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

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

On 3/16/24 16:14, 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?

> they can't use sr->done_io at that point. I'll ponder this a bit and see
> what the best generic solution is.

-- 
Pavel Begunkov