netdev - Re: [PATCH net-next v2 2/6] 6lowpan: fix fragmentation on sending side

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Message-Id: <20140224.185213.1069306984692624378.davem@davemloft.net>
Date:	Mon, 24 Feb 2014 18:52:13 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	alex.aring@...il.com
Cc:	alex.bluesman.smirnov@...il.com, dbaryshkov@...il.com,
	linux-zigbee-devel@...ts.sourceforge.net, netdev@...r.kernel.org,
	martin.townsend@...lon.com
Subject: Re: [PATCH net-next v2 2/6] 6lowpan: fix fragmentation on sending
 side

From: Alexander Aring <alex.aring@...il.com>
Date: Fri, 21 Feb 2014 19:58:58 +0100

> +	err = lowpan_fragment_xmit(skb, head, header_length,
> +				   frag_plen + lowpan_size, 0,
> +				   LOWPAN_DISPATCH_FRAG1);
>  	if (err) {
>  		pr_debug("%s unable to send FRAG1 packet (tag: %d)",
>  			 __func__, tag);
>  		goto exit;
>  	}
>  
> -	offset = LOWPAN_FRAG_SIZE;
> +	offset = lowpan_size + frag_plen;
> +	dgram_offset = mac_cb(skb)->frag_info.d_offset + frag_plen;

This isn't going to work.

Once you call dev_queue_xmit(), the skb control block can be reused by
other layers.  In particular the device qdisc layer is going to
scramble the control block on you.

Once a packet travels from one layer to another, SKB control block
is likewise transferred.
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