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

[Arseniy Krasnov]

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).

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.

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.


Thanks, Arseniy

> 
> Thanks,
> Stefano
> 
>> +
>>     release_sock(sk);
>>     return err;
>> }
>> -- 
>> 2.25.1
>>
>