Message-ID: <20250311-iov-iter-v1-2-f6c9134ea824@google.com>
Date: Tue, 11 Mar 2025 14:25:13 +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 2/5] rust: iov: add iov_iter abstractions for ITER_DEST

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@...gle.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

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