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

[Jens Axboe <axboe@kernel.dk>]

On 3/16/24 10:46 AM, Pavel Begunkov wrote:
> On 3/16/24 16:42, Jens Axboe wrote:
>> On 3/16/24 10:36 AM, Pavel Begunkov wrote:
>>> 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
>>
>> I did try both ways, fails if we just have:
> 
> Ok, but the point is that the sendzc's ->fail doesn't
> need to be called unless you've done ->prep first.

But it fails, not sure how else to say it.

FWIW, the current io_uring-6.9 branch has two patches on top, looks fine
for me so far. We'll see if syzbot agrees. I'll send them out later
today, unless I change my mind and try a different approach.

-- 
Jens Axboe