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

[Alexander Lobakin <alobakin@pm.me>]

From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: Mon, 9 Nov 2020 09:36:12 -0500

> On Sat, Nov 7, 2020 at 8:11 PM Alexander Lobakin <alobakin@pm.me> wrote:
>>
>> 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;
>
> Good catch. skb_gro_header_slow may fail and return NULL. Need to
> check that before dereferencing uh below in

Ah, sure. It's very unlikely condition, but "better safe than sorry"
(c). Thanks, will do v2 soon.

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

Al