netdev - Re: skb_segment() questions

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Message-ID: <20090329020701.GA9983@gondor.apana.org.au>
Date:	Sun, 29 Mar 2009 10:07:01 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	James Huang <jamesclhuang@...il.com>,
	"David S. Miller" <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: skb_segment() questions

On Wed, Mar 04, 2009 at 07:12:40PM -0800, James Huang wrote:
>
> (2) What is the purpose of the this check?
> 
> `    if (pos >= offset + len)
>         continue;
> 
>      If the payload in the head buffer of skb has at least mss bytes, this
> check will succeed and no payload in skb’s head buffer will be copy into
> nskb
> through a call to skb_copy_from_linear_data_offset(). Something seems to be
> wrong here.

Indeed.  This breaks linear packets, which unfortunately older
versions of tun likes to construct.

gso: Fix support for linear packets

When GRO/frag_list support was added to GSO, I made an error
which broke the support for segmenting linear GSO packets (GSO
packets are normally non-linear in the payload).

These days most of these packets are constructed by the tun
driver, which prefers to allocate linear memory if possible.
This is fixed in the latest kernel, but for 2.6.29 and earlier
it is still the norm.

Therefore this bug causes failures with GSO when used with tun
in 2.6.29.

Reported-by: James Huang <jamesclhuang@...il.com>
Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 6acbf9e..ce6356c 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -2579,7 +2579,7 @@ struct sk_buff *skb_segment(struct sk_buff *skb, int features)
 					  skb_network_header_len(skb));
 		skb_copy_from_linear_data(skb, nskb->data, doffset);
 
-		if (pos >= offset + len)
+		if (fskb != skb_shinfo(skb)->frag_list)
 			continue;
 
 		if (!sg) {

Thanks,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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