Re: [PATCHv2] io_uring: Support calling io_uring_register with a registered ring fd

[Josh Triplett]

On Wed, Feb 15, 2023 at 10:44:38AM -0700, Jens Axboe wrote:
> On 2/14/23 5:42 PM, Josh Triplett wrote:
> > Add a new flag IORING_REGISTER_USE_REGISTERED_RING (set via the high bit
> > of the opcode) to treat the fd as a registered index rather than a file
> > descriptor.
> > 
> > This makes it possible for a library to open an io_uring, register the
> > ring fd, close the ring fd, and subsequently use the ring entirely via
> > registered index.
> 
> This looks pretty straight forward to me, only real question I had
> was whether using the top bit of the register opcode for this is the
> best choice. But I can't think of better ways to do it, and the space
> is definitely big enough to do that, so looks fine to me.

It seemed like the cleanest way available given the ABI of
io_uring_register, yeah.

> One more comment below:
> 
> > +	if (use_registered_ring) {
> > +		/*
> > +		 * Ring fd has been registered via IORING_REGISTER_RING_FDS, we
> > +		 * need only dereference our task private array to find it.
> > +		 */
> > +		struct io_uring_task *tctx = current->io_uring;
> 
> I need to double check if it's guaranteed we always have current->io_uring
> assigned here. If the ring is registered we certainly will have it, but
> what if someone calls io_uring_register(2) without having a ring setup
> upfront?
> 
> IOW, I think we need a NULL check here and failing the request at that
> point.

The next line is:

+               if (unlikely(!tctx || fd >= IO_RINGFD_REG_MAX))

The first part of that condition is the NULL check you're looking for,
right?

- Josh Triplett