netdev - Re: [PATCH 10/10] tcp: handle shift/merge of cloned skbs too

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Date:	Mon, 24 Nov 2008 21:32:02 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	ilpo.jarvinen@...sinki.fi
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH 10/10] tcp: handle shift/merge of cloned skbs too

From: "Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
Date: Mon, 24 Nov 2008 16:22:07 +0200

> This caused me to get repeatably:
> 
>   tcpdump: pcap_loop: recvfrom: Bad address
> 
> Happens occassionally when I tcpdump my for-looped test xfers:
>   while [ : ]; do echo -n "$(date '+%s.%N') "; ./sendfile; sleep 20; done
> 
> Rest of the relevant commands:
>   ethtool -K eth0 tso off
>   tc qdisc add dev eth0 root netem drop 4%
>   tcpdump -n -s0 -i eth0 -w sacklog.all

I'm applying this in any event.

What could be happening is some bad clone handling
elsewhere (AF_PACKET, for example) and thus for some
reason libpcap reads stale data then performs a packet
read using incorrect lengths and this leads to reading
past the end of it's user buffer and we -EFAULT.

Just capturing the -EFAULT'ing call arguments with strace
would be enough to give some deeper clues.  Do you have
that?
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