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

[Jens Axboe]

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.

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.

-- 
Jens Axboe