Message-ID: <20250722-iov-iter-v3-2-3efc9c2c2893@google.com>
Date: Tue, 22 Jul 2025 12:33:10 +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>, 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>, 
	Benno Lossin <lossin@...nel.org>
Subject: [PATCH v3 2/4] 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.

This patch adds an IovIterDest struct that is very similar to the
IovIterSource from the previous patch. However, as the methods on the
two structs have very little overlap (just getting the length and
advance/revert), I do not think it is worth it to try and deduplicate
this logic.

Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
---
 rust/kernel/iov.rs | 143 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 143 insertions(+)

diff --git a/rust/kernel/iov.rs b/rust/kernel/iov.rs
index a92fa22c856a506f836a15c74a29e82dc90a4721..e31a7947677c0603764648c06418697b2be2af18 100644
--- a/rust/kernel/iov.rs
+++ b/rust/kernel/iov.rs
@@ -16,6 +16,15 @@
 use core::{marker::PhantomData, mem::MaybeUninit, ptr, 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 _: () = {
+    build_assert!(
+        ITER_SOURCE != ITER_DEST,
+        "ITER_DEST and ITER_SOURCE should be different."
+    );
+};
 
 /// An IO vector that acts as a source of data.
 ///
@@ -165,3 +174,137 @@ pub fn copy_from_iter_raw(&mut self, out: &mut [MaybeUninit<u8>]) -> &mut [u8] {
         unsafe { slice::from_raw_parts_mut(out, len) }
     }
 }
+
+/// An IO vector that acts as a destination for data.
+///
+/// IO vectors support many different types of destinations. This includes both buffers in
+/// kernel-space and writing to userspace. It's possible that the destination buffer is mapped in a
+/// thread-local manner using e.g. `kmap_local_page()`, so this type is not `Send` to ensure that
+/// the mapping is written to the right context in that scenario.
+///
+/// # Invariants
+///
+/// Must hold a valid `struct iov_iter` with `data_source` set to `ITER_DEST`. For the duration of
+/// `'data`, it must be safe to write to this IO vector using the standard C methods for this
+/// purpose.
+#[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]>,
+}
+
+impl<'data> IovIterDest<'data> {
+    /// Obtain an `IovIterDest` from a raw pointer.
+    ///
+    /// # Safety
+    ///
+    /// * The referenced `struct iov_iter` must be valid and must only be accessed through the
+    ///   returned reference for the duration of `'iov`.
+    /// * The referenced `struct iov_iter` must have `data_source` set to `ITER_DEST`.
+    /// * For the duration of `'data`, it must be safe to write to this IO vector using the
+    ///   standard C methods for this purpose.
+    #[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, and
+        // `IovIterSource` is layout compatible with `struct iov_iter`.
+        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: We have shared access to this IO vector, so we can read its `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: By the struct invariants, `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: By the struct invariants, `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:
+        // * By the struct invariants, it is still valid to write to this IO vector.
+        // * `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)
+    }
+}

-- 
2.50.0.727.gbf7dc18ff4-goog

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