Re: [PATCH 2/4] ublk: enhance check for register/unregister io buffer command

[Ming Lei <ming.lei@redhat.com>]

On Sat, Apr 26, 2025 at 01:38:14PM -0700, Caleb Sander Mateos wrote:
> On Sat, Apr 26, 2025 at 2:41 AM Ming Lei <ming.lei@redhat.com> wrote:
> >
> > The simple check of UBLK_IO_FLAG_OWNED_BY_SRV can avoid incorrect
> > register/unregister io buffer easily, so check it before calling
> > starting to register/un-register io buffer.
> >
> > Also only allow io buffer register/unregister uring_cmd in case of
> > UBLK_F_SUPPORT_ZERO_COPY.
> 
> Indeed, both these checks make sense. (Hopefully there aren't any
> applications depending on the ability to use ublk zero-copy without
> setting the flag.) I too was thinking of adding the
> UBLK_IO_FLAG_OWNED_BY_SRV check because it could allow the
> kref_get_unless_zero() to be replaced with the cheaper kref_get(). I
> think the checks could be split into 2 separate commits, but up to
> you.

Let's do it in single patch for making everyone easier.

> 
> >
> > Fixes: 1f6540e2aabb ("ublk: zc register/unregister bvec")
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >  drivers/block/ublk_drv.c | 23 ++++++++++++++++++++++-
> >  1 file changed, 22 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> > index 40f971a66d3e..347790b3a633 100644
> > --- a/drivers/block/ublk_drv.c
> > +++ b/drivers/block/ublk_drv.c
> > @@ -609,6 +609,11 @@ static void ublk_apply_params(struct ublk_device *ub)
> >                 ublk_dev_param_zoned_apply(ub);
> >  }
> >
> > +static inline bool ublk_support_zero_copy(const struct ublk_queue *ubq)
> > +{
> > +       return ubq->flags & UBLK_F_SUPPORT_ZERO_COPY;
> > +}
> > +
> >  static inline bool ublk_support_user_copy(const struct ublk_queue *ubq)
> >  {
> >         return ubq->flags & (UBLK_F_USER_COPY | UBLK_F_SUPPORT_ZERO_COPY);
> > @@ -1950,9 +1955,16 @@ static int ublk_register_io_buf(struct io_uring_cmd *cmd,
> >                                 unsigned int index, unsigned int issue_flags)
> >  {
> >         struct ublk_device *ub = cmd->file->private_data;
> > +       struct ublk_io *io = &ubq->ios[tag];
> 
> I thought you had mentioned in
> https://lore.kernel.org/linux-block/aAmYJxaV1-yWEMRo@fedora/ wanting
> to the ability to offload the ublk zero-copy buffer registration to a
> thread other than ubq_daemon. Are you still planning to do that, or
> does the "auto-register" feature supplant the need for that?

The auto-register idea is actually thought of when I was working on ublk
selftest offload function.

If this auto-register feature is supported, it becomes less important to
relax the ubq_daemon limit for register_io_buffer command, then I jump
on this feature & post put the patch.

But I will continue to work on the offload test code and finally relax
the limit for register/unregister io buffer command, hope it can be
done in next week.

> Accessing
> the ublk_io here only seems safe when on the ubq_daemon thread.

Both ublk_register_io_buf()/ublk_unregister_io_buf() just reads ublk_io or
the request buffer only, so it is just fine for the two to run from other
contexts.

> 
> >         struct request *req;
> >         int ret;
> >
> > +       if (!ublk_support_zero_copy(ubq))
> > +               return -EINVAL;
> > +
> > +       if (!(io->flags & UBLK_IO_FLAG_OWNED_BY_SRV))
> > +               return -EINVAL;
> 
> Every opcode except UBLK_IO_FETCH_REQ now checks io->flags &
> UBLK_IO_FLAG_OWNED_BY_SRV. Maybe it would make sense to lift the check
> up to __ublk_ch_uring_cmd() to avoid duplicating it?

Good point.


Thanks, 
Ming