netdev - Re: [PATCH net-next] net: Check frag_lists first to prevent data out of order

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Message-ID: <1440642184.8932.37.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 26 Aug 2015 19:23:04 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	chenweilong@...wei.com
Cc:	davem@...emloft.net, edumazet@...gle.com,
	alexander.h.duyck@...hat.com, willemb@...gle.com,
	hannes@...essinduktion.org, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: Check frag_lists first to prevent data
 out of order

On Wed, 2015-08-26 at 19:12 -0700, Eric Dumazet wrote:
> On Thu, 2015-08-27 at 08:56 +0800, chenweilong@...wei.com wrote:
> > From: Weilong Chen <chenweilong@...wei.com>
> > 
> > When try to merge several skbs to prior one, if the frag_list is
> > used and the the last one is a small packet, once the condition
> > "len <= skb_tailroom(to)" is satisfied, we will get a wrong
> > packet!
> > This patch just check frag_lists before the condtion to prevent
> > this from happening.
> > 
> > Signed-off-by: Weilong Chen <chenweilong@...wei.com>
> > ---
> >  net/core/skbuff.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > index 8a725cc..d08edcb 100644
> > --- a/net/core/skbuff.c
> > +++ b/net/core/skbuff.c
> > @@ -4133,6 +4133,9 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
> >  	if (skb_cloned(to))
> >  		return false;
> >  
> > +	if (skb_has_frag_list(to) || skb_has_frag_list(from))
> > +		return false;
> > +
> >  	if (len <= skb_tailroom(to)) {
> >  		if (len)
> >  			BUG_ON(skb_copy_bits(from, 0, skb_put(to, len), len));
> > @@ -4140,9 +4143,6 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
> >  		return true;
> >  	}
> >  
> > -	if (skb_has_frag_list(to) || skb_has_frag_list(from))
> > -		return false;
> > -
> >  	if (skb_headlen(from) != 0) {
> >  		struct page *page;
> >  		unsigned int offset;
> 
> Sigh.
> 
> No idea what problem you tried to solve.
> 
> This patch is not needed.
> 
> If (len <= skb_tailroom()), then it is obviously correct to copy_bits()
> the bytes.
> 
> Hints :
> 
> - If @to has a fraglist, then skb_tailroom(to) is 0 so the copy can not
> be done.
> 
> - If @from has a fraglist, it is not relevant as we copy it into @to and
> will free @from.

This is going to be a FAQ 

http://www.spinics.net/lists/netdev/msg315090.html




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