings::BITS_PER_LONG as usize,
> +            )
> +        };
> +        assert_eq!(2 * bindings::BITS_PER_LONG as usize, b.len());
> +        assert_eq!(None, b.next_bit(0));
> +    }
> +
> +    #[test]
> +    fn cbitmap_copy() {
> +        let fake_c_bitmap: usize = 0xFF;
> +        // SAFETY: `fake_c_bitmap` can be used as one-element array of expected length.
> +        let b = unsafe { CBitmap::from_raw(core::ptr::addr_of!(fake_c_bitmap), 8) };
> +        assert_eq!(8, b.len());
> +        assert_eq!(None, b.next_zero_bit(0));
> +    }
> +
> +    #[test]
> +    fn bitmap_new() {
> +        let b = Bitmap::new(0, GFP_KERNEL).unwrap();
> +        assert_eq!(0, b.len());
> +
> +        let b = Bitmap::new(3, GFP_KERNEL).unwrap();
> +        assert_eq!(3, b.len());
> +
> +        let b = Bitmap::new(1024, GFP_KERNEL).unwrap();
> +        assert_eq!(1024, b.len());
> +
> +        // Requesting too large values results in [`AllocError`].
> +        let b = Bitmap::new(1 << 31, GFP_KERNEL);
> +        assert!(b.is_err());
> +    }
> +
> +    #[test]
> +    fn bitmap_set_clear_find() {
> +        let mut b = Bitmap::new(128, GFP_KERNEL).unwrap();
> +
> +        // Zero-initialized
> +        assert_eq!(None, b.next_bit(0));
> +        assert_eq!(Some(0), b.next_zero_bit(0));
> +        assert_eq!(None, b.last_bit());
> +
> +        b.set_bit(17);
> +
> +        assert_eq!(Some(17), b.next_bit(0));
> +        assert_eq!(Some(17), b.next_bit(17));
> +        assert_eq!(None, b.next_bit(18));
> +        assert_eq!(Some(17), b.last_bit());
> +
> +        b.set_bit(107);
> +
> +        assert_eq!(Some(17), b.next_bit(0));
> +        assert_eq!(Some(17), b.next_bit(17));
> +        assert_eq!(Some(107), b.next_bit(18));
> +        assert_eq!(Some(107), b.last_bit());
> +
> +        b.clear_bit(17);
> +
> +        assert_eq!(Some(107), b.next_bit(0));
> +        assert_eq!(Some(107), b.last_bit());
> +    }
> +
> +    #[cfg(not(CONFIG_RUST_BITMAP_HARDENED))]
> +    #[test]
> +    fn bitmap_out_of_bounds() {
> +        let mut b = Bitmap::new(128, GFP_KERNEL).unwrap();
> +
> +        b.set_bit(2048);
> +        b.set_bit_atomic(2048);
> +        b.clear_bit(2048);
> +        b.clear_bit_atomic(2048);
> +        assert_eq!(None, b.next_bit(2048));
> +        assert_eq!(None, b.next_zero_bit(2048));
> +        assert_eq!(None, b.last_bit());
> +    }
> +
> +    #[cfg(CONFIG_RUST_BITMAP_HARDENED)]
> +    #[test]
> +    #[should_panic]
> +    fn bitmap_out_of_bounds() {

I don't think we have #[should_panic] support in Rust KUnit yet.

> +        let mut b = Bitmap::new(128, GFP_KERNEL).unwrap();
> +
> +        b.set_bit(2048);
> +    }
> +
> +    #[test]
> +    fn bitmap_copy_and_extend() {
> +        let mut long_bitmap = Bitmap::new(256, GFP_KERNEL).unwrap();
> +
> +        long_bitmap.set_bit(3);
> +        long_bitmap.set_bit(200);
> +
> +        let mut short_bitmap = Bitmap::new(32, GFP_KERNEL).unwrap();
> +
> +        short_bitmap.set_bit(17);
> +
> +        long_bitmap.copy_and_extend(&short_bitmap);
> +
> +        // Previous bits have been cleared.
> +        assert_eq!(Some(17), long_bitmap.next_bit(0));
> +        assert_eq!(Some(17), long_bitmap.last_bit());
> +    }
> +}
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index de07aadd1ff5..8c4161cd82ac 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -38,6 +38,7 @@
>  pub use ffi;
>
>  pub mod alloc;
> +pub mod bitmap;
>  #[cfg(CONFIG_BLOCK)]
>  pub mod block;
>  #[doc(hidden)]
> diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
> index 3fe9d7b945c4..d77a39aef554 100644
> --- a/security/Kconfig.hardening
> +++ b/security/Kconfig.hardening
> @@ -324,6 +324,16 @@ config LIST_HARDENED
>
>           If unsure, say N.
>
> +config RUST_BITMAP_HARDENED
> +       bool "Check integrity of bitmap Rust API"
> +       depends on RUST
> +       help
> +         Enables additional assertions in the Rust Bitmap API to catch
> +         arguments that are not guaranteed to result in an immediate access
> +         fault.
> +
> +         If unsure, say N.
> +
>  config BUG_ON_DATA_CORRUPTION
>         bool "Trigger a BUG when data corruption is detected"
>         select LIST_HARDENED
> --
> 2.50.0.rc1.591.g9c95f17f64-goog
>

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