fix clone(CLONE_NEWPID)

fix clone(CLONE_NEWPID)

Currently we are complicating the code in copy_process, the clone ABI, and
if we fix the bugs sys_setsid itself, with an unnecessary open coded
version of sys_setsid.

So just simplify everything and don't special case the session and pgrp of
the initial process in a pid namespace.

Having this special case actually presents to user space the classic linux
startup conditions with session == pgrp == 0 for /sbin/init.

We already handle sending signals to processes in a child pid namespace.

We need to handle sending signals to processes in a parent pid namespace
for cases like SIGCHILD and SIGIO.

This makes nothing extra visible inside a pid namespace.  So this extra
special case appears to have no redeeming merits.

Further removing this special case increases the flexibility of how we can
use pid namespaces, by not requiring the initial process in a pid namespace
to be a daemon.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>