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

[Arseniy Krasnov]

On 22.08.2023 12:39, Stefano Garzarella wrote:
> On Mon, Aug 14, 2023 at 10:46:05PM +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.
>>
>> Update about sending SIGPIPE for SOCK_SEQPACKET, I checked POSIX doc and kernel sources more deeply:
>>
>>
>> 1)
>>
>> I checked four types of sockets, which sends SIGPIPE for SOCK_SEQPACKET or not ('YES' if
>> this socket sends SIGPIPE in SOCK_SEQPACKET case):
>>
>> net/kcm/: YES
>> net/unix/: NO
>> net/sctp/: YES
>> net/caif/: NO
>>
>> Looking for this, I think it is impossible to get the right answer, as there is some
>> mess - everyone implements it as wish.
> 
> Eheh, I had the same impression!
> 
>>
>> 2)
>>
>> I opened POSIX spec again, and here are details about returning EPIPE from pages
>> for 'send()', 'sendto()', 'sendmsg()':
>>
>> [EPIPE] The socket is shut down for writing, or the socket is connection-mode and is
>> no longer connected. In the latter case, and if the socket is of type
>> SOCK_STREAM, the SIGPIPE signal is generated to the calling thread
>>
>> So my opinion is that we need to send SIGPIPE only for SOCK_STREAM. Another question
>> is how to interpret this from above (but again - SIGPIPE is related for SOCK_STREAM
>> only):
>>
>> **" and is no longer connected"**
>>
>> IIUC, if we follow POSIX strictly, this check must be like:
>>
>> /* socket is shut down for writing or no longer connected. */
>> if (sk->sk_shutdown & SEND_SHUTDOWN ||
>>    vsk->peer_shutdown & RCV_SHUTDOWN ||
>>    sock_flag(SOCK_DONE)) {
>>     err = -EPIPE;
>>     goto out;
>> }
>>
>> ...
>>
>> out:
>>     /* Handle -EPIPE for stream socket which is no longer connected. */
>>     if (sk->sk_type == SOCK_STREAM &&
>>         sock_flag(SOCK_DONE))
>>         err = sk_stream_error();
>>
>>
>>
>> From the other side, we can just follow TCP/AF_UNIX implementations as both are
>> popular types of socket. In this case I suggest to implement this check like
>> (e.g. without sock_flag(SOCK_DONE)):
>>
>>
>> if (sk->sk_shutdown & SEND_SHUTDOWN ||
>>    vsk->peer_shutdown & RCV_SHUTDOWN) {
>>     err = -EPIPE;
>>     goto out;
>> }
>>
>> ...
>>
>> out:
>>     if (sk->sk_type == SOCK_STREAM)
>>         err = sk_stream_error();
>>
>> What do you think?
> 
> I'd follow TCP/AF_UNIX implementations, but it is up to you ;-)

Got it, I'll use this approach

Thanks, Arseniy

> 
> Thanks,
> Stefano
>