Subject: Re: [PATCHv2 2/2] Don't compute checksum value for SCTP skb with, CHECKSUM_PARTIAL set

From: Fan Du <[email-masked]>
Date: Mon, 14 Oct 2013 15:16:14 +0800

On 2013年10月12日 21:06, Vlad Yasevich wrote:
>
>
> Fan Du<[email-masked]>  wrote:
>
>>
>>
>> On 2013年10月11日 22:25, Vlad Yasevich wrote:
>>> On 10/11/2013 03:08 AM, Fan Du wrote:
>>>>
>>>>
>>>> On 2013年10月10日 21:11, Neil Horman wrote:
>>>>> On Thu, Oct 10, 2013 at 01:51:36PM +0800, Fan Du wrote:
>>>>>> igb/ixgbe have hardware sctp checksum support, when this feature
>> is
>>>>>> enabled
>>>>>> and also IPsec is armed to protect sctp traffic, ugly things
>> happened as
>>>>>> xfrm_output checks CHECKSUM_PARTIAL to do check sum operation(sum
>>>>>> every thing
>>>>>> up and pack the 16bits result in the checksum field). The result
>> is fail
>>>>>> establishment of sctp communication.
>>>>>>
>>>>> Shouldn't this be fixed in the xfrm code then? E.g. check the
>> device
>>>>> features
>>>>> for SCTP checksum offloading and and skip the checksum during xfrm
>>>>> output if its
>>>>> available?
>>>>>
>>>>> Or am I missing something?
>>>>> Neil
>>>>>
>>>>>
>>>>
>>>>
>>>>  From cff25810910603ff991f0c56441d6de8dc33a822 Mon Sep 17 00:00:00
>> 2001
>>>> From: Fan Du<[email-masked]>
>>>> Date: Fri, 11 Oct 2013 14:31:57 +0800
>>>> Subject: [PATCH 2/2] Don't compute checksum value for SCTP skb with
>>>> CHECKSUM_PARTIAL set
>>>> MIME-Version: 1.0
>>>> Content-Type: text/plain; charset=UTF-8
>>>> Content-Transfer-Encoding: 8bit
>>>>
>>>> IPsec is not enabled in this scenario:
>>>>
>>>> SCTP skb set CHECKSUM_PARTIAL to indicate hardware is capable of
>> doing
>>>> SCTP checksum(crc32-c) scoping the whole SCTP packet range. However
>> when
>>>> such kind of skb is delivered through IPv4/v6 output handler,
>> IPv4/v6 stack
>>>> interpret CHECKSUM_PARTIAL by calling skb_checksum_help to compute
>> 16bits
>>>> checksum value by summing everything up, the whole SCTP packet in
>> software
>>>> manner！ After this skb reach NIC, after hardware doing its SCTP
>> checking
>>>> business, a crc32-c value will overwrite the value IPv4/v6 stack
>> computed
>>>> before.
>>>>
>>>> This patch solves this by introducing skb_is_sctpv4/6 to optimize
>> such
>>>> case.
>>>>
>>>> Signed-off-by: Fan Du<[email-masked]>
>>>> ---
>>>> v2:
>>>> Rework this problem by introducing skb_is_scktv4/6
>>>>
>>>> ---
>>>> include/linux/ip.h | 5 +++++
>>>> include/linux/ipv6.h | 6 ++++++
>>>> include/linux/skbuff.h | 1 -
>>>> net/core/skbuff.c | 1 +
>>>> net/ipv4/ip_output.c | 4 +++-
>>>> net/ipv6/ip6_output.c | 1 +
>>>> net/xfrm/xfrm_output.c | 20 +++++++++++++++-----
>>>> 7 files changed, 31 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/include/linux/ip.h b/include/linux/ip.h
>>>> index 492bc65..f556292 100644
>>>> --- a/include/linux/ip.h
>>>> +++ b/include/linux/ip.h
>>>> @@ -19,6 +19,7 @@
>>>>
>>>> #include<linux/skbuff.h>
>>>> #include<uapi/linux/ip.h>
>>>> +#include<uapi/linux/in.h>
>>>>
>>>> static inline struct iphdr *ip_hdr(const struct sk_buff *skb)
>>>> {
>>>> @@ -34,4 +35,8 @@ static inline struct iphdr *ipip_hdr(const struct
>>>> sk_buff *skb)
>>>> {
>>>> return (struct iphdr *)skb_transport_header(skb);
>>>> }
>>>> +static inline int skb_is_sctpv4(const struct sk_buff *skb)
>>>> +{
>>>> + return ip_hdr(skb)->protocol == IPPROTO_SCTP;
>>>> +}
>>>> #endif /* _LINUX_IP_H */
>>>> diff --git a/include/linux/ipv6.h b/include/linux/ipv6.h
>>>> index 28ea384..6e17c04 100644
>>>> --- a/include/linux/ipv6.h
>>>> +++ b/include/linux/ipv6.h
>>>> @@ -2,6 +2,7 @@
>>>> #define _IPV6_H
>>>>
>>>> #include<uapi/linux/ipv6.h>
>>>> +#include<uapi/linux/in.h>
>>>>
>>>> #define ipv6_optlen(p) (((p)->hdrlen+1)<<  3)
>>>> /*
>>>> @@ -387,4 +388,9 @@ static inline struct raw6_sock *raw6_sk(const
>> struct
>>>> sock *sk)
>>>> ((__sk)->sk_bound_dev_if == (__dif)))&&  \
>>>> net_eq(sock_net(__sk), (__net)))
>>>>
>>>> +static inline int skb_is_sctpv6(const struct sk_buff *skb)
>>>> +{
>>>> + return ipv6_hdr(skb)->nexthdr == IPPROTO_SCTP;
>>>> +}
>>>> +
>>>> #endif /* _IPV6_H */
>>>> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
>>>> index 2ddb48d..b36d0cc 100644
>>>> --- a/include/linux/skbuff.h
>>>> +++ b/include/linux/skbuff.h
>>>> @@ -2393,7 +2393,6 @@ extern void skb_split(struct sk_buff *skb,
>>>> extern int skb_shift(struct sk_buff *tgt, struct sk_buff *skb,
>>>> int shiftlen);
>>>> extern void skb_scrub_packet(struct sk_buff *skb, bool xnet);
>>>> -
>>>> extern struct sk_buff *skb_segment(struct sk_buff *skb,
>>>> netdev_features_t features);
>>>>
>>>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>>>> index d81cff1..54d6172 100644
>>>> --- a/net/core/skbuff.c
>>>> +++ b/net/core/skbuff.c
>>>> @@ -3526,3 +3526,4 @@ void skb_scrub_packet(struct sk_buff *skb,
>> bool xnet)
>>>> nf_reset_trace(skb);
>>>> }
>>>> EXPORT_SYMBOL_GPL(skb_scrub_packet);
>>>> +
>>>> diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
>>>> index a04d872..8676b2c 100644
>>>> --- a/net/ipv4/ip_output.c
>>>> +++ b/net/ipv4/ip_output.c
>>>> @@ -587,7 +587,9 @@ slow_path_clean:
>>>>
>>>> slow_path:
>>>> /* for offloaded checksums cleanup checksum before fragmentation */
>>>> - if ((skb->ip_summed == CHECKSUM_PARTIAL)&&
>> skb_checksum_help(skb))
>>>> + if ((skb->ip_summed == CHECKSUM_PARTIAL)&&
>>>> + !skb_is_sctpv4(skb)&&
>>>> + skb_checksum_help(skb))
>>>> goto fail;
>>>> iph = ip_hdr(skb);
>>>>
>>>> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
>>>> index 3a692d5..9b27d95 100644
>>>> --- a/net/ipv6/ip6_output.c
>>>> +++ b/net/ipv6/ip6_output.c
>>>> @@ -671,6 +671,7 @@ slow_path_clean:
>>>>
>>>> slow_path:
>>>> if ((skb->ip_summed == CHECKSUM_PARTIAL)&&
>>>> + !skb_is_sctpv6(skb)&&
>>>> skb_checksum_help(skb))
>>>> goto fail;
>>>>
>>>
>>> No, this isn't right. This is a case where IP fragmentation is
>>> required, and the above code will cause SCTP checksum to not be
>>> computed.
>>
>> Ok, I got my ball, ten bucks bet this correct ;)
>>
>> IPv4:
>> after skb reach fragmentation part, CHECKSUM_PARTIAL denotes
>> L4 layer protocol need to be checksummed, then IPv4 checksum is
>> recomputed for each fragmented IPv4 packet.
>>
>> IPv6:
>> Here IPv6 doesn't need checksum for its header, again
>> CHECKSUM_PARTIAL denotes L4 layer protocol need to be checksummed.
>>
>> So all in all, this is the right place to distinguish SCTP skb out,
>> and skip checksum operation as hw does it thereafter.
>>
>
>   How does HW compute SCTP checksum when the data is split between skb?
> Each skb will be submitted separately to the HW.
> I think we need to fall back to SW checksum when packer will be fragmented.

Hi, Vlad

I understand your argument now, I finally applied a 82576 NIC with SCTP CHECKSUM
supported to test this. It seems when sending this super-sized packet,
sctp_datamsg_from_user fragments each packet to 1480 size already using pathmtu,
this pathmtu is equal or less than the interface device mtu, which means
ip_fragment/ip6_fragment didn't fragments any SCTP skb.

Host A(with 82576):
sctp_test -h 128.224.162.161 -p 5001 -H 128.224.162.220 -P 500 -x 1 -c 5 -s -T
                                                                      ^^^
                                                 set each packet to 32768 bytes

I cannot picture any scenario where a SCTP skb with CHECK_PARTIAL set to reach
ip_fragment/ip6_fragment. FWIW, the fix for xfrm_output part is definitely a
valid one.

> -vlad
>
>> Q.E.D.
>>
>>
>>> Looks like SCTP needs to compute the checksum in the case where
>>> skb will be fragmented.
>>>
>>> An alternative, that will also allow us to get rid of patch 1
>>> in the serices is to have a checksum handler offload function
>>> that can be used to compute checksum in this case.
>>>
>>> -vlad
>>>
>>>> diff --git a/net/xfrm/xfrm_output.c b/net/xfrm/xfrm_output.c
>>>> index 3bb2cdc..ddef94a 100644
>>>> --- a/net/xfrm/xfrm_output.c
>>>> +++ b/net/xfrm/xfrm_output.c
>>>> @@ -180,6 +180,14 @@ static int xfrm_output_gso(struct sk_buff *skb)
>>>> return 0;
>>>> }
>>>>
>>>> +static int skb_is_sctp(const struct sk_buff *skb)
>>>> +{
>>>> + if (skb->protocol == __constant_htons(ETH_P_IP))
>>>> + return skb_is_sctpv4(skb);
>>>> + else
>>>> + return skb_is_sctpv6(skb);
>>>> +}
>>>> +
>>>> int xfrm_output(struct sk_buff *skb)
>>>> {
>>>> struct net *net = dev_net(skb_dst(skb)->dev);
>>>> @@ -189,11 +197,13 @@ int xfrm_output(struct sk_buff *skb)
>>>> return xfrm_output_gso(skb);
>>>>
>>>> if (skb->ip_summed == CHECKSUM_PARTIAL) {
>>>> - err = skb_checksum_help(skb);
>>>> - if (err) {
>>>> - XFRM_INC_STATS(net, LINUX_MIB_XFRMOUTERROR);
>>>> - kfree_skb(skb);
>>>> - return err;
>>>> + if (!skb_is_sctp(skb)) {
>>>> + err = skb_checksum_help(skb);
>>>> + if (err) {
>>>> + XFRM_INC_STATS(net, LINUX_MIB_XFRMOUTERROR);
>>>> + kfree_skb(skb);
>>>> + return err;
>>>> + }
>>>> }
>>>> }
>>>>
>>>
>>>
>

-- 
浮沉随浪只记今朝笑

--fan