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, 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

We can't post a notif unless we allocated it, which couldn't
possibly happen without ->prep being called fist.

Let's better call it UNPREPPED_FAIL or somehow more meaningfully,
I expect a lot of confusion around "EARLY_FAIL"

-- 
Pavel Begunkov