[PATCH 2/5] rust: iov: add iov_iter abstractions for ITER_DEST

Alice Ryhl
2025-03-11T14:26:04Z

This adds abstractions for the iov_iter type in the case where
data_source is ITER_DEST. This will make Rust implementations of
fops->read_iter possible.

This series only has support for using existing IO vectors created by C
code. Additional abstractions will be needed to support the creation of
IO vectors in Rust code.

These abstractions make the assumption that `struct iov_iter` does not
have internal self-references, which implies that it is valid to move it
between different local variables, and that you can make a copy of it to
get two IO vectors into the same buffers.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
rust/kernel/iov.rs | 140 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 139 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/iov.rs b/rust/kernel/iov.rs
index 4498f65e1f65bd964909810c020db3a9f8fae389..dc32c27c5c76d059562fd7c6b9d4b178a8ea7c81 100644
--- a/rust/kernel/iov.rs
+++ b/rust/kernel/iov.rs
@@ -7,7 +7,12 @@
//! C headers: [`include/linux/iov_iter.h`](srctree/include/linux/iov_iter.h),
//! [`include/linux/uio.h`](srctree/include/linux/uio.h)

-use crate::{bindings, prelude::*, types::Opaque};
+use crate::{
+ alloc::{Allocator, Flags},
+ bindings,
+ prelude::*,
+ types::Opaque,
+};
use core::{marker::PhantomData, mem::MaybeUninit, slice};

const ITER_SOURCE: bool = bindings::ITER_SOURCE != 0;
@@ -168,3 +173,136 @@ fn clone(&self) -> IovIterSource<'data> {
unsafe { core::ptr::read(self) }
}
}
+
+/// An IO vector that acts as a destination for data.
+///
+/// # Invariants
+///
+/// Must hold a valid `struct iov_iter` with `data_source` set to `ITER_DEST`. The buffers
+/// referenced by the IO vector must be valid for writing for the duration of `'data`.
+///
+/// Note that if the IO vector is backed by a userspace pointer, it is always considered valid for
+/// writing.
+#[repr(transparent)]
+pub struct IovIterDest<'data> {
+ iov: Opaque<bindings::iov_iter>,
+ /// Represent to the type system that this value contains a pointer to writable data it does
+ /// not own.
+ _source: PhantomData<&'data mut [u8]>,
+}
+
+// SAFETY: This struct is essentially just a fancy `std::io::Cursor<&mut [u8]>`, and that type is
+// safe to send across thread boundaries.
+unsafe impl<'data> Send for IovIterDest<'data> {}
+// SAFETY: This struct is essentially just a fancy `std::io::Cursor<&mut [u8]>`, and that type is
+// safe to share across thread boundaries.
+unsafe impl<'data> Sync for IovIterDest<'data> {}
+
+impl<'data> IovIterDest<'data> {
+ /// Obtain an `IovIterDest` from a raw pointer.
+ ///
+ /// # Safety
+ ///
+ /// * For the duration of `'iov`, the `struct iov_iter` must remain valid and must not be
+ /// accessed except through the returned reference.
+ /// * For the duration of `'data`, the buffers backing this IO vector must be valid for
+ /// writing.
+ #[track_caller]
+ #[inline]
+ pub unsafe fn from_raw<'iov>(ptr: *mut bindings::iov_iter) -> &'iov mut IovIterDest<'data> {
+ // SAFETY: The caller ensures that `ptr` is valid.
+ let data_source = unsafe { (*ptr).data_source };
+ assert_eq!(data_source, ITER_DEST);
+
+ // SAFETY: The caller ensures the struct invariants for the right durations.
+ unsafe { &mut *ptr.cast::<IovIterDest<'data>>() }
+ }
+
+ /// Access this as a raw `struct iov_iter`.
+ #[inline]
+ pub fn as_raw(&mut self) -> *mut bindings::iov_iter {
+ self.iov.get()
+ }
+
+ /// Returns the number of bytes available in this IO vector.
+ ///
+ /// Note that this may overestimate the number of bytes. For example, reading from userspace
+ /// memory could fail with EFAULT, which will be treated as the end of the IO vector.
+ #[inline]
+ pub fn len(&self) -> usize {
+ // SAFETY: It is safe to access the `count` field.
+ unsafe {
+ (*self.iov.get())
+ .__bindgen_anon_1
+ .__bindgen_anon_1
+ .as_ref()
+ .count
+ }
+ }
+
+ /// Returns whether there are any bytes left in this IO vector.
+ ///
+ /// This may return `true` even if there are no more bytes available. For example, reading from
+ /// userspace memory could fail with EFAULT, which will be treated as the end of the IO vector.
+ #[inline]
+ pub fn is_empty(&self) -> bool {
+ self.len() == 0
+ }
+
+ /// Advance this IO vector by `bytes` bytes.
+ ///
+ /// If `bytes` is larger than the size of this IO vector, it is advanced to the end.
+ #[inline]
+ pub fn advance(&mut self, bytes: usize) {
+ // SAFETY: `self.iov` is a valid IO vector.
+ unsafe { bindings::iov_iter_advance(self.as_raw(), bytes) };
+ }
+
+ /// Advance this IO vector backwards by `bytes` bytes.
+ ///
+ /// # Safety
+ ///
+ /// The IO vector must not be reverted to before its beginning.
+ #[inline]
+ pub unsafe fn revert(&mut self, bytes: usize) {
+ // SAFETY: `self.iov` is a valid IO vector, and `bytes` is in bounds.
+ unsafe { bindings::iov_iter_revert(self.as_raw(), bytes) };
+ }
+
+ /// Write data to this IO vector.
+ ///
+ /// Returns the number of bytes that were written. If this is shorter than the provided slice,
+ /// then no more bytes can be written.
+ #[inline]
+ pub fn copy_to_iter(&mut self, input: &[u8]) -> usize {
+ // SAFETY: `input` is valid for `input.len()` bytes.
+ unsafe { bindings::_copy_to_iter(input.as_ptr().cast(), input.len(), self.as_raw()) }
+ }
+
+ /// Utility for implementing `read_iter` given the full contents of the file.
+ ///
+ /// The full contents of the file being read from is represented by `contents`. This call will
+ /// write the appropriate sub-slice of `contents` and update the file position in `ppos` so
+ /// that the file will appear to contain `contents` even if takes multiple reads to read the
+ /// entire file.
+ #[inline]
+ pub fn simple_read_from_buffer(&mut self, ppos: &mut i64, contents: &[u8]) -> Result<usize> {
+ if *ppos < 0 {
+ return Err(EINVAL);
+ }
+ let Ok(pos) = usize::try_from(*ppos) else {
+ return Ok(0);
+ };
+ if pos >= contents.len() {
+ return Ok(0);
+ }
+
+ // BOUNDS: We just checked that `pos < contents.len()` above.
+ let num_written = self.copy_to_iter(&contents[pos..]);
+
+ // OVERFLOW: pos+num_written <= contents.len() <= isize::MAX <= i64::MAX
+ *ppos = (pos + num_written) as i64;
+
+ Ok(num_written)
+ }
+}

--
2.49.0.rc0.332.g42c0ae87b1-goog