Re: [PATCH v2 net] net: udp: fix Fast/frag0 UDP GRO

From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: Mon, 9 Nov 2020 14:06:07 -0500

On Mon, Nov 9, 2020 at 1:54 PM Eric Dumazet [off-list ref] wrote:

quoted

On 11/9/20 7:28 PM, Willem de Bruijn wrote:

quoted

On Mon, Nov 9, 2020 at 12:37 PM Eric Dumazet [off-list ref] wrote:

quoted

On 11/9/20 5:56 PM, Alexander Lobakin wrote:

quoted

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 v1 [1]:
 - added a NULL pointer check for "uh" as suggested by Willem.

[1] 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_offload.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index e67a66fbf27b..7f6bd221880a 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c

@@ -366,13 +366,18 @@ 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;
      unsigned int ulen;
      int ret = 0;

+     if (unlikely(!uh)) {

How uh could be NULL here ?

My understanding is that udp_gro_receive() is called
only after udp4_gro_receive() or udp6_gro_receive()
validated that udp_gro_udphdr(skb) was not NULL.

Oh indeed. This has already been checked before.

quoted

quoted

+             NAPI_GRO_CB(skb)->flush = 1;
+             return NULL;
+     }
+
      /* requires non zero csum, for symmetry with GSO */
      if (!uh->check) {
              NAPI_GRO_CB(skb)->flush = 1;

Why uh2 is left unchanged ?

    uh2 = udp_hdr(p);

Isn't that the same as th2 = tcp_hdr(p) in tcp_gro_receive? no frag0
optimization to worry about for packets on the list.

My feeling was that tcp_gro_receive() is terminal in the GRO stack.

While UDP could be encapsulated in UDP :)

I guess we do not support this yet.

Years ago we made sure to propagate the current header offset into GRO stack
(when we added SIT/IPIP/GRE support to GRO)
299603e8370a93dd5d8e8d800f0dff1ce2c53d36 ("net-gro: Prepare GRO stack for the upcoming tunneling support")

On which note, and Alexander's mention of udp4_lib_lookup_skb(), that
performs a standard ip_hdr on the possibly frag0 optimized incoming
packet:

struct sock *udp4_lib_lookup_skb(struct sk_buff *skb,
                                 __be16 sport, __be16 dport)
{
        const struct iphdr *iph = ip_hdr(skb);

        return __udp4_lib_lookup(dev_net(skb->dev), iph->saddr, sport,
                                 iph->daddr, dport, inet_iif(skb),
                                 inet_sdif(skb), &udp_table, NULL);
}

This should use skb_gro_header_.. too, then?

Well, both of udp{4,6}_lib_lookup_skb() and a couple of neighbour
functions use ip_hdr(skb). I didn't check this path before as my
test case was NAT/forwarding with no sk.
I suspect they should use skb_gro_network_header() to obtain an IP
header. I'll fix them in v3, thanks for pointing this out!

Al