Re: [PATCH 2/5] rust: iov: add iov_iter abstractions for ITER_DEST

[Andreas Hindborg <a.hindborg@kernel.org>]

"Christian Schrefl" <chrisi.schrefl@gmail.com> writes:

> Hi Alice
>
> On 11.03.25 3:25 PM, Alice Ryhl wrote:
>> 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@google.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]>,
>> +}
>
> It might be a bit nicer to add a (private) struct 'IovIter' that implements the common operations.
> Then 'IovIterDest' and 'IovIterSource' could store that struct and forward the implementations to
> it.
> But I'm not sure if that's really much better.

Yea, I was thinking the same. Maybe we could have an `IovInner` and a
local `AsIovInner` trait that would give all the shared methods?

I am sure Alice knows the idiomatic way to achieve code sharing here.


Best regards,
Andreas Hindborg