linux-kernel - RE: [Question]Why a skb with frag_list causes BUG_ON in function skb

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Date:	Thu, 30 Jul 2015 05:56:40 +0000
From:	"Zhangjie (HZ)" <zhangjie14@...wei.com>
To:	Eric Dumazet <eric.dumazet@...il.com>,
	Jason Wang <jasowang@...hat.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mst@...hat.com" <mst@...hat.com>,
	Qinchuanyu <qinchuanyu@...wei.com>, Yewudi <yewudi@...wei.com>,
	liuyongan 00175866 <l00175866@...esmail.huawei.com.cn>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [Question]Why a skb with frag_list causes BUG_ON in function
 skb_segment

Get it.
Eric, thanks for your explaination. :-)

Jason, thank you! I will remember cc netdev@...r.kernel.org next time.

-----Original Message-----
From: Eric Dumazet [mailto:eric.dumazet@...il.com] 
Sent: Thursday, July 30, 2015 1:01 PM
To: Jason Wang
Cc: Zhangjie (HZ); linux-kernel@...r.kernel.org; mst@...hat.com; Qinchuanyu; Yewudi; liuyongan 00175866; netdev@...r.kernel.org
Subject: Re: [Question]Why a skb with frag_list causes BUG_ON in function skb_segment

On Thu, 2015-07-30 at 12:26 +0800, Jason Wang wrote:
> cc netdev for more experts
> 
> On 07/28/2015 04:53 PM, Zhangjie (HZ) wrote:
> >
> > Hi,
> >
> > I generate a skb as follows:
> >
> > It has a linear data region, 17 frags and the last fragment is in 
> > skb_shinfo(skb)->frag_list.
> >
> > Before this skb is sent to driver, dev_hard_start_xmit() will 
> > segment it first(skb has frag_list,
> >
> > so we get true from netif_needs_gso()), then the skb is passed to 
> > function skb_segment().
> >
> > Then, BUG_ON() happened.
> >
> > while (pos < offset + len) {
> >
> >         if (i >= nfrags) {
> >
> >                 BUG_ON(skb_headlen(list_skb));    (skbuff.c:3120)
> >
> >                 …
> >
> >         }
> >
> >         …
> >
> > }
> >
> > A skb that has no frags but frag_list also causes BUG_ON().
> >
> > I wonder if skb like follows is legal? Could skb in frag_list have 
> > linear data region?

The answer is : skb_segment() is very complex but does not handle all possible cases.

skb found in skb_shinfo(skb)->frag_list must not have anything in their
skb->head. This would require very expensive logic and memory
allocations and copies.

Make sure you follow this rule in your driver, or even better leave this work to GRO engine.