[PATCH] pids: sys_getsid: make sure we don't return 0 instead of -ESRCH

[Oleg Nesterov <oleg@tv-sign.ru>]

The task can exit between find_task_by_vpid() and task_session_vnr(), in
that unlikely case sys_getsid() returns 0 instead of -ESRCH.

The bug was introduced by me in ac9a8e3f0f43d20fc316162e8e5f9186d295ff49.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- 25/kernel/sys.c~1_GETSID	2008-03-20 18:21:17.000000000 +0300
+++ 25/kernel/sys.c	2008-03-23 19:25:36.000000000 +0300
@@ -1019,19 +1019,19 @@ asmlinkage long sys_getsid(pid_t pid)
 	if (!pid)
 		return task_session_vnr(current);
 	else {
-		int retval;
+		int retval = 0;
 		struct task_struct *p;
 
 		rcu_read_lock();
 		p = find_task_by_vpid(pid);
-		retval = -ESRCH;
 		if (p) {
 			retval = security_task_getsid(p);
 			if (!retval)
 				retval = task_session_vnr(p);
 		}
 		rcu_read_unlock();
-		return retval;
+
+		return retval ?: -ESRCH;
 	}
 }