linux-kernel - inconsistency in kernel/time.c with jiffies

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Date:	Wed, 11 Jul 2012 11:24:53 +0200
From:	Andreas Herz <andi@...kosphere.org>
To:	linux-kernel@...r.kernel.org
Subject: inconsistency in kernel/time.c with jiffies

I wrote this testcase module to prove a bug in kernel/time.c.
The problem i found is in the msecs_to_jiffies() function. In my
testcase the bug occurs with kernel 3.2 on a 32-Bit system.
The problem is this first part of the function:

> if ((int)m < 0)
>   return MAX_JIFFY_OFFSET;

When the function has const unsigned int as parameter so it's fine to
call it with a unsigned int value, for example 2147483647 for 32-Bit as
the maximum value the return value is 2147483647 on a system with
HZ=1000. But when the value is increased by 1 to 2147483648 the return
value is 1073741822 (MAX_JIFFY_OFFSET). This means, a even higher value
results in a smaller return value.

But time.c also says:

* - 'too large' values [that would result in larger than
* MAX_JIFFY_OFFSET values] mean 'infinite timeout' too.

If you read this it would mean that even 2147483647 should result in
1073741822 (MAX_JIFFY_OFFSET) as it's larger then MAX_JIFFY_OFFSET.

So there are several suggestions i would discuss:

1. Change MAX_JIFFY_OFFSET ((ULONG_MAX >> 1)-1 as i guess unsigned makes
more sense, because jiffies are never < 0
2. Change the if((int)m < 0) to return MAX_JIFFY_OFFSET even with values
that are valid but higher then MAX_JIFFY_OFFSET
> if (((int)m < 0) || (m > MAX_JIFFY_OFFSET))
3. Change the parameter to signed int.

This issue came up when i worked with ipset and higher values resulted
in smaller timeouts.

Any feedback is appreciated.

Here the testcase source code:

#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/jiffies.h>
#include <linux/time.h>

static int jiffiestestcase_init(void)
{
        printk(KERN_ALERT "Last working msecs_to_jiffies value with 2147483647 %ld\n", msecs_to_jiffies(2147483647));
        printk(KERN_ALERT "First wrong msecs_to_jiffies value with 2147483648 %ld\n", msecs_to_jiffies(2147483648));
        printk(KERN_ALERT "int casts %d and %d\n", (int)2147483647, (int)2147483648);
        printk(KERN_ALERT "MAX JIFFIES: %ld\n", MAX_JIFFY_OFFSET);
        return 0;
}

static void jiffiestestcase_exit(void)
{
        printk(KERN_ALERT "ending jiffiestestcase!\n");
}

module_init(jiffiestestcase_init);
module_exit(jiffiestestcase_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Andreas Herz <andi@...kosphere.org>");
MODULE_DESCRIPTION("Jiffies Testcase Module");

-- 
Andreas Herz
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