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

[Dongseok Yi]

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.

> 
> We copy only the MAC header in skb_segment_list(), so I think
> this is a valid bug when NAT changed the UDP header.
> 
> >
> > Update dport, daddr and checksums of each skb of the segment list
> > after __udp_gso_segment.
> >
> > Fixes: 9fd1ff5d2ac7 (udp: Support UDP fraglist GRO/GSO.)
> > Signed-off-by: Dongseok Yi <dseok.yi@xxxxxxxxxxx>
> > ---
> > Steffen Klassert said, there could be 2 options.
> > https://lore.kernel.org/patchwork/patch/1362257/
> >
> > I was trying to write a quick fix, but it was not easy to forward
> > segmented list. Currently, assuming DNAT only. Should we consider
> > SNAT too?
> 
> If it is broken, then it is broken for both, so yes.

Okay.

> 
> >
> >  net/ipv4/udp_offload.c | 45 +++++++++++++++++++++++++++++++++++++++++----
> >  1 file changed, 41 insertions(+), 4 deletions(-)
> >
> > diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> > index ff39e94..7e24928 100644
> > --- a/net/ipv4/udp_offload.c
> > +++ b/net/ipv4/udp_offload.c
> > @@ -309,10 +309,12 @@ static struct sk_buff *udp4_ufo_fragment(struct sk_buff *skb,
> >  					 netdev_features_t features)
> >  {
> >  	struct sk_buff *segs = ERR_PTR(-EINVAL);
> > +	struct sk_buff *seg;
> >  	unsigned int mss;
> >  	__wsum csum;
> > -	struct udphdr *uh;
> > -	struct iphdr *iph;
> > +	struct udphdr *uh, *uh2;
> > +	struct iphdr *iph, *iph2;
> > +	bool is_fraglist = false;
> >
> >  	if (skb->encapsulation &&
> >  	    (skb_shinfo(skb)->gso_type &
> > @@ -327,8 +329,43 @@ static struct sk_buff *udp4_ufo_fragment(struct sk_buff *skb,
> >  	if (!pskb_may_pull(skb, sizeof(struct udphdr)))
> >  		goto out;
> >
> > -	if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4)
> > -		return __udp_gso_segment(skb, features);
> > +	if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4) {
> > +		if (skb_shinfo(skb)->gso_type & SKB_GSO_FRAGLIST)
> > +			is_fraglist = true;
> > +
> > +		segs = __udp_gso_segment(skb, features);
> > +		if (IS_ERR_OR_NULL(segs) || !is_fraglist)
> > +			return segs;
> > +
> > +		seg = segs;
> > +		uh = udp_hdr(seg);
> > +		iph = ip_hdr(seg);
> > +
> > +		while ((seg = seg->next)) {
> > +			uh2 = udp_hdr(seg);
> > +			iph2 = ip_hdr(seg);
> > +
> > +			if (uh->dest == uh2->dest && iph->daddr == iph2->daddr)
> > +				continue;
> > +
> > +			if (uh2->check) {
> > +				inet_proto_csum_replace4(&uh2->check, seg,
> > +							 iph2->daddr,
> > +							 iph->daddr, true);
> > +				inet_proto_csum_replace2(&uh2->check, seg,
> > +							 uh2->dest, uh->dest,
> > +							 false);
> > +				if (!uh2->check)
> > +					uh2->check = CSUM_MANGLED_0;
> > +			}
> > +			uh2->dest = uh->dest;
> > +
> > +			csum_replace4(&iph2->check, iph2->daddr, iph->daddr);
> > +			iph2->daddr = iph->daddr;
> > +		}
> > +
> > +		return segs;
> > +	}
> 
> 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?

> 
> 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?