sys_setpgid(): simplify pid/ns interaction

sys_setpgid() does unneeded conversions from pid_t to "struct pid" and vice
versa.  Use "struct pid" more consistently.  Saves one find_vpid() and
eliminates the explicit usage of ->nsproxy->pid_ns.  Imho, cleanups the
code.

Also use the same_thread_group() helper.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/sys.c

@@ -916,8 +916,8 @@ asmlinkage long sys_setpgid(pid_t pid, pid_t pgid)
 {
 	struct task_struct *p;
 	struct task_struct *group_leader = current->group_leader;
-	int err = -EINVAL;
-	struct pid_namespace *ns;
+	struct pid *pgrp;
+	int err;
 
 	if (!pid)
 		pid = task_pid_vnr(group_leader);
@@ -929,12 +929,10 @@ asmlinkage long sys_setpgid(pid_t pid, pid_t pgid)
 	/* From this point forward we keep holding onto the tasklist lock
 	 * so that our parent does not change from under us. -DaveM
 	 */
-	ns = current->nsproxy->pid_ns;
-
 	write_lock_irq(&tasklist_lock);
 
 	err = -ESRCH;
-	p = find_task_by_pid_ns(pid, ns);
+	p = find_task_by_vpid(pid);
 	if (!p)
 		goto out;
 
@@ -942,7 +940,7 @@ asmlinkage long sys_setpgid(pid_t pid, pid_t pgid)
 	if (!thread_group_leader(p))
 		goto out;
 
-	if (p->real_parent->tgid == group_leader->tgid) {
+	if (same_thread_group(p->real_parent, group_leader)) {
 		err = -EPERM;
 		if (task_session(p) != task_session(group_leader))
 			goto out;
@@ -959,10 +957,12 @@ asmlinkage long sys_setpgid(pid_t pid, pid_t pgid)
 	if (p->signal->leader)
 		goto out;
 
+	pgrp = task_pid(p);
 	if (pgid != pid) {
 		struct task_struct *g;
 
-		g = find_task_by_pid_type_ns(PIDTYPE_PGID, pgid, ns);
+		pgrp = find_vpid(pgid);
+		g = pid_task(pgrp, PIDTYPE_PGID);
 		if (!g || task_session(g) != task_session(group_leader))
 			goto out;
 	}
@@ -971,13 +971,10 @@ asmlinkage long sys_setpgid(pid_t pid, pid_t pgid)
 	if (err)
 		goto out;
 
-	if (task_pgrp_nr_ns(p, ns) != pgid) {
-		struct pid *pid;
-
+	if (task_pgrp(p) != pgrp) {
 		detach_pid(p, PIDTYPE_PGID);
-		pid = find_vpid(pgid);
-		attach_pid(p, PIDTYPE_PGID, pid);
-		set_task_pgrp(p, pid_nr(pid));
+		attach_pid(p, PIDTYPE_PGID, pgrp);
+		set_task_pgrp(p, pid_nr(pgrp));
 	}
 
 	err = 0;