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Subject[RFC PATCH net] udp: check sk for UDP GRO fraglist
It is a workaround patch.

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 any UDP/IP header of the segment list.

It might make sense because each skb of frag_skbs is converted to a
list of regular packets. Header update with checksum calculation may
be not needed for UDP GROed frag_skbs.

But UDP GRO frag_list is started from udp_gro_receive, we don't know
whether the skb will be NAT forwarded at that time. For workaround,
try to get sock always when call udp4_gro_receive -> udp_gro_receive
to check if the skb is for local.

I'm still not sure if UDP GRO frag_list is really designed for local
session only. Can kernel support NAT forward for UDP GRO frag_list?
What am I missing?

Fixes: 9fd1ff5d2ac7 (udp: Support UDP fraglist GRO/GSO.)
Signed-off-by: Dongseok Yi <>
net/ipv4/udp_offload.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index ff39e94..d476216 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c
@@ -457,7 +457,7 @@ struct sk_buff *udp_gro_receive(struct list_head *head, struct sk_buff *skb,
int flush = 1;

NAPI_GRO_CB(skb)->is_flist = 0;
- if (skb->dev->features & NETIF_F_GRO_FRAGLIST)
+ if (sk && (skb->dev->features & NETIF_F_GRO_FRAGLIST))
NAPI_GRO_CB(skb)->is_flist = sk ? !udp_sk(sk)->gro_enabled: 1;

if ((sk && udp_sk(sk)->gro_enabled) || NAPI_GRO_CB(skb)->is_flist) {
@@ -537,8 +537,7 @@ struct sk_buff *udp4_gro_receive(struct list_head *head, struct sk_buff *skb)
NAPI_GRO_CB(skb)->is_ipv6 = 0;

- if (static_branch_unlikely(&udp_encap_needed_key))
- sk = udp4_gro_lookup_skb(skb, uh->source, uh->dest);
+ sk = udp4_gro_lookup_skb(skb, uh->source, uh->dest);

pp = udp_gro_receive(head, skb, uh, sk);
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