[PATCH 1/1] oom: fix the unsafe usage of badness() in proc_oom_score()

[Oleg Nesterov <oleg@redhat.com>]

proc_oom_score(task) have a reference to task_struct, but that is all.
If this task was already released before we take tasklist_lock

	- we can't use task->group_leader, it points to nowhere

	- it is not safe to call badness() even if this task is
	  ->group_leader, has_intersects_mems_allowed() assumes
	  it is safe to iterate over ->thread_group list.

	- even worse, badness() can hit ->signal == NULL

Add the pid_alive() check to ensure __unhash_process() was not called.

Also, use "task" instead of task->group_leader. badness() should return
the same result for any sub-thread. Currently this is not true, but
this should be changed anyway.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---

 fs/proc/base.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- TTT/fs/proc/base.c~PROC_OOM_SCORE	2010-03-11 13:11:50.000000000 +0100
+++ TTT/fs/proc/base.c	2010-04-01 14:41:17.000000000 +0200
@@ -442,12 +442,13 @@ static const struct file_operations proc
 unsigned long badness(struct task_struct *p, unsigned long uptime);
 static int proc_oom_score(struct task_struct *task, char *buffer)
 {
-	unsigned long points;
+	unsigned long points = 0;
 	struct timespec uptime;
 
 	do_posix_clock_monotonic_gettime(&uptime);
 	read_lock(&tasklist_lock);
-	points = badness(task->group_leader, uptime.tv_sec);
+	if (pid_alive(task))
+		points = badness(task, uptime.tv_sec);
 	read_unlock(&tasklist_lock);
 	return sprintf(buffer, "%lu\n", points);
 }

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