RE: [PATCH] af_unix: closed SOCK_SEQPACKET socketpair must get SIGPIPE

From: Alexander Potapenko

Sent: 15 March 2016 09:04
According to IEEE Std 1003.1, 2013, sending data to a SOCK_SEQPACKET
socketpair with MSG_NOSIGNAL flag set must result in a SIGPIPE if the
socket is no longer connected.

...

Without the below patch the behavior is as follows:

$ ./sock seqpacket
sendmsg: Broken pipe

...

The behavior of the patched kernel complies with POSIX:

$  ./sock seqpacket
Killed by SIGPIPE

...

While POSIX might specify this behaviour, changing the behaviour
could easily break applications.
Basically this change (more or less) require every application that
uses SOCK_SEQPACKED to be audited to ensure that MSG_NOSIGNAL is set
on ever send/write to the socket.

Personally I think the whole SIGPIPE on sockets should never have been
allowed to get into the standard.
I don't remember MSG_NOSIGNAL being present in SYSV.

The only time you want a write into a pipe to generate SIGPIPE is
for pipes generates by the shell that feed stdout to stdin of the
next process in the pipeline.
If pipes are implemented as unix-domain socketpairs (no one does that
any more) then it would require the SIGPIPE for write().

	David