[PATCH] setpgid: should work for sub-threads

setsid() does not work unless the calling process is a
thread_group_leader().

'man setpgid' does not tell anything about that, so I consider this
behaviour is a bug.
Signed-off-by: Oren Laadan <orenl@cs.columbia.edu>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/exit.c

@@ -257,7 +257,7 @@ static inline void reparent_to_init(void)
 
 void __set_special_pids(pid_t session, pid_t pgrp)
 {
-	struct task_struct *curr = current;
+	struct task_struct *curr = current->group_leader;
 
 	if (curr->signal->session != session) {
 		detach_pid(curr, PIDTYPE_SID);

kernel/sys.c

@@ -1215,24 +1215,22 @@ asmlinkage long sys_getsid(pid_t pid)
 
 asmlinkage long sys_setsid(void)
 {
+	struct task_struct *group_leader = current->group_leader;
 	struct pid *pid;
 	int err = -EPERM;
 
-	if (!thread_group_leader(current))
-		return -EINVAL;
-
 	down(&tty_sem);
 	write_lock_irq(&tasklist_lock);
 
-	pid = find_pid(PIDTYPE_PGID, current->pid);
+	pid = find_pid(PIDTYPE_PGID, group_leader->pid);
 	if (pid)
 		goto out;
 
-	current->signal->leader = 1;
-	__set_special_pids(current->pid, current->pid);
-	current->signal->tty = NULL;
-	current->signal->tty_old_pgrp = 0;
-	err = process_group(current);
+	group_leader->signal->leader = 1;
+	__set_special_pids(group_leader->pid, group_leader->pid);
+	group_leader->signal->tty = NULL;
+	group_leader->signal->tty_old_pgrp = 0;
+	err = process_group(group_leader);
 out:
 	write_unlock_irq(&tasklist_lock);
 	up(&tty_sem);