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

[Stefano Garzarella]

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 ;-)

Thanks,
Stefano