linux-kernel - [PATCH] sched: Fix potential call to __ffs(0) in sched_show

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Message-Id: <201412052131.GCE35924.FVHFOtLOJOMQFS@I-love.SAKURA.ne.jp>
Date:	Fri, 5 Dec 2014 21:31:14 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	mingo@...hat.com, peterz@...radead.org
Cc:	linux-kernel@...r.kernel.org
Subject: [PATCH] sched: Fix potential call to __ffs(0) in sched_show_task()

>>From 8579fc072fda603c4ae472cc3f0390d61556ac01 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Date: Fri, 5 Dec 2014 21:22:22 +0900
Subject: [PATCH] sched: Fix potential call to __ffs(0) in sched_show_task()

"struct task_struct"->state is "volatile long" and __ffs() warns that
"Undefined if no bit exists, so code should check against 0 first."

Therefore, at expression

  state = p->state ? __ffs(p->state) + 1 : 0;

in sched_show_task(), CPU might see "p->state" before "?" as "non-zero"
but "p->state" after "?" as "zero", which could result in
"state >= sizeof(stat_nam)" being true and bogus '?' is printed.

This patch changes "state" from "unsigned int" to "unsigned long" and
save "p->state" before calling __ffs(), in order to avoid potential call
to __ffs(0).

Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
---
 kernel/sched/core.c |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 89e7283..8cb5f9b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4528,9 +4528,10 @@ void sched_show_task(struct task_struct *p)
 {
 	unsigned long free = 0;
 	int ppid;
-	unsigned state;
+	unsigned long state = p->state;
 
-	state = p->state ? __ffs(p->state) + 1 : 0;
+	if (state)
+		state = __ffs(state) + 1;
 	printk(KERN_INFO "%-15.15s %c", p->comm,
 		state < sizeof(stat_nam) - 1 ? stat_nam[state] : '?');
 #if BITS_PER_LONG == 32
-- 
1.7.1
--
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