linux-kernel - Re: [PATCH] io_uring: fix deferred req iovec leak

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Date:   Thu, 6 Feb 2020 13:58:51 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Pavel Begunkov <asml.silence@...il.com>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] io_uring: fix deferred req iovec leak

On 2/6/20 1:39 PM, Pavel Begunkov wrote:
> On 06/02/2020 23:16, Jens Axboe wrote:
>> On 2/6/20 1:00 PM, Pavel Begunkov wrote:
>>> On 06/02/2020 22:56, Jens Axboe wrote:
>>>> On 2/6/20 10:16 AM, Pavel Begunkov wrote:
>>>>> On 06/02/2020 20:04, Pavel Begunkov wrote:
>>>>>> On 06/02/2020 19:51, Pavel Begunkov wrote:
>>>>>>> After defer, a request will be prepared, that includes allocating iovec
>>>>>>> if needed, and then submitted through io_wq_submit_work() but not custom
>>>>>>> handler (e.g. io_rw_async()/io_sendrecv_async()). However, it'll leak
>>>>>>> iovec, as it's in io-wq and the code goes as follows:
>>>>>>>
>>>>>>> io_read() {
>>>>>>> 	if (!io_wq_current_is_worker())
>>>>>>> 		kfree(iovec);
>>>>>>> }
>>>>>>>
>>>>>>> Put all deallocation logic in io_{read,write,send,recv}(), which will
>>>>>>> leave the memory, if going async with -EAGAIN.
>>>>>>>
>>>>>> Interestingly, this will fail badly if it returns -EAGAIN from io-wq context.
>>>>>> Apparently, I need to do v2.
>>>>>>
>>>>> Or not...
>>>>> Jens, can you please explain what's with the -EAGAIN handling in
>>>>> io_wq_submit_work()? Checking the code, it seems neither of
>>>>> read/write/recv/send can return -EAGAIN from async context (i.e.
>>>>> force_nonblock=false). Are there other ops that can do it?
>>>>
>>>> Nobody should return -EAGAIN with force_nonblock=false, they should
>>>> end the io_kiocb inline for that.
>>>>
>>>
>>> If so for those 4, then the patch should work well.
>>
>> Maybe I'm dense, but I'm not seeing the leak? We have two cases here:
>>
> 
> There is an example:
> 
> 1. submit a read, which need defer.
> 
> 2. io_req_defer() allocates ->io and goes io_req_defer_prep() -> io_read_prep().
> Let #vecs > UIO_FASTIOV, so the prep() in the presence of ->io will allocate iovec.
> Note: that work.func is left io_wq_submit_work
> 
> 3. At some point @io_wq calls io_wq_submit_work() -> io_issue_sqe() -> io_read(),
> 
> 4. actual reading succeeds, and it's coming to finalisation and the following
> code in particular.
> 
> if (!io_wq_current_is_worker())
> 	kfree(iovec);
> 
> 5. Because we're in io_wq, the cleanup will not be performed, even though we're
> returning with success. And that's a leak.
> 
> Do you see anything wrong with it?

That's my bad, I didn't read the subject fully, this is specific to
a deferred request. Patch looks good to me, and it cleans it up too
which is always a nice win!

-- 
Jens Axboe