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

[Stefano Garzarella]

On Mon, Aug 14, 2023 at 10:40:17PM +0300, Arseniy Krasnov wrote:
> 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.

Yep, I agree!

However, I think it's a fairly borderline case, so unless we have a
specific request, I wouldn't spend too much time on it.

Thanks for looking at it!

Stefano