[PATCH] bacct_add_tsk: fix unsafe and wrong parent/group_leader dereference

1. ts = timespec_sub(uptime, current->group_leader->start_time);

   It is possible that current != tsk. Probably it was supposed
   to be 'tsk->group_leader->start_time. But why we are reading
   group_leader's start_time ? This accounting is per thread,
   not per procees, I changed this to 'tsk->start_time.
   Please corect me.

2. stats->ac_ppid = (tsk->parent) ? tsk->parent->pid : 0;

   tsk->parent never == NULL, and it is unsafe to dereference it.
   Both the task and it's parent may exit after the caller unlocks
   tasklist_lock, the memory could be unmapped (DEBUG_SLAB).
   (And we should use ->real_parent->tgid in fact).

Q: I don't understand the 'if (thread_group_leader(tsk))' check.
Why it is needed ?
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Shailabh Nagar <nagar@watson.ibm.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Acked-by: Jay Lan <jlan@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/tsacct.c

@@ -36,7 +36,7 @@ void bacct_add_tsk(struct taskstats *stats, struct task_struct *tsk)
 
 	/* calculate task elapsed time in timespec */
 	do_posix_clock_monotonic_gettime(&uptime);
-	ts = timespec_sub(uptime, current->group_leader->start_time);
+	ts = timespec_sub(uptime, tsk->start_time);
 	/* rebase elapsed time to usec */
 	ac_etime = timespec_to_ns(&ts);
 	do_div(ac_etime, NSEC_PER_USEC);
@@ -58,7 +58,10 @@ void bacct_add_tsk(struct taskstats *stats, struct task_struct *tsk)
 	stats->ac_uid	 = tsk->uid;
 	stats->ac_gid	 = tsk->gid;
 	stats->ac_pid	 = tsk->pid;
-	stats->ac_ppid	 = (tsk->parent) ? tsk->parent->pid : 0;
+	rcu_read_lock();
+	stats->ac_ppid	 = pid_alive(tsk) ?
+				rcu_dereference(tsk->real_parent)->tgid : 0;
+	rcu_read_unlock();
 	stats->ac_utime	 = cputime_to_msecs(tsk->utime) * USEC_PER_MSEC;
 	stats->ac_stime	 = cputime_to_msecs(tsk->stime) * USEC_PER_MSEC;
 	stats->ac_minflt = tsk->min_flt;