Add UserSliceWriter::write_dma() to copy data from a CoherentAllocation<u8> to userspace. This provides a safe interface for copying DMA buffer contents to userspace without requiring callers to work with raw pointers.
The method handles bounds checking and offset calculation internally, wrapping the unsafe copy_to_user() call. Signed-off-by: Timur Tabi <[email protected]> --- rust/kernel/uaccess.rs | 41 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 41 insertions(+) diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs index f989539a31b4..f9bb88e47408 100644 --- a/rust/kernel/uaccess.rs +++ b/rust/kernel/uaccess.rs @@ -7,6 +7,7 @@ use crate::{ alloc::{Allocator, Flags}, bindings, + dma::CoherentAllocation, error::Result, ffi::{c_char, c_void}, fs::file, @@ -481,6 +482,46 @@ pub fn write_slice(&mut self, data: &[u8]) -> Result { Ok(()) } + /// Writes raw data to this user pointer from a DMA coherent allocation. + /// + /// Returns error if the offset+count exceeds the allocation size. + /// + /// Fails with [`EFAULT`] if the write happens on a bad address, or if the write goes out of + /// bounds of this [`UserSliceWriter`]. This call may modify the associated userspace slice + /// even if it returns an error. + /// + /// Note: The memory may be concurrently modified by hardware (e.g., DMA). In such cases, + /// the copied data may be inconsistent, but this does not cause undefined behavior. + pub fn write_dma( + &mut self, + alloc: &CoherentAllocation<u8>, + offset: usize, + count: usize, + ) -> Result { + let len = alloc.count(); + if offset.checked_add(count).ok_or(EOVERFLOW)? > len { + return Err(ERANGE); + } + + // SAFETY: `start_ptr()` returns a valid pointer to a memory region of `count()` bytes, + // as guaranteed by the `CoherentAllocation` invariants. The check above ensures + // `offset + count <= len`. + let src_ptr = unsafe { alloc.start_ptr().add(offset) }; + + // SAFETY: `src_ptr` is valid for reads of `count` bytes per the above. + let res = unsafe { + bindings::copy_to_user(self.ptr.as_mut_ptr(), src_ptr.cast::<c_void>(), count) + }; + if res != 0 { + return Err(EFAULT); + } + + self.ptr = self.ptr.wrapping_byte_add(count); + self.length -= count; + + Ok(()) + } + /// Writes raw data to this user pointer from a kernel buffer partially. /// /// This is the same as [`Self::write_slice`] but considers the given `offset` into `data` and -- 2.52.0
