netdev - Re: Bug in computing data_len in tcp

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Message-Id: <20111202.134040.638198723589712419.davem@davemloft.net>
Date:	Fri, 02 Dec 2011 13:40:40 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	subramanian.vijay@...il.com, therbert@...gle.com,
	netdev@...r.kernel.org
Subject: Re: Bug in computing data_len in tcp_sendmsg?

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Fri, 02 Dec 2011 19:36:29 +0100

> What I ask now is following problem (even prior to my frag allocation
> patch) :
> 
> 1) We allocate a linear skb (SG being off) to cook a tcp frame of length
> XXX bytes.
> 
> 2) We send it.
> 
> 3) We receive an ACK for first 31 bytes.
> 
> 4) We trim 31 bytes from the head of skb.  (skb_pull(skb, 31))
>     skb->data is now not anymore aligned to a 4 bytes boundary.
> 
> 5) Later, we need to retransmit skb (XXX minus 31 bytes already ACKed)
>    We push TCP header, and skb->data is not aligned.
>    TCP header is not aligned anymore. x86 doesnt care, but what about
> other arches with misalign traps ?

Yes, for non-SG this always was technically possible.

But now you've made it possible for SG too.

Frankly, I think we should:

1) Revert your patch, so it's not possible for SG once more.

2) Add code to reallocate the SKB linear data when this happens in
   the non-SG case.
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