Message-ID: <20250311-iov-iter-v1-1-f6c9134ea824@google.com>
Date: Tue, 11 Mar 2025 14:25:12 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Arnd Bergmann <arnd@...db.de>, Miguel Ojeda <ojeda@...nel.org>
Cc: Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <benno.lossin@...ton.me>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, Matthew Maurer <mmaurer@...gle.com>, Lee Jones <lee@...nel.org>, 
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, 
	Alice Ryhl <aliceryhl@...gle.com>
Subject: [PATCH 1/5] rust: iov: add iov_iter abstractions for ITER_SOURCE

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@...gle.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

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