linux-kernel - Re: kernel oops

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Date:	Wed, 23 Jul 2008 19:11:44 +0200
From:	Vegard Nossum <vegard.nossum@...il.com>
To:	Andrei Popa <andrei.popa@...eo.ro>,
	Oliver Neukum <oneukum@...e.de>,
	Greg Kroah-Hartman <gregkh@...e.de>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: kernel oops

Hi,

On Wed, Jul 23, 2008 at 2:52 PM, Andrei Popa <andrei.popa@...eo.ro> wrote:
>
> Hello,
>
> I installed gnokii-0.6.22-r2 and gave the command "gnokii --identify"
> and the kernel oopsed:
>
> BUG: unable to handle kernel NULL pointer dereference at 00000458
> IP: [<c0444b52>] mutex_unlock+0x0/0xb
> *pde = 00000000
> Oops: 0002 [#1] PREEMPT SMP
>
> Pid: 19043, comm: gnokii Not tainted (2.6.26-ineo7 #2)
> EIP: 0060:[<c0444b52>] EFLAGS: 00010246 CPU: 0
> EIP is at mutex_unlock+0x0/0xb
...
>  [<c03830ae>] acm_tty_open+0x4c/0x214

This shouldn't be too hard; the code is trying to unlock the mutex
&acm->mutex even when "acm" is NULL. It seems that the label "err_out"
is otherwise unused, so it makes sense to move this one step further
down, so that it doesn't try to unlock the non-existant mutex.

If the problem is reproducible, you could try the patch below!


Vegard

PS: I actually think the code has some other problems too. Shouldn't
&acm->mutex be locked before we even inspect acm->dev?


Reported-by: Andrei Popa <andrei.popa@...eo.ro>
Signed-off-by: Vegard Nossum <vegard.nossum@...il.com>

diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index 63c3404..74d03a7 100644
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -525,8 +525,8 @@ static int acm_tty_open(struct tty_struct *tty, struct file *filp)
 	tasklet_schedule(&acm->urb_task);
 
 done:
-err_out:
 	mutex_unlock(&acm->mutex);
+err_out:
 	mutex_unlock(&open_mutex);
 	return rv;
 
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