Re: [PATCH v1 1/2] io-uring: add io_uring_cmd_get_buffer_info()

[Joanne Koong <joannelkoong@gmail.com>]

On Thu, Oct 23, 2025 at 3:20 PM Joanne Koong <joannelkoong@gmail.com> wrote:
>
> On Wed, Oct 22, 2025 at 8:17 PM Caleb Sander Mateos
> <csander@purestorage.com> wrote:
> >
> > On Wed, Oct 22, 2025 at 1:23 PM Joanne Koong <joannelkoong@gmail.com> wrote:
> > > diff --git a/include/linux/io_uring/cmd.h b/include/linux/io_uring/cmd.h
> > > index 7509025b4071..a92e810f37f9 100644
> > > --- a/io_uring/rsrc.c
> > > +++ b/io_uring/rsrc.c
> > > @@ -1569,3 +1569,24 @@ int io_prep_reg_iovec(struct io_kiocb *req, struct iou_vec *iv,
> > >         req->flags |= REQ_F_IMPORT_BUFFER;
> > >         return 0;
> > >  }
> > > +
> > > +int io_uring_cmd_get_buffer_info(struct io_uring_cmd *cmd, u64 *ubuf,
> > > +                                unsigned int *len)
> > > +{
> > > +       struct io_ring_ctx *ctx = cmd_to_io_kiocb(cmd)->ctx;
> > > +       struct io_rsrc_data *data = &ctx->buf_table;
> > > +       struct io_mapped_ubuf *imu;
> > > +       unsigned int buf_index;
> > > +
> > > +       if (!data->nr)
> > > +               return -EINVAL;
> > > +
> > > +       buf_index = cmd->sqe->buf_index;
> >
> > This is reading userspace-mapped memory, it should use READ_ONCE().
> > But why not just use cmd_to_io_kiocb(cmd)->buf_index? That's already
> > sampled from the SQE in io_uring_cmd_prep() if the
> > IORING_URING_CMD_FIXED flag is set. And it seems like the fuse
> > uring_cmd implementation requires that flag to be set.
>
> Thanks, I didn't realize the cmd->sqe is userspace-mapped. I'll look
> at what io_uring_cmd_prep() does.
>
> For v2 I am going to drop this patch entirely and pass in the ubuf
> address and len through the 80B sqe command area when the server
> registers the ring.

Actually, after looking at this some more and realizing that we can't
get away with recycling iters across requests, I like your
io_uring_cmd_import_fixed_relative() helper idea. I think I'll just do
it that way for v2 instead of having the libfuse server pass in the
buffer address and length, since otherwise we would need to pass that
info for every request (including commit/fetch, not just when
registering) or have to stash it somewhere.

Thanks,
Joanne

>
> Thanks,
> Joanne