On Fri, Jan 08, 2016 at 12:13:49PM +0000, David Laight wrote:
> From: Of Konstantin Khlebnikov
> > Sent: 08 January 2016 12:01
> > Skb_gso_segment() uses skb control block during segmentation.
> > This patch adds 32-bytes room for previous control block which
> > will be copied into all resulting segments.
> >
> > This patch fixes kernel crash during fragmenting forwarded packets.
> > Fragmentation requires valid IP CB in skb for clearing ip options.
> > Also patch removes custom save/restore in ovs code, now it's redundant.
> >
> ...
> > diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> > index 4355129fff91..9147f9f34cbe 100644
> > --- a/include/linux/skbuff.h
> > +++ b/include/linux/skbuff.h
> > @@ -3446,7 +3446,8 @@ struct skb_gso_cb {
> > int encap_level;
> > __u16 csum_start;
> > };
> > -#define SKB_GSO_CB(skb) ((struct skb_gso_cb *)(skb)->cb)
> > +#define SKB_SGO_CB_OFFSET 32
> > +#define SKB_GSO_CB(skb) ((struct skb_gso_cb *)((skb)->cb +
> > SKB_SGO_CB_OFFSET))
>
> You could set SKB_SGO_CB_OFFSET to sizeof ((skb)->cb) - sizeof (struct
> skb_gso_cb)
> so that the end of 'cb' is always used.
> (Assuming the former is a multiple of 4.)
>
> It might be worth using an on-stack structure passed through as a separate
> parameter - it doesn't look as though it has to be queued with the skb.
> (Clearly a bigger change.)
>
I considered that as an option. But the bigger change and the use of the extra
stack for all users, plus the extra parameters indicated I should go the other
way.
In my opinion, at least in the IP fragmentation case, saving/restoring cb is not
such a big problem since we are in slow path already.
Cascardo.
> David
>
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