Re: [PATCH v2 1/4] rust: iov: add iov_iter abstractions for ITER_SOURCE

[Alice Ryhl <aliceryhl@google.com>]

On Tue, Jul 08, 2025 at 04:45:14PM +0200, Andreas Hindborg wrote:
> "Alice Ryhl" <aliceryhl@google.com> writes:
> > +/// # Invariants
> > +///
> > +/// Must hold a valid `struct iov_iter` with `data_source` set to `ITER_SOURCE`. For the duration
> > +/// of `'data`, it must be safe to read the data in this IO vector.
> 
> In my opinion, the phrasing you had in v1 was better:
> 
>   The buffers referenced by the IO vector must be valid for reading for
>   the duration of `'data`.
> 
> That is, I would prefer "must be valid for reading" over "it must be
> safe to read ...".

If it's backed by userspace data, then technically there aren't any
buffers that are valid for reading in the usual sense. We need to call
into special assembly to read it, and a normal pointer dereference would
be illegal.

> > +    /// 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.
> 
> Reiterating my comment from v1: Why?

It's the same reason as why this is safe:

struct HasLength {
    length: usize,
}
impl HasLength {
    fn len(&self) -> usize {
        // why is this safe?
        self.length
    }
}

I'm not sure how to say it concisely. I guess it's because all access to
the iov_iter goes through the &IovIterSource.

> > +        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`.
> 
> Can you provide something to back this claim?

I guess the logic could go along these lines:

* If the iov_iter reads from userspace, then it's because we always
  consider such reads to produce initialized data.
* If the iov_iter reads from a kernel buffer, then the creator of the
  iov_iter must provide an initialized buffer.

Ultimately, if we don't know that the bytes are initialized, then it's
impossible to use the API correctly because you can never inspect the
bytes in any way. I.e., any implementation of copy_from_iter that
produces uninit data is necessarily buggy.

Alice