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

[Alexander Lobakin]

From: Eric Dumazet <eric.dumazet@xxxxxxxxx>
Date: Mon, 9 Nov 2020 18:37:36 +0100

> On 11/9/20 5:56 PM, Alexander Lobakin 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.
>>
>> Since v1 [1]:
>>  - added a NULL pointer check for "uh" as suggested by Willem.
>>
>> [1] https://lore.kernel.org/netdev/YazU6GEzBdpyZMDMwJirxDX7B4sualpDG68ADZYvJI@xxxxxxxxxxxxxxxxxxxx
>>
>> Fixes: e20cf8d3f1f7 ("udp: implement GRO for plain UDP sockets.")
>> Signed-off-by: Alexander Lobakin <alobakin@xxxxx>
>> ---
>>  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.

Right, but only after udp{4,6}_lib_lookup_skb() in certain cases.
I don't know for sure if their logic can actually edit skb->data,
so it's better to check from my point of view.

>> +		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);
>
>...

Packets from list_head *head have their headers already pulled to
skb->data in 100% cases, no need to change anything here.
I double-checked that udp_hdr(p) always returns the same pointer as
"p->data + network offset" and left it as it is.

Thanks,
Al