Re: [PATCH v2] io_uring/net: ensure async prep handlers always initialize ->done_io

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

On 3/16/24 16:31, Jens Axboe wrote:
> On 3/16/24 10:28 AM, Pavel Begunkov wrote:
>> On 3/16/24 16:14, Jens Axboe wrote:
>>> On 3/15/24 5:28 PM, Pavel Begunkov wrote:
>>>> On 3/15/24 23:25, Jens Axboe wrote:
>>>>> On 3/15/24 5:19 PM, Pavel Begunkov wrote:
>>>>>> On 3/15/24 23:13, Pavel Begunkov wrote:
>>>>>>> On 3/15/24 23:09, Pavel Begunkov wrote:
>>>>>>>> On 3/15/24 22:48, Jens Axboe wrote:
>>>>>>>>> If we get a request with IOSQE_ASYNC set, then we first run the prep
>>>>>>>>> async handlers. But if we then fail setting it up and want to post
>>>>>>>>> a CQE with -EINVAL, we use ->done_io. This was previously guarded with
>>>>>>>>> REQ_F_PARTIAL_IO, and the normal setup handlers do set it up before any
>>>>>>>>> potential errors, but we need to cover the async setup too.
>>>>>>>>
>>>>>>>> You can hit io_req_defer_failed() { opdef->fail(); }
>>>>>>>> off of an early submission failure path where def->prep has
>>>>>>>> not yet been called, I don't think the patch will fix the
>>>>>>>> problem.
>>>>>>>>
>>>>>>>> ->fail() handlers are fragile, maybe we should skip them
>>>>>>>> if def->prep() wasn't called. Not even compile tested:
>>>>>>>>
>>>>>>>>
>>>>>>>> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
>>>>>>>> index 846d67a9c72e..56eed1490571 100644
>>>>>>>> --- a/io_uring/io_uring.c
>>>>>>>> +++ b/io_uring/io_uring.c
>>>>>> [...]
>>>>>>>>              def->fail(req);
>>>>>>>>          io_req_complete_defer(req);
>>>>>>>>      }
>>>>>>>> @@ -2201,8 +2201,7 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
>>>>>>>>              }
>>>>>>>>              req->flags |= REQ_F_CREDS;
>>>>>>>>          }
>>>>>>>> -
>>>>>>>> -    return def->prep(req, sqe);
>>>>>>>> +    return 0;
>>>>>>>>      }
>>>>>>>>
>>>>>>>>      static __cold int io_submit_fail_init(const struct io_uring_sqe *sqe,
>>>>>>>> @@ -2250,8 +2249,15 @@ static inline int io_submit_sqe(struct io_ring_ctx *ctx, struct io_kiocb *req,
>>>>>>>>          int ret;
>>>>>>>>
>>>>>>>>          ret = io_init_req(ctx, req, sqe);
>>>>>>>> -    if (unlikely(ret))
>>>>>>>> +    if (unlikely(ret)) {
>>>>>>>> +fail:
>>>>>>
>>>>>> Obvious the diff is crap, but still bugging me enough to write
>>>>>> that the label should've been one line below, otherwise we'd
>>>>>> flag after ->prep as well.
>>>>>
>>>>> It certainly needs testing :-)
>>>>>
>>>>> We can go either way - patch up the net thing, or do a proper EARLY_FAIL
>>>>> and hopefully not have to worry about it again. Do you want to clean it
>>>>> up, test it, and send it out?
>>>>
>>>> I'd rather leave it to you, I suspect it wouldn't fix the syzbot
>>>> report w/o fiddling with done_io as in your patch.
>>>
>>> I gave this a shot, but some fail handlers do want to get called. But
>>
>> Which one and/or which part of it?
> 
> send zc

I don't think so. If prep wasn't called there wouldn't be
a notif allocated, and so no F_MORE required. If you take
at the code path it's under REQ_F_NEED_CLEANUP, which is only
set by opcode handlers


> 
> I think the sanest is:
> 
> 1) Opcode handlers should always initialize whatever they need before
>     failure
> 2) If we fail before ->prep, don't call ->fail
> 
> Yes that doesn't cover the case where opcode handlers do stupid things
> like use opcode members in fail if they fail the prep, but that should
> be the smallest part.
> 

-- 
Pavel Begunkov