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

From: Vlad Yasevich <[email-masked]>
Date: Fri, 11 Oct 2013 10:25:44 -0400

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.

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;
> +            }
>           }
>       }
>