linux-kernel - Re: pskb_expand_head: skb

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Date:	Fri, 6 Mar 2015 14:27:44 +1100
From:	Chris Dunlop <chris@...he.net.au>
To:	dev@...nvswitch.org, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Cc:	"Xu (Simon) Chen" <xchenum@...il.com>, Thomas Graf <tgraf@...g.ch>,
	Pravin Shelar <pshelar@...ira.com>, discuss@...nvswitch.org
Subject: Re: pskb_expand_head: skb_shared BUG

On Mon, Mar 02, 2015 at 11:45:11AM +1100, Chris Dunlop wrote:
> Heads up...
> 
> We've hit this BUG() in v3.10.70, v3.14.27 and v3.18.7:
> 
> net/core/skbuff.c:
> 1027 int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
> 1028                      gfp_t gfp_mask)
> 1029 {
> 1030         int i;
> 1031         u8 *data;
> 1032         int size = nhead + skb_end_offset(skb) + ntail;
> 1033         long off;
> 1034 
> 1035         BUG_ON(nhead < 0);
> 1036 
> 1037         if (skb_shared(skb))
> 1038                 BUG();             <<< BOOM!!!
> 
> This appears to be a regression in the 3.10.x stable series:
> we've been running for 11 months on v3.10.33 without problem, we
> upgraded to v3.14.27 and hit the BUG(), than again on upgrading
> to v3.18.7, then again after downgrading to v3.10.70. 

Apologies, this was a false alarm.

There was indeed a regression, but it's in the upstream openvswitch code
rather than linux core. (Further details: a sharing of an
otherwise-unshared skb, causing us to hit the BUG() above, introduced in
v2.3, will be fixed in upcoming v2.3.2)

Cheers,

Chris
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