netdev - Re: Change in alloc_skb() behavior in 3.2+ kernels?

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Date:	Wed, 06 Jun 2012 21:42:22 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	grant.b.edwards@...il.com, netdev@...r.kernel.org
Subject: Re: Change in alloc_skb() behavior in 3.2+ kernels?

On Wed, 2012-06-06 at 11:51 -0700, David Miller wrote:
> From: Grant Edwards <grant.b.edwards@...il.com>
> Date: Wed, 6 Jun 2012 18:32:57 +0000 (UTC)
> 
> > The kernel module that's started failing fills the allocated sk_buff
> > until tailroom() indicates it is full and then sends it.  The problem
> > is that sending a packet with a length of 1850 won't work (it's a
> > MAC-layer Ethernet packet).
> 
> The amount of tailroom an SKB has is implementation dependent.
> 
> It's incredibly poor form to rely upon it to determine whether a fully
> sized frame has been constructed or not.
> 
> Please fix the code that does this.


By the way, we had a similar problem, and the fix was :

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=a21d45726acacc963d8baddf74607d9b74e2b723

Grant, depending on the context, you might use skb->avail_size and
skb_availroom() as well.

Beware skb->avail_size is unioned with skb->{mark|dropcount}



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