Re: [PATCH v3 net] net: udp: fix Fast/frag0 UDP GRO

While testing UDP GSO fraglists forwarding through driver that uses
Fast GRO (via napi_gro_frags()), I was observing lots of out-of-order
iperf packets:

[ ID] Interval           Transfer     Bitrate         Jitter
[SUM]  0.0-40.0 sec  12106 datagrams received out-of-order

Simple switch to napi_gro_receive() any other method without frag0
shortcut completely resolved them.

I've found that UDP GRO uses udp_hdr(skb) in its .gro_receive()
callback. While it's probably OK for non-frag0 paths (when all
headers or even the entire frame are already in skb->data), this
inline points to junk when using Fast GRO (napi_gro_frags() or
napi_gro_receive() with only Ethernet header in skb->data and all
the rest in shinfo->frags) and breaks GRO packet compilation and
the packet flow itself.
To support both modes, skb_gro_header_fast() + skb_gro_header_slow()
are typically used. UDP even has an inline helper that makes use of
them, udp_gro_udphdr(). Use that instead of troublemaking udp_hdr()
to get rid of the out-of-order delivers.

Present since the introduction of plain UDP GRO in 5.0-rc1.

Since v2 [1]:
 - dropped redundant check introduced in v2 as it's performed right
   before (thanks to Eric);
 - udp_hdr() switched to data + off for skbs from list (also Eric);
 - fixed possible malfunction of {,__}udp{4,6}_lib_lookup_skb() with
   Fast/frag0 due to ip{,v6}_hdr() usage (Willem).

Since v1 [2]:
 - added a NULL pointer check for "uh" as suggested by Willem.

[1] https://lore.kernel.org/netdev/0eaG8xtbtKY1dEKCTKUBubGiC9QawGgB3tVZtNqVdY@cp4-web-030.plabs.ch (local)
[2] https://lore.kernel.org/netdev/YazU6GEzBdpyZMDMwJirxDX7B4sualpDG68ADZYvJI@cp4-web-034.plabs.ch (local)

Fixes: e20cf8d3f1f7 ("udp: implement GRO for plain UDP sockets.")
Signed-off-by: Alexander Lobakin <redacted>
---
 net/ipv4/udp.c         | 4 ++--
 net/ipv4/udp_offload.c | 9 ++++++---
 net/ipv6/udp.c         | 4 ++--
 3 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 09f0a23d1a01..948ddc9a0212 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c

@@ -534,7 +534,7 @@ static inline struct sock *__udp4_lib_lookup_skb(struct sk_buff *skb,
                                                 __be16 sport, __be16 dport,
                                                 struct udp_table *udptable)
 {
-       const struct iphdr *iph = ip_hdr(skb);
+       const struct iphdr *iph = skb_gro_network_header(skb);

        return __udp4_lib_lookup(dev_net(skb->dev), iph->saddr, sport,
                                 iph->daddr, dport, inet_iif(skb),

@@ -544,7 +544,7 @@ static inline struct sock *__udp4_lib_lookup_skb(struct sk_buff *skb,
 struct sock *udp4_lib_lookup_skb(struct sk_buff *skb,
                                 __be16 sport, __be16 dport)
 {
-       const struct iphdr *iph = ip_hdr(skb);
+       const struct iphdr *iph = skb_gro_network_header(skb);