Re: [PATCH net] udp: ipv4: manipulate network header of NATed UDP GRO fraglist

[Steffen Klassert]

On Fri, Jan 15, 2021 at 05:55:22PM +0900, Dongseok Yi wrote:
> On 2021-01-15 17:12, Steffen Klassert wrote:
> > On Fri, Jan 15, 2021 at 02:58:24PM +0900, Dongseok Yi wrote:
> > > UDP/IP header of UDP GROed frag_skbs are not updated even after NAT
> > > forwarding. Only the header of head_skb from ip_finish_output_gso ->
> > > skb_gso_segment is updated but following frag_skbs are not updated.
> > >
> > > A call path skb_mac_gso_segment -> inet_gso_segment ->
> > > udp4_ufo_fragment -> __udp_gso_segment -> __udp_gso_segment_list
> > > does not try to update UDP/IP header of the segment list.
> > 
> > We still need to find out why it works for Alexander, but not for you.
> > Different usecases?
> 
> This patch is not for
> https://lore.kernel.org/patchwork/patch/1364544/
> Alexander might want to call udp_gro_receive_segment even when
> !sk and ~NETIF_F_GRO_FRAGLIST.

Yes, I know. But he said that fraglist GRO + NAT works for him.
I want to find out why it works for him, but not for you.

> > 
> > I would not like to add this to a generic codepath. I think we can
> > relatively easy copy the full headers in skb_segment_list().
> 
> I tried to copy the full headers with the similar approach, but it
> copies length too. Can we keep the length of each skb of the fraglist?

Ah yes, good point.

Then maybe you can move your approach into __udp_gso_segment_list()
so that we dont touch generic code.

> 
> > 
> > I think about something like the (completely untested) patch below:
> > 
> > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > index f62cae3f75d8..63ae7f79fad7 100644
> > --- a/net/core/skbuff.c
> > +++ b/net/core/skbuff.c
> > @@ -3651,13 +3651,14 @@ struct sk_buff *skb_segment_list(struct sk_buff *skb,
> >  				 unsigned int offset)
> >  {
> >  	struct sk_buff *list_skb = skb_shinfo(skb)->frag_list;
> > +	unsigned int doffset = skb->data - skb_mac_header(skb);
> >  	unsigned int tnl_hlen = skb_tnl_header_len(skb);
> >  	unsigned int delta_truesize = 0;
> >  	unsigned int delta_len = 0;
> >  	struct sk_buff *tail = NULL;
> >  	struct sk_buff *nskb;
> > 
> > -	skb_push(skb, -skb_network_offset(skb) + offset);
> > +	skb_push(skb, doffset);
> > 
> >  	skb_shinfo(skb)->frag_list = NULL;
> > 
> > @@ -3675,7 +3676,7 @@ struct sk_buff *skb_segment_list(struct sk_buff *skb,
> >  		delta_len += nskb->len;
> >  		delta_truesize += nskb->truesize;
> > 
> > -		skb_push(nskb, -skb_network_offset(nskb) + offset);
> > +		skb_push(nskb, doffset);
> > 
> >  		skb_release_head_state(nskb);
> >  		 __copy_skb_header(nskb, skb);
> > diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> > index ff39e94781bf..1181398378b8 100644
> > --- a/net/ipv4/udp_offload.c
> > +++ b/net/ipv4/udp_offload.c
> > @@ -190,9 +190,22 @@ EXPORT_SYMBOL(skb_udp_tunnel_segment);
> >  static struct sk_buff *__udp_gso_segment_list(struct sk_buff *skb,
> >  					      netdev_features_t features)
> >  {
> > +	struct sk_buff *list_skb = skb_shinfo(skb)->frag_list;
> >  	unsigned int mss = skb_shinfo(skb)->gso_size;
> > +	unsigned int offset;
> > 
> > -	skb = skb_segment_list(skb, features, skb_mac_header_len(skb));
> > +	skb_headers_offset_update(list_skb, skb_headroom(list_skb) - skb_headroom(skb));
> > +
> > +	/* Check for header changes and copy the full header in that case. */
> > +	if ((udp_hdr(skb)->dest == udp_hdr(list_skb)->dest) &&
> > +	    (udp_hdr(skb)->source == udp_hdr(list_skb)->source) &&
> > +	    (ip_hdr(skb)->daddr == ip_hdr(list_skb)->daddr) &&
> > +	    (ip_hdr(skb)->saddr == ip_hdr(list_skb)->saddr))
> > +		offset = skb_mac_header_len(skb);
> > +	else
> > +		offset = skb->data - skb_mac_header(skb);
> > +
> > +	skb = skb_segment_list(skb, features, offset);
> >  	if (IS_ERR(skb))
> >  		return skb;
> > 
> > 
> > After that you can apply the CSUM magic in __udp_gso_segment_list().
> 
> Sorry, I don't know CSUM magic well. Is it used for checksum
> incremental update too?

With that I meant the checksum updating you did in your patch.