All members of the process group have the same sid and it can't be == 0.
NOTE: this code (and a similar one in sys_setpgid) was needed because it
was possibe to have ->session == 0. It's not possible any longer since
[PATCH] pidhash: don't use zero pids
Commit: c7c6464117
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>