linux-kernel - Re: Why exported const value modified by another driver not updated in original driver

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Message-ID: <CAJKgH8CNJM2SyR7bD8nXae13-Fq1MP1Xn7+383YU-HPwKBe+Zg@mail.gmail.com>
Date:	Tue, 4 Sep 2012 15:58:20 +0530
From:	Manavendra Nath Manav <mnm.kernel@...il.com>
To:	kernelnewbies@...nelnewbies.org
Cc:	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org,
	Greg KH <gregkh@...uxfoundation.org>
Subject: Re: Why exported const value modified by another driver not updated
 in original driver

On Tue, Sep 4, 2012 at 3:00 PM, Manavendra Nath Manav
<mnm.kernel@...il.com> wrote:
> Hi,
>
> I have declared a static const int variable in one driver and exported
> that variable symbol. In another driver i am modifying that variable.
> The other driver prints the modified value but the original driver
> retains the original value. When both virtual and physical addresses
> of the variable as seen by both drivers are same, how is this even
> possible. Is it a kernel bug?
>
> [root@...alhost bug]# uname -a
> Linux localhost.localdomain 3.5.3 #3 SMP Mon Sep 3 21:52:12 IST 2012
> i686 i686 i386 GNU/Linux
>
> ================================
> [root@...alhost bug]# cat driver.c
> #include <linux/module.h>
>
> static const int value = 123;
>
> int init_module()
> {
>         printk("Base driver (init): value                     = %d\n", value);
>         printk("Base driver (init): virtual address of value  = %p\n", (void *)&value);
>         printk("Base driver (init): physical address of value = %p\n", (void
> *)__pa(&value));
>         return 0;
> }
>
> void cleanup_module()
> {
>         printk("Base driver (exit): value                     = %d\n", value);
>         printk("Base driver (exit): virtual address of value  = %p\n", (void *)&value);
>         printk("Base driver (exit): physical address of value = %p\n", (void
> *)__pa(&value));
> }
> EXPORT_SYMBOL(value);
>
> ==============================================
> [root@...alhost bug]# cat hacker.c
> #include <linux/module.h>
>
> extern int value;
>
> int init_module()
> {
>         value = 987;
>         printk("Hacker driver (init): value                     = %d\n", value);
>         printk("Hacker Base driver (init): virtual address of value  = %p\n",
> (void *)&value);
>         printk("Hacker Base driver (init): physical address of value = %p\n",
> (void *)__pa(&value));
>         return 0;
> }
>
> void cleanup_module()
> {
>         printk("Hacker driver (exit): value                     = %d\n", value);
>         printk("Hacker driver (exit): virtual address of value  = %p\n",
> (void *)&value);
>         printk("Hacker driver (exit): physical address of value = %p\n",
> (void *)__pa(&value));
>         return;
> }
>
> =========================================
> [root@...alhost bug]# insmod driver.ko
> Base driver (init): value                     = 123
> Base driver (init): virtual address of value  = f89d61c8
> Base driver (init): physical address of value = 389d61c8
>
> [root@...alhost bug]# insmod hacker.ko
> Hacker driver (init): value                     = 987
> Hacker Base driver (init): virtual address of value  = f89d61c8
> Hacker Base driver (init): physical address of value = 389d61c8
>
> [root@...alhost bug]# rmmod hacker.ko
> Hacker driver (exit): value                     = 987
> Hacker driver (exit): virtual address of value  = f89d61c8
> Hacker driver (exit): physical address of value = 389d61c8
>
> [root@...alhost bug]# rmmod driver.ko
> Base driver (exit): value                     = 123
> Base driver (exit): virtual address of value  = f89d61c8
> Base driver (exit): physical address of value = 389d61c8
>
> [root@...alhost bug]# cat /proc/kallsyms | grep value
> f89f91c8 R value        [driver]
> f89f9088 r __ksymtab_value      [driver]
> f89f91cc r __kstrtab_value      [driver]
>
> Interestingly, when i change the declaration of variable to "volatile"
> in driver.c as "static const volatile int" then the driver.ko prints
> modified value "987" during rmmod and the original value "123" is
> lost. What is the difference between this and previous declaration?
> Please forgive if you find this question silly?
> --
> Manavendra Nath Manav

Is the above a genuine kernel bug, or i am missing something out here. Pls help.

Thanks,
Manavendra Nath Manav
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