[PATCH 1/5] rust: iov: add iov_iter abstractions for ITER_SOURCE

Alice Ryhl
2025-03-11T14:26:01Z

This adds abstractions for the iov_iter type in the case where
data_source is ITER_SOURCE. This will make Rust implementations of
fops->write_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 | 170 +++++++++++++++++++++++++++++++++++++++++++++++++++++
rust/kernel/lib.rs | 1 +
2 files changed, 171 insertions(+)

diff --git a/rust/kernel/iov.rs b/rust/kernel/iov.rs
new file mode 100644
index 0000000000000000000000000000000000000000..4498f65e1f65bd964909810c020db3a9f8fae389
--- /dev/null
+++ b/rust/kernel/iov.rs
@@ -0,0 +1,170 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2025 Google LLC.
+
+//! IO vectors.
+//!
+//! 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 core::{marker::PhantomData, mem::MaybeUninit, slice};
+
+const ITER_SOURCE: bool = bindings::ITER_SOURCE != 0;
+const ITER_DEST: bool = bindings::ITER_DEST != 0;
+
+// Compile-time assertion for the above constants.
+const _: () = {
+ if ITER_SOURCE == ITER_DEST {
+ panic!("ITER_DEST and ITER_SOURCE should be different.");
+ }
+};
+
+/// An IO vector that acts as a source of data.
+///
+/// # Invariants
+///
+/// Must hold a valid `struct iov_iter` with `data_source` set to `ITER_SOURCE`. The buffers
+/// referenced by the IO vector must be valid for reading for the duration of `'data`.
+///
+/// Note that if the IO vector is backed by a userspace pointer, it is always considered valid for
+/// reading.
+#[repr(transparent)]
+pub struct IovIterSource<'data> {
+ iov: Opaque<bindings::iov_iter>,
+ /// Represent to the type system that this value contains a pointer to readable data it does
+ /// not own.
+ _source: PhantomData<&'data [u8]>,
+}
+
+// SAFETY: This struct is essentially just a fancy `std::io::Cursor<&[u8]>`, and that type is safe
+// to send across thread boundaries.
+unsafe impl<'data> Send for IovIterSource<'data> {}
+// SAFETY: This struct is essentially just a fancy `std::io::Cursor<&[u8]>`, and that type is safe
+// to share across thread boundaries.
+unsafe impl<'data> Sync for IovIterSource<'data> {}
+
+impl<'data> IovIterSource<'data> {
+ /// Obtain an `IovIterSource` 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
+ /// reading.
+ #[track_caller]
+ #[inline]
+ pub unsafe fn from_raw<'iov>(ptr: *mut bindings::iov_iter) -> &'iov mut IovIterSource<'data> {
+ // SAFETY: The caller ensures that `ptr` is valid.
+ let data_source = unsafe { (*ptr).data_source };
+ assert_eq!(data_source, ITER_SOURCE);
+
+ // SAFETY: The caller ensures the struct invariants for the right durations.
+ unsafe { &mut *ptr.cast::<IovIterSource<'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) };
+ }
+
+ /// Read data from this IO vector.
+ ///
+ /// Returns the number of bytes that have been copied.
+ #[inline]
+ pub fn copy_from_iter(&mut self, out: &mut [u8]) -> usize {
+ // SAFETY: We will not write uninitialized bytes to `out`.
+ let out = unsafe { &mut *(out as *mut [u8] as *mut [MaybeUninit<u8>]) };
+
+ self.copy_from_iter_raw(out).len()
+ }
+
+ /// Read data from this IO vector and append it to a vector.
+ ///
+ /// Returns the number of bytes that have been copied.
+ #[inline]
+ pub fn copy_from_iter_vec<A: Allocator>(
+ &mut self,
+ out: &mut Vec<u8, A>,
+ flags: Flags,
+ ) -> Result<usize> {
+ out.reserve(self.len(), flags)?;
+ let len = self.copy_from_iter_raw(out.spare_capacity_mut()).len();
+ // SAFETY: The next `len` bytes of the vector have been initialized.
+ unsafe { out.set_len(out.len() + len) };
+ Ok(len)
+ }
+
+ /// Read data from this IO vector into potentially uninitialized memory.
+ ///
+ /// Returns the sub-slice of the output that has been initialized. If the returned slice is
+ /// shorter than the input buffer, then the entire IO vector has been read.
+ #[inline]
+ pub fn copy_from_iter_raw(&mut self, out: &mut [MaybeUninit<u8>]) -> &mut [u8] {
+ // SAFETY: `out` is valid for `out.len()` bytes.
+ let len =
+ unsafe { bindings::_copy_from_iter(out.as_mut_ptr().cast(), out.len(), self.as_raw()) };
+
+ // SAFETY: We just initialized the first `len` bytes of `out`.
+ unsafe { slice::from_raw_parts_mut(out.as_mut_ptr().cast(), len) }
+ }
+}
+
+impl<'data> Clone for IovIterSource<'data> {
+ #[inline]
+ fn clone(&self) -> IovIterSource<'data> {
+ // SAFETY: This duplicates the bytes inside the `Opaque` value exactly. Since `struct
+ // iov_iter` does not have any internal self references, that is okay.
+ //
+ // Since this IO vector only reads from the backing buffers, having multiple IO vectors to
+ // the same source can't lead to data races on the backing buffers.
+ unsafe { core::ptr::read(self) }
+ }
+}
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 398242f92a961c3a445d681c65449047a847968a..de9d6e797953cb69637eef127997b1f357f3f90b 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -53,6 +53,7 @@
pub mod init;
pub mod io;
pub mod ioctl;
+pub mod iov;
pub mod jump_label;
#[cfg(CONFIG_KUNIT)]
pub mod kunit;

--
2.49.0.rc0.332.g42c0ae87b1-goog