netdev - kernel newbie question on kernel panic

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Message-ID: <4474723F928CA640ADB2172099AEAB111B603F@exalfa.stromtelecom.cz>
Date:	Mon, 21 Jul 2008 17:24:50 +0200
From:	"Filka Michal" <Michal.Filka@...ronicsts.com>
To:	<netdev@...r.kernel.org>
Subject: kernel newbie question on kernel panic

Hi all,

I try to create my first kernel module.
I have following problem. I have a function rp ... interesting part
follows (just some initialization is ommited - nothing special - the
function is just a skeleton)

int rp( struct sk_buff ** skb)
{
	_filter_rule * rule = NULL;

...

	rule = placebo();
-->	if( rule == NULL)
		printk( KERN_DEBUG "null rule\n");
	return 1;
}

The rp is called (subsequently) in netfilter (prerouting) hook context,
so as far as I know it runs in a SoftIRQ.

The placebo function does nothing - just returns NULL. The problem is in
line selected (-->).Everything goes until I try to somehow access
returned value. The access raises kernel panic, seems that some page
fault happent. If I comment out the line bellow selected (the test), no
kernel panic happens.

Questions are:
- why the page fault happens (some trouble in memory usage, or so?)
- how to avoid it?

Thanks for any help
Michal Filka
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