netdev - Re: [PATCH] ipv4: ip_check_defrag must not modify skb before unsharing

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Date:	Mon, 10 Dec 2012 13:41:46 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	johannes@...solutions.net
Cc:	eric@...it.org, netdev@...r.kernel.org,
	linux-wireless@...r.kernel.org, linville@...driver.com,
	eric.dumazet@...il.com
Subject: Re: [PATCH] ipv4: ip_check_defrag must not modify skb before
 unsharing

From: Johannes Berg <johannes@...solutions.net>
Date: Mon, 10 Dec 2012 10:41:06 +0100

> From: Johannes Berg <johannes.berg@...el.com>
> 
> ip_check_defrag() might be called from af_packet within the
> RX path where shared SKBs are used, so it must not modify
> the input SKB before it has unshared it for defragmentation.
> Use skb_copy_bits() to get the IP header and only pull in
> everything later.
> 
> The same is true for the other caller in macvlan as it is
> called from dev->rx_handler which can also get a shared SKB.
> 
> Reported-by: Eric Leblond <eric@...it.org>
> Cc: stable@...r.kernel.org
> Signed-off-by: Johannes Berg <johannes.berg@...el.com>
> ---
> For some versions of the kernel, this code goes into af_packet.c

So the bug is that ip_check_defrag() has a precondition which is met
properly by all callers except AF_PACKET.

If this is the case, remind me why are we changing ip_check_defrag()
rather than the violator of the precondition?
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