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

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

On 3/16/24 16:36, Jens Axboe wrote:
> On 3/16/24 10:32 AM, Pavel Begunkov wrote:
>> 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'm not making this up, your test case will literally fail as it doesn't
> get to flag MORE for that case. FWIW, this was done with EARLY_FAIL
> being flagged, and failing if we fail during or before prep.

Maybe the test is too strict, but your approach is different
from what I mentioned yesterday

-	return def->prep(req, sqe);
+	ret = def->prep(req, sqe);
+	if (unlikely(ret)) {
+		req->flags |= REQ_F_EARLY_FAIL;
+		return ret;
+	}
+
+	return 0;

It should only set REQ_F_EARLY_FAIL if we fail
_before_ prep is called

-- 
Pavel Begunkov