Message-ID: <kernljom75y4nzle7s5cfid6pm6rvhf7t3zf4mrdes7oxjry23@pm7cdshzvemn>
Date:   Tue, 22 Aug 2023 11:36:54 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Arseniy Krasnov <avkrasnov@...rdevices.ru>
Cc:     Arseniy Krasnov <oxffffaa@...il.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Bobby Eshleman <bobby.eshleman@...edance.com>,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel@...rdevices.ru
Subject: Re: [RFC PATCH v1 1/2] vsock: send SIGPIPE on write to shutdowned
 socket

On Mon, Aug 14, 2023 at 10:40:17PM +0300, Arseniy Krasnov wrote:
>
>
>On 04.08.2023 18:02, Stefano Garzarella wrote:
>> On Fri, Aug 04, 2023 at 05:34:20PM +0300, Arseniy Krasnov wrote:
>>>
>>>
>>> On 04.08.2023 17:28, Stefano Garzarella wrote:
>>>> On Fri, Aug 04, 2023 at 03:46:47PM +0300, Arseniy Krasnov wrote:
>>>>> Hi Stefano,
>>>>>
>>>>> On 02.08.2023 10:46, Stefano Garzarella wrote:
>>>>>> On Tue, Aug 01, 2023 at 05:17:26PM +0300, Arseniy Krasnov wrote:
>>>>>>> POSIX requires to send SIGPIPE on write to SOCK_STREAM socket which was
>>>>>>> shutdowned with SHUT_WR flag or its peer was shutdowned with SHUT_RD
>>>>>>> flag. Also we must not send SIGPIPE if MSG_NOSIGNAL flag is set.
>>>>>>>
>>>>>>> Signed-off-by: Arseniy Krasnov <AVKrasnov@...rdevices.ru>
>>>>>>> ---
>>>>>>> net/vmw_vsock/af_vsock.c | 3 +++
>>>>>>> 1 file changed, 3 insertions(+)
>>>>>>>
>>>>>>> diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
>>>>>>> index 020cf17ab7e4..013b65241b65 100644
>>>>>>> --- a/net/vmw_vsock/af_vsock.c
>>>>>>> +++ b/net/vmw_vsock/af_vsock.c
>>>>>>> @@ -1921,6 +1921,9 @@ static int vsock_connectible_sendmsg(struct socket *sock, struct msghdr *msg,
>>>>>>>             err = total_written;
>>>>>>>     }
>>>>>>> out:
>>>>>>> +    if (sk->sk_type == SOCK_STREAM)
>>>>>>> +        err = sk_stream_error(sk, msg->msg_flags, err);
>>>>>>
>>>>>> Do you know why we don't need this for SOCK_SEQPACKET and SOCK_DGRAM?
>>>>>
>>>>> Yes, here is my explanation:
>>>>>
>>>>> This function checks that input error is SIGPIPE, and if so it sends SIGPIPE to the 'current' thread
>>>>> (except case when MSG_NOSIGNAL flag is set). This behaviour is described in POSIX:
>>>>>
>>>>> Page 367 (description of defines from sys/socket.h):
>>>>> MSG_NOSIGNAL: No SIGPIPE generated when an attempt to send is made on a stream-
>>>>> oriented socket that is no longer connected.
>>>>>
>>>>> Page 497 (description of SOCK_STREAM):
>>>>> A SIGPIPE signal is raised if a thread sends on a broken stream (one that is
>>>>> no longer connected).
>>>>
>>>> Okay, but I think we should do also for SEQPACKET:
>>>>
>>>> https://pubs.opengroup.org/onlinepubs/009696699/functions/xsh_chap02_10.html
>>>>
>>>> In 2.10.6 Socket Types:
>>>>
>>>> "The SOCK_SEQPACKET socket type is similar to the SOCK_STREAM type, and
>>>> is also connection-oriented. The only difference between these types is
>>>> that record boundaries ..."
>>>>
>>>> Then in  2.10.14 Signals:
>>>>
>>>> "The SIGPIPE signal shall be sent to a thread that attempts to send data
>>>> on a socket that is no longer able to send. In addition, the send
>>>> operation fails with the error [EPIPE]."
>>>>
>>>> It's honestly not super clear, but I assume the problem is similar with
>>>> seqpacket since it's connection-oriented, or did I miss something?
>>>>
>>>> For example in sctp_sendmsg() IIUC we raise a SIGPIPE regardless of
>>>> whether the socket is STREAM or SEQPACKET.
>>>
>>> Hm, yes, you're right. Seems check for socket type is not needed in this case,
>>> as this function is only for connection oriented sockets.
>>
>> Ack!
>>
>>>
>>>>
>>>>>
>>>>> Page 1802 (description of 'send()' call):
>>>>> MSG_NOSIGNAL
>>>>>
>>>>> Requests not to send the SIGPIPE signal if an attempt to
>>>>> send is made on a stream-oriented socket that is no
>>>>> longer connected. The [EPIPE] error shall still be
>>>>> returned
>>>>>
>>>>> And the same for 'sendto()' and 'sendmsg()'
>>>>>
>>>>> Link to the POSIX document:
>>>>> https://www.open-std.org/jtc1/sc22/open/n4217.pdf
>>>>>
>>>>> TCP (I think we must rely on it), KCM, SMC sockets (all of them are stream) work in the same
>>>>> way by calling this function. AF_UNIX also works in the same way, but it implements SIGPIPE handling
>>>>> without this function.
>>>>
>>>> I'm okay calling this function.
>>>>
>>>>>
>>>>> The only thing that confused me a little bit, that sockets above returns EPIPE when
>>>>> we have only SEND_SHUTDOWN set, but for AF_VSOCK EPIPE is returned for RCV_SHUTDOWN
>>>>> also, but I think it is related to this patchset.
>>>>
>>>> Do you mean that it is NOT related to this patchset?
>>>
>>> Yes, **NOT**
>>
>> Got it, so if you have time when you're back, let's check also that
>> (not for this series as you mentioned).
>
>^^^
>Hello Stefano, so:
>
>there is some confusion with check for RCV_SHUTDOWN: it presents in AF_UNIX, but missed
>in TCP (it checks only for SEND_SHUTDOWN). I performed simple test which tries
>to send data to peer which already called shutdown(SHUT_RD) - AF_UNIX and TCP behave
>differently. AF_UNIX sends SIGPIPE, while TCP allows to send data.
>
>I suggest to not touch this check for AF_VSOCK (e.g. continue work as AF_UNIX),
>because I don't see strong motivation/argument to remove it.

Yep, I agree!

However, I think it's a fairly borderline case, so unless we have a
specific request, I wouldn't spend too much time on it.

Thanks for looking at it!

Stefano

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