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

[Jens Axboe <axboe@kernel.dk>]

On 2/15/23 1:33?PM, Josh Triplett wrote:
> 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?

Ah yeah, I'm just blind... Looks fine!

-- 
Jens Axboe