linux-kernel - Re: [RFC PATCH v1 1/2] vsock: send SIGPIPE on write to shutdowned socket

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Date:   Fri, 4 Aug 2023 17:02:12 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Arseniy Krasnov <oxffffaa@...il.com>
Cc:     Arseniy Krasnov <AVKrasnov@...rdevices.ru>,
        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 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).

Thanks,
Stefano