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

[Arseniy Krasnov]

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@xxxxxxxxxxxxxx>
>>>>>> ---
>>>>>> 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.

Thanks, Arseniy

> 
> Thanks,
> Stefano
>