linux-kernel - Re: io_uring_enter() with opcode IORING_OP_RECV ignores MSG_WAITALL in msg

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Message-ID: <8f3cc170-1c24-da8a-37ec-f2e73eade46c@gmail.com>
Date:   Wed, 23 Mar 2022 19:40:18 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>,
        Constantine Gavrilov <CONSTG@...ibm.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     io-uring <io-uring@...r.kernel.org>
Subject: Re: io_uring_enter() with opcode IORING_OP_RECV ignores MSG_WAITALL
 in msg_flags

On 3/23/22 12:19, Jens Axboe wrote:
> On 3/23/22 4:31 AM, Constantine Gavrilov wrote:
>> I get partial receives on TCP socket, even though I specify
>> MSG_WAITALL with IORING_OP_RECV opcode. Looking at tcpdump in
>> wireshark, I see entire reassambled packet (+4k), so it is not a
>> disconnect. The MTU is smaller than 4k.
>>
>>  From the mailing list history, looks like this was discussed before
>> and it seems the fix was supposed to be in. Can someone clarify the
>> expected behavior?
>>
>> I do not think rsvmsg() has this issue.
> 
> Do you have a test case? I added the io-uring list, that's the
> appropriate forum for this kind of discussion.

MSG_WAITALL (since Linux 2.2)
        This flag requests that the operation block until the full
        request is satisfied.  However, the call may still return
        less data than requested if a signal is caught

My guess would be that it's either due to signals (including
task_works actively used by io_uring) or because of some
interoperability problem with NOWAIT.

-- 
Pavel Begunkov