linux-kernel - [PATCH] avoid return NULL on root rb_node in rb_next/rb

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Date:	Mon, 28 Jun 2010 21:17:03 +0800
From:	shenghui <crosslonelyover@...il.com>
To:	kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org,
	Greg KH <greg@...ah.com>, linux-mm@...ck.org, mingo@...e.hu,
	peterz@...radead.org
Subject: [PATCH] avoid return NULL on root rb_node in rb_next/rb_prev in 
	lib/rbtree.c

Hi,

       I'm reading cfs code, and get the following potential bug.

In kernel/sched_fair.c, we can get the following call thread:

1778static struct task_struct *pick_next_task_fair(struct rq *rq)
1779{
...
1787        do {
1788                se = pick_next_entity(cfs_rq);
1789                set_next_entity(cfs_rq, se);
1790                cfs_rq = group_cfs_rq(se);
1791        } while (cfs_rq);
...
1797}

 925static struct sched_entity *pick_next_entity(struct cfs_rq *cfs_rq)
 926{
 927        struct sched_entity *se = __pick_next_entity(cfs_rq);
...
 941        return se;
 942}

 377static struct sched_entity *__pick_next_entity(struct cfs_rq *cfs_rq)
 378{
 379        struct rb_node *left = cfs_rq->rb_leftmost;
 380
 381        if (!left)
 382                return NULL;
 ...
 385}

To manipulate cfs_rq->rb_leftmost, __dequeue_entity does the following:

 365static void __dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
 366{
 367        if (cfs_rq->rb_leftmost == &se->run_node) {
 368                struct rb_node *next_node;
 369
 370                next_node = rb_next(&se->run_node);
 371                cfs_rq->rb_leftmost = next_node;
 372        }
 373
 374        rb_erase(&se->run_node, &cfs_rq->tasks_timeline);
 375}

Here, if se->run_node is the root rb_node, next_node will be set NULL
by rb_next.
Then __pick_next_entity may get NULL on some call, and set_next_entity
may deference
NULL value.

 892static void
 893set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
 894{
 895        /* 'current' is not kept within the tree. */
 896        if (se->on_rq) {
...
 919        se->prev_sum_exec_runtime = se->sum_exec_runtime;
 920}

Following is my patch. Please check it.


>From cb076095b9c93860afb037c50bb70c4a957c9118 Mon Sep 17 00:00:00 2001
From: Wang Sheng-Hui <crosslonelyover@...il.com>
Date: Mon, 28 Jun 2010 19:13:33 +0800
Subject: [PATCH] avoid return NULL on root rb_node in rb_next/rb_prev
in lib/rbtree.c


Signed-off-by: Wang Sheng-Hui <crosslonelyover@...il.com>
---
 lib/rbtree.c |    6 ------
 1 files changed, 0 insertions(+), 6 deletions(-)

diff --git a/lib/rbtree.c b/lib/rbtree.c
index 15e10b1..474e9fd 100644
--- a/lib/rbtree.c
+++ b/lib/rbtree.c
@@ -356,9 +356,6 @@ struct rb_node *rb_next(const struct rb_node *node)
 {
 	struct rb_node *parent;

-	if (rb_parent(node) == node)
-		return NULL;
-
 	/* If we have a right-hand child, go down and then left as far
 	   as we can. */
 	if (node->rb_right) {
@@ -385,9 +382,6 @@ struct rb_node *rb_prev(const struct rb_node *node)
 {
 	struct rb_node *parent;

-	if (rb_parent(node) == node)
-		return NULL;
-
 	/* If we have a left-hand child, go down and then right as far
 	   as we can. */
 	if (node->rb_left) {
-- 
1.6.3.3



-- 


Thanks and Best Regards,
shenghui
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