[PATCHv7 2/6] io_uring: add support for kernel registered bvecs

[Keith Busch]

On Thu, Feb 27, 2025 at 03:54:31PM +0000, Pavel Begunkov wrote:
> On 2/26/25 18:20, Keith Busch wrote:
> > From: Keith Busch <kbusch at kernel.org>
> > 
> > Provide an interface for the kernel to leverage the existing
> > pre-registered buffers that io_uring provides. User space can reference
> > these later to achieve zero-copy IO.
> > 
> > User space must register an empty fixed buffer table with io_uring in
> > order for the kernel to make use of it.
> 
> Can you also fail rw.c:loop_rw_iter()? Something like:
> 
> loop_rw_iter() {
> 	if ((req->flags & REQ_F_BUF_NODE) &&
> 	    req->buf_node->buf->release)
> 		return -EFAULT;
> }

For posterity: the suggestion is because this function uses the
file_operations' .read/.write callbacks, which expect __user pointers.

Playing devil's advocate here, I don't see how user space might know
ahead of time if the file they opened implements the supported _iter
versions. I think only esoteric and legacy interfaces still use it, so
maybe we don't care.