daemonize() calls set_special_pids(1,1), while init and kernel threads spawned
from init/main.c:init() run with 0,0 special pids.  This patch changes
INIT_SIGNALS() so that that they run with ->pgrp == ->session == 1 also.  This
patch relies on fact that swapper's pid == 1.

Now we have no hashed zero pids in pid_hash[].

User-space visibible change is that now /sbin/init runs with (1,1) special
pids and becomes a session leader.

Quoting Eric W. Biederman:
>
> daemonize consuming pids (1,1) then consumes pgrp 1.  So that when
> /sbin/init calls setsid() it thinks /sbin/init is a process group
> leader and setsid() fails.  So /sbin/init wants pgrp 1 session 1
> but doesn't get it.  I am pretty certain daemonize did not exist so
> /sbin/init got pgrp 1 session 1 in 2.4.
>
> That is the bug that is being fixed.
>
> This patch takes things one step farther and essentially calls
> setsid() for pid == 1 before init is execed.  That is new behavior
> but it cleans up the kernel as we now do not need to support the
> case of a process without a process group or a session.
>
> The only process that could have possibly cared was /sbin/init
> and it already calls setsid() because it doesn't want that.
>
> If this was going to break anything noticeable the change in behavior
> from 2.4 to 2.6 would have already done that.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>