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Subject[PATCH 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.

Fixes: e20cf8d3f1f7 ("udp: implement GRO for plain UDP sockets.")
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
---
net/ipv4/udp_offload.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index e67a66fbf27b..13740e9fe6ec 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c
@@ -366,7 +366,7 @@ static struct sk_buff *udp4_ufo_fragment(struct sk_buff *skb,
static struct sk_buff *udp_gro_receive_segment(struct list_head *head,
struct sk_buff *skb)
{
- struct udphdr *uh = udp_hdr(skb);
+ struct udphdr *uh = udp_gro_udphdr(skb);
struct sk_buff *pp = NULL;
struct udphdr *uh2;
struct sk_buff *p;
--
2.29.2

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