[PATCH v4 2/3] Adds Rust Bitmap API.

[Burak Emir]

Provides an abstraction for C bitmap API and bitops operations.
This includes enough functionality to reimplementing a Binder
data structure (drivers/android/dbitmap.h). More methods can be
added later. We offer a safe API through bounds checks which
panic if violated.

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.

Suggested-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
Signed-off-by: Burak Emir <bqe@xxxxxxxxxx>
---
 MAINTAINERS           |   2 +
 rust/kernel/bitmap.rs | 234 ++++++++++++++++++++++++++++++++++++++++++
 rust/kernel/lib.rs    |   1 +
 3 files changed, 237 insertions(+)
 create mode 100644 rust/kernel/bitmap.rs

diff --git a/MAINTAINERS b/MAINTAINERS
index 50f44d7e5c6e..b3bbce9274f0 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -4036,9 +4036,11 @@ F:	rust/helpers/bitmap.c
 F:	rust/helpers/cpumask.c
 
 BITMAP API [RUST]
+M:	Alice Ryhl <aliceryhl@xxxxxxxxxx> (bitmap)
 M:	Viresh Kumar <viresh.kumar@xxxxxxxxxx> (cpumask)
 R:	Yury Norov <yury.norov@xxxxxxxxx>
 S:	Maintained
+F:	rust/kernel/bitmap.rs
 F:	rust/kernel/cpumask.rs
 
 BITOPS API
diff --git a/rust/kernel/bitmap.rs b/rust/kernel/bitmap.rs
new file mode 100644
index 000000000000..e8117e0dbe05
--- /dev/null
+++ b/rust/kernel/bitmap.rs
@@ -0,0 +1,234 @@
+// 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;
+use core::ptr::NonNull;
+
+/// Represents a bitmap.
+///
+/// Wraps underlying C bitmap API.
+///
+/// # 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(1), b.find_next_zero_bit(0));
+/// assert_eq!(Some(5), b.find_next_zero_bit(5));
+/// assert_eq!(Some(12), b.find_last_bit());
+/// # Ok::<(), Error>(())
+/// ```
+///
+/// # Invariants
+///
+/// `ptr` is obtained from a successful call to `bitmap_zalloc` and
+/// holds the address of an initialized array of `unsigned long`
+/// that is large enough to hold `nbits` bits.
+/// `nbits` is `<= u32::MAX` and never changes.
+pub struct Bitmap {
+    /// Pointer to an array of `unsigned long`.
+    ptr: NonNull<usize>,
+    /// How many bits this bitmap stores. Must be `<= u32::MAX`.
+    nbits: usize,
+}
+
+impl Drop for Bitmap {
+    fn drop(&mut self) {
+        // 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.as_mut_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 `u32::MAX`.
+    #[inline]
+    pub fn new(nbits: usize, flags: Flags) -> Result<Self, AllocError> {
+        if let Ok(nbits_u32) = u32::try_from(nbits) {
+            // SAFETY: `nbits == 0` is permitted and `nbits <= u32::MAX`.
+            let ptr = unsafe { bindings::bitmap_zalloc(nbits_u32, flags.as_raw()) };
+            // Zero-size allocation is ok and yields a dangling pointer.
+            let ptr = NonNull::new(ptr).ok_or(AllocError)?;
+            // INVARIANT: ptr returned by C `bitmap_zalloc` and nbits checked.
+            Ok(Bitmap { ptr, nbits })
+        } else {
+            Err(AllocError)
+        }
+    }
+
+    /// Returns how many bits this [`Bitmap`] holds.
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.nbits
+    }
+
+    /// Returns true if this [`Bitmap`] has length `0`.
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.nbits == 0
+    }
+
+    /// Returns a mutable raw pointer to the backing [`Bitmap`].
+    #[inline]
+    fn as_mut_ptr(&mut self) -> *mut usize {
+        self.ptr.as_ptr()
+    }
+
+    /// Returns a raw pointer to the backing [`Bitmap`].
+    #[inline]
+    fn as_ptr(&self) -> *const usize {
+        self.ptr.as_ptr()
+    }
+
+    /// Sets bit with index `index`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `index` is greater than or equal to `self.nbits`.
+    #[inline]
+    pub fn set_bit(&mut self, index: usize) {
+        assert!(
+            index < self.nbits,
+            "Bit `index` must be < {}, was {}",
+            self.nbits,
+            index
+        );
+        // SAFETY: Bit `index` is within bounds.
+        unsafe { bindings::__set_bit(index as u32, self.as_mut_ptr()) };
+    }
+
+    /// Clears bit with index `index`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `index` is greater than or equal to `self.nbits`.
+    #[inline]
+    pub fn clear_bit(&mut self, index: usize) {
+        assert!(
+            index < self.nbits,
+            "Bit `index` must be < {}, was {}",
+            self.nbits,
+            index
+        );
+        // SAFETY: Bit `index` is within bounds.
+        unsafe { bindings::__clear_bit(index as u32, self.as_mut_ptr()) };
+    }
+
+    /// Replaces this [`Bitmap`] with `src` and sets any remaining bits to zero.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `src` is longer than this [`Bitmap`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::alloc::{AllocError, flags::GFP_KERNEL};
+    /// use kernel::bitmap::Bitmap;
+    ///
+    /// let mut long_bitmap = Bitmap::new(256, GFP_KERNEL)?;
+    /// let short_bitmap = Bitmap::new(16, GFP_KERNEL)?;
+    /// long_bitmap.copy_from_bitmap_and_extend(&short_bitmap);
+    /// # Ok::<(), AllocError>(())
+    /// ```
+    #[inline]
+    pub fn copy_from_bitmap_and_extend(&mut self, src: &Bitmap) {
+        assert!(
+            src.nbits <= self.nbits,
+            "Length of `src` must be <= {}, was {}",
+            self.nbits,
+            src.nbits
+        );
+        // SAFETY: `nbits == 0` is supported and access to `self` and `src` is within bounds.
+        unsafe {
+            bindings::bitmap_copy_and_extend(
+                self.as_mut_ptr(),
+                src.as_ptr(),
+                src.nbits as u32,
+                self.nbits as u32,
+            )
+        };
+    }
+
+    /// Replaces this bitmap with a prefix of `src` that fits into this [`Bitmap`].
+    ///
+    /// # Panics
+    ///
+    /// Panics if `src` is shorter than this [`Bitmap`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::alloc::{AllocError, flags::GFP_KERNEL};
+    /// use kernel::bitmap::Bitmap;
+    ///
+    /// let mut short_bitmap = Bitmap::new(16, GFP_KERNEL)?;
+    /// let long_bitmap = Bitmap::new(256, GFP_KERNEL)?;
+    /// short_bitmap.copy_prefix_from_bitmap(&long_bitmap);
+    /// # Ok::<(), AllocError>(())
+    /// ```
+    #[inline]
+    pub fn copy_prefix_from_bitmap(&mut self, src: &Bitmap) {
+        assert!(
+            self.nbits <= src.nbits,
+            "Length of `src` must be >= {}, was {}",
+            self.nbits,
+            src.nbits
+        );
+        // SAFETY: `nbits == 0` is supported and access to `self` and `src` is within bounds.
+        unsafe {
+            bindings::bitmap_copy_and_extend(
+                self.as_mut_ptr(),
+                src.as_ptr(),
+                self.nbits as u32,
+                self.nbits as u32,
+            )
+        };
+    }
+
+    /// Finds the last bit that is set.
+    #[inline]
+    pub fn find_last_bit(&self) -> Option<usize> {
+        // SAFETY: `nbits == 0` is supported and access is within bounds.
+        let index = unsafe { bindings::_find_last_bit(self.as_ptr(), self.nbits) };
+        if index == self.nbits {
+            None
+        } else {
+            Some(index)
+        }
+    }
+
+    /// Finds the next zero bit, starting from `offset`.
+    #[inline]
+    pub fn find_next_zero_bit(&self, offset: usize) -> Option<usize> {
+        // SAFETY: `nbits == 0` and out-of-bounds offset is supported.
+        let index = unsafe { bindings::_find_next_zero_bit(self.as_ptr(), self.nbits, offset) };
+        if index == self.nbits {
+            None
+        } else {
+            Some(index)
+        }
+    }
+}
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index efbd7be98dab..be06ffc47473 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -36,6 +36,7 @@
 pub use ffi;
 
 pub mod alloc;
+pub mod bitmap;
 #[cfg(CONFIG_BLOCK)]
 pub mod block;
 #[doc(hidden)]
-- 
2.49.0.rc1.451.g8f38331e32-goog