Re: [PATCH 3/5] io_uring: let fast poll support multishot

[Pavel Begunkov <asml.silence@gmail.com>]

On 5/7/22 08:08, Hao Xu wrote:
> 在 2022/5/7 上午1:19, Pavel Begunkov 写道:
>> On 5/6/22 08:01, Hao Xu wrote:
[...]
>> That looks dangerous, io_queue_sqe() usually takes the request ownership
>> and doesn't expect that someone, i.e. io_poll_check_events(), may still be
>> actively using it.
>>
>> E.g. io_accept() fails on fd < 0, return an error,
>> io_queue_sqe() -> io_queue_async() -> io_req_complete_failed()
>> kills it. Then io_poll_check_events() and polling in general
>> carry on using the freed request => UAF. Didn't look at it
>> too carefully, but there might other similar cases.
>>
> I checked this when I did the coding, it seems the only case is
> while (atomic_sub_return(v & IO_POLL_REF_MASK, &req->poll_refs));
> uses req again after req recycled in io_queue_sqe() path like you
> pointed out above, but this case should be ok since we haven't
> reuse the struct req{} at that point.

Replied to another message with an example that I think might
be broken, please take a look.

The issue is that io_queue_sqe() was always consuming / freeing /
redirecting / etc. requests, i.e. call it and forget about the req.
With io_accept now it may or may not free it and not even returning
any return code about that. This implicit knowledge is quite tricky
to maintain.

might make more sense to "duplicate" io_queue_sqe()

ret = io_issue_sqe(req, IO_URING_F_NONBLOCK|IO_URING_F_COMPLETE_DEFER);
// REQ_F_COMPLETE_INLINE should never happen, no check for that
// don't care about io_arm_ltimeout(), should already be armed
// ret handling here



> In my first version, I skiped the do while{} in io_poll_check_events()
> for multishot apoll and do the reap in io_req_task_submit()
> 
> static void io_apoll_task_func(struct io_kiocb *req, bool *locked)
>    {
>            int ret;
> 
> 
>            ret = io_poll_check_events(req, locked);
>            if (ret > 0)
>                    return;
> 
> 
>            __io_poll_clean(req);
> 
> 
>            if (!ret)
>                    io_req_task_submit(req, locked);   <------here
>            else
>                    io_req_complete_failed(req, ret);
>    }
> 
> But the disadvantage is in high frequent workloads case, it may loop in
> io_poll_check_events for long time, then finally generating cqes in the
> above io_req_task_submit() which is not good in terms of latency.
>>

-- 
Pavel Begunkov