Re: [PATCH] rust: dma: add `Range` type
From: Vasileios Almpanis
Date: Thu Aug 06 2026 - 09:40:49 EST
On 8/6/26 1:23 PM, Robin Murphy wrote:
On 2026-08-06 8:46 am, Vasileios Almpanis wrote:You are right, lengths come from size_t taking APIs anyway, so usize is the right type. Will change len and offset together with some things sashiko found in v2
The base dma address of `Coherent` and `CoherentHandle` is a bare
`DmaAddress` integer so any arithmetic, a driver does on it is unchecked
and can potentially go past the end of the allocation or overflow.
Add a `dma::Range` type that couples a base `DmaAddress` with a length
and only hands out addresses and sub-ranges within `[start, start + len)`
with all arithmetics checked against both the length and the overflow of
of the underlying `dma_addr_t`. Let `Coherent` and `CoherentHandle`
provide the `Range` covering their allocation.
Suggested-by: Danilo Krummrich <dakr@xxxxxxxxxx>
Link: https://github.com/Rust-for-Linux/linux/issues/1248
Signed-off-by: Vasileios Almpanis <vasilisalmpanis@xxxxxxxxx>
---
This is one of my first rust-for-linux patches, so any suggestions are
extremely welcome. Some words about the decisions taken:
- I chose lengths offsets to be `DmaAddress` instead of usize since the
bus space can be 64-bit on 32-bit CPUS.
The base address may be 64-bit, but the underlying dma_alloc_*/dma_map_* APIs all express lengths in size_t, so we shouldn't need to accommodate anything larger in Rust either. Sizes-as-address-types always seem a bit weird and clunky (lookin' at you, resource_size_t...), so probably better avoided if not absolutely necessary, IMO.
Thanks,
Robin.
- The constructor returns EOVERFLOW for `dma_addr_t` overflow, while
out-of-bounds requests return EINVAL; happy to use a single error
code if preferred.
- `dma_range()` is added to both `Coherent` and `CoherentHandle` so the
type has users from the start. I could split it into a follow-up if
that is preferred.
Tested with `make rustdoc`, `make rustfmtcheck`, `CLIPPY=1`, and the
KUnit doctests `rust_doctests_kernel`.
---
rust/kernel/dma.rs | 174 +++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 174 insertions(+)
diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
index 200def84fb69e006bca0b1c578bac9f1dc8da708..e65e99ae4d915ac2e385df8b72fc518c07e4c82d 100644
--- a/rust/kernel/dma.rs
+++ b/rust/kernel/dma.rs
@@ -41,6 +41,142 @@
/// Note that this may be `u64` even on 32-bit architectures.
pub type DmaAddress = bindings::dma_addr_t;
+/// A range of DMA addresses.
+///
+/// Couples a base [`DmaAddress`] with the length in bytes of the region it belongs to,
+/// representing the half-open range `[start, start + len)` of DMA addresses.
+///
+/// Unlike a bare [`DmaAddress`], a [`Range`] only hands out addresses and sub-ranges that are
+/// guaranteed to lie within `[start, start + len)`; all arithmetic is checked against both the
+/// length of the range and overflow of the underlying [`DmaAddress`].
+///
+/// # Invariants
+///
+/// `start + len` does not overflow [`DmaAddress`].
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct Range {
+ start: DmaAddress,
+ len: DmaAddress,
+}
+
+impl Range {
+ /// Creates a new [`Range`] of `len` bytes, starting at `start`.
+ ///
+ /// Returns [`EOVERFLOW`] if `start + len` overflows [`DmaAddress`].
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::dma::{DmaAddress, Range};
+ ///
+ /// let range = Range::new(0x1000, 0x200)?;
+ /// assert_eq!(range.start(), 0x1000);
+ /// assert_eq!(range.end(), 0x1200);
+ /// assert_eq!(range.len(), 0x200);
+ ///
+ /// assert!(Range::new(DmaAddress::MAX, 1).is_err());
+ /// # Ok::<(), Error>(())
+ /// ```
+ #[inline]
+ pub const fn new(start: DmaAddress, len: DmaAddress) -> Result<Self> {
+ if start.checked_add(len).is_none() {
+ return Err(EOVERFLOW);
+ }
+
+ // INVARIANT: We just checked that `start + len` does not overflow `DmaAddress`.
+ Ok(Self { start, len })
+ }
+
+ /// Returns the first address of the range.
+ #[inline]
+ pub const fn start(&self) -> DmaAddress {
+ self.start
+ }
+
+ /// Returns the first address after the end of the range.
+ #[inline]
+ pub const fn end(&self) -> DmaAddress {
+ // By the type invariant, `start + len` does not overflow `DmaAddress`.
+ self.start + self.len
+ }
+
+ /// Returns the length of the range in bytes.
+ #[inline]
+ pub const fn len(&self) -> DmaAddress {
+ self.len
+ }
+
+ /// Returns `true` if the range is empty.
+ #[inline]
+ pub const fn is_empty(&self) -> bool {
+ self.len == 0
+ }
+
+ /// Returns the address at `offset` bytes into the range.
+ ///
+ /// The returned address is guaranteed to lie within the range; returns [`EINVAL`] if `offset`
+ /// is not smaller than the length of the range.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::dma::Range;
+ ///
+ /// let range = Range::new(0x1000, 0x200)?;
+ ///
+ /// assert_eq!(range.address(0)?, 0x1000);
+ /// assert_eq!(range.address(0x1ff)?, 0x11ff);
+ /// assert!(range.address(0x200).is_err());
+ /// # Ok::<(), Error>(())
+ /// ```
+ #[inline]
+ pub const fn address(&self, offset: DmaAddress) -> Result<DmaAddress> {
+ if offset >= self.len {
+ return Err(EINVAL);
+ }
+
+ // By the type invariant, `start + offset < start + len` does not overflow `DmaAddress`.
+ Ok(self.start + offset)
+ }
+
+ /// Returns the sub-range of `len` bytes, starting `offset` bytes into the range.
+ ///
+ /// The returned range is guaranteed to lie within the range; returns [`EINVAL`] if
+ /// `offset + len` overflows [`DmaAddress`] or exceeds the length of the range.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::dma::Range;
+ ///
+ /// let range = Range::new(0x1000, 0x200)?;
+ ///
+ /// let sub = range.subrange(0x100, 0x80)?;
+ /// assert_eq!(sub.start(), 0x1100);
+ /// assert_eq!(sub.end(), 0x1180);
+ ///
+ /// assert!(range.subrange(0x100, 0x101).is_err());
+ /// # Ok::<(), Error>(())
+ /// ```
+ #[inline]
+ pub const fn subrange(&self, offset: DmaAddress, len: DmaAddress) -> Result<Self> {
+ let Some(end) = offset.checked_add(len) else {
+ return Err(EINVAL);
+ };
+
+ if end > self.len {
+ return Err(EINVAL);
+ }
+
+ // INVARIANT: `start + offset + len <= start + self.len`, which by the type invariant of
+ // `self` does not overflow `DmaAddress`.
+ Ok(Self {
+ start: self.start + offset,
+ len,
+ })
+ }
+}
+
/// Trait to be implemented by DMA capable bus devices.
///
/// The [`dma::Device`](Device) trait should be implemented by bus specific device representations,
@@ -626,6 +762,25 @@ pub fn dma_handle(&self) -> DmaAddress {
self.dma_handle
}
+ /// Returns the [`Range`] of DMA addresses covering this allocation.
+ ///
+ /// Unlike [`Self::dma_handle`], which hands out the base address as a bare integer, the
+ /// returned [`Range`] couples the base address with the size of the allocation, such that
+ /// any offset arithmetic performed on it is checked.
+ #[inline]
+ pub fn dma_range(&self) -> Range {
+ // INVARIANT: By the type invariants of `Self`, `dma_handle` is the DMA address base of an
+ // allocated region of `self.size()` bytes; the DMA API guarantees that a mapped region
+ // never wraps the DMA address space, hence `dma_handle + size` does not overflow
+ // `DmaAddress`.
+ Range {
+ start: self.dma_handle,
+ // CAST: `usize` always fits in `DmaAddress`, which is at least 32 bits wide and
+ // always 64 bits wide on 64-bit architectures.
+ len: self.size() as DmaAddress,
+ }
+ }
+
/// Returns a reference to the data in the region.
///
/// # Safety
@@ -1101,6 +1256,25 @@ pub fn dma_handle(&self) -> DmaAddress {
self.dma_handle
}
+ /// Returns the [`Range`] of DMA addresses covering this allocation.
+ ///
+ /// Unlike [`Self::dma_handle`], which hands out the base address as a bare integer, the
+ /// returned [`Range`] couples the base address with the size of the allocation, such that
+ /// any offset arithmetic performed on it is checked.
+ #[inline]
+ pub fn dma_range(&self) -> Range {
+ // INVARIANT: By the type invariants of `Self`, `dma_handle` is the DMA address base of an
+ // allocated region of `self.size` bytes; the DMA API guarantees that a mapped region
+ // never wraps the DMA address space, hence `dma_handle + size` does not overflow
+ // `DmaAddress`.
+ Range {
+ start: self.dma_handle,
+ // CAST: `usize` always fits in `DmaAddress`, which is at least 32 bits wide and
+ // always 64 bits wide on 64-bit architectures.
+ len: self.size as DmaAddress,
+ }
+ }
+
/// Returns the size in bytes of this allocation.
#[inline]
pub fn size(&self) -> usize {
---
base-commit: dc01dfb37b34beeefcfe1c3055364d41a4070c7e
change-id: 20260805-dma-7c5330baadb6
Best regards,