[RFC,net-next,v2,3/4] net: more accurate checksumming in validate_xmit_skb

Introduce skb->csum_not_inet to identify not-yet-checksummed SCTP packets.
Use this bit in combination with netdev feature bit in validate_xmit_skb,
to discriminate whether skb needs crc32c or 2-complement Internet Checksum
(or none of the two, when the underlying device can do checksum offload).

Signed-off-by: Davide Caratti <dcaratti@redhat.com>
---
 include/linux/skbuff.h                |  1 +
 net/core/dev.c                        | 14 ++++++++++++--
 net/netfilter/ipvs/ip_vs_proto_sctp.c |  1 +
 net/netfilter/nf_nat_proto_sctp.c     |  1 +
 net/sched/act_csum.c                  |  1 +
 net/sctp/output.c                     |  1 +
 6 files changed, 17 insertions(+), 2 deletions(-)

On Tue, Feb 28, 2017 at 2:32 AM, Davide Caratti <dcaratti@redhat.com> wrote:
> Introduce skb->csum_not_inet to identify not-yet-checksummed SCTP packets.
> Use this bit in combination with netdev feature bit in validate_xmit_skb,
> to discriminate whether skb needs crc32c or 2-complement Internet Checksum
> (or none of the two, when the underlying device can do checksum offload).
>
> Signed-off-by: Davide Caratti <dcaratti@redhat.com>
> ---
>  include/linux/skbuff.h                |  1 +
>  net/core/dev.c                        | 14 ++++++++++++--
>  net/netfilter/ipvs/ip_vs_proto_sctp.c |  1 +
>  net/netfilter/nf_nat_proto_sctp.c     |  1 +
>  net/sched/act_csum.c                  |  1 +
>  net/sctp/output.c                     |  1 +
>  6 files changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 0671131..236b7d9 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -759,6 +759,7 @@ struct sk_buff {
>         __u8                    tc_redirected:1;
>         __u8                    tc_from_ingress:1;
>  #endif
> +       __u8                    csum_not_inet:1;
>
Unfortunately this potentially pushes the skbuf flags over 32 bits if
I count correctly. I suggest that you rename csum_bad to
csum_not_inet. Looks like csum_bad is only set by a grand total of one
driver and I don't believe that is enough to justify its existence.
It's probably a good time to remove it.

>  #ifdef CONFIG_NET_SCHED
>         __u16                   tc_index;       /* traffic control index */
> diff --git a/net/core/dev.c b/net/core/dev.c
> index b9fb843..fae3217 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -2614,6 +2614,7 @@ int skb_sctp_csum_help(struct sk_buff *skb)
>         }
>         *(__le32 *)(skb->data + offset) = crc32c_csum;
>         skb->ip_summed = CHECKSUM_NONE;
> +       skb->csum_not_inet = 0;
>  out:
>         return ret;
>  }
> @@ -2960,6 +2961,16 @@ static struct sk_buff *validate_xmit_vlan(struct sk_buff *skb,
>         return skb;
>  }
>
> +static int skb_csum_hwoffload_help(struct sk_buff *skb,
> +                                  netdev_features_t features)
> +{
> +       if (unlikely(skb->csum_not_inet))
> +               return !(features & NETIF_F_SCTP_CRC) ?
> +                               skb_sctp_csum_help(skb) : 0;
> +
Return value looks complex. Maybe we should just change
skb_csum_*_help to return bool, true of checksum was handled false if
not.

> +       return !(features & NETIF_F_CSUM_MASK) ? skb_checksum_help(skb) : 0;
> +}
> +
>  static struct sk_buff *validate_xmit_skb(struct sk_buff *skb, struct net_device *dev)
>  {
>         netdev_features_t features;
> @@ -2995,8 +3006,7 @@ static struct sk_buff *validate_xmit_skb(struct sk_buff *skb, struct net_device
>                         else
>                                 skb_set_transport_header(skb,
>                                                          skb_checksum_start_offset(skb));
> -                       if (!(features & NETIF_F_CSUM_MASK) &&
> -                           skb_checksum_help(skb))
> +                       if (skb_csum_hwoffload_help(skb, features))
>                                 goto out_kfree_skb;
>                 }
>         }
> diff --git a/net/netfilter/ipvs/ip_vs_proto_sctp.c b/net/netfilter/ipvs/ip_vs_proto_sctp.c
> index d952d67..4972a60 100644
> --- a/net/netfilter/ipvs/ip_vs_proto_sctp.c
> +++ b/net/netfilter/ipvs/ip_vs_proto_sctp.c
> @@ -81,6 +81,7 @@ static void sctp_nat_csum(struct sk_buff *skb, sctp_sctphdr_t *sctph,
>                           unsigned int sctphoff)
>  {
>         sctph->checksum = sctp_compute_cksum(skb, sctphoff);
> +       skb->csum_not_inet = 0;
>         skb->ip_summed = CHECKSUM_UNNECESSARY;
>  }
>
> diff --git a/net/netfilter/nf_nat_proto_sctp.c b/net/netfilter/nf_nat_proto_sctp.c
> index 31d3586..9459b88 100644
> --- a/net/netfilter/nf_nat_proto_sctp.c
> +++ b/net/netfilter/nf_nat_proto_sctp.c
> @@ -49,6 +49,7 @@ sctp_manip_pkt(struct sk_buff *skb,
>
>         if (skb->ip_summed != CHECKSUM_PARTIAL) {
>                 hdr->checksum = sctp_compute_cksum(skb, hdroff);
> +               skb->csum_not_inet = 0;
>                 skb->ip_summed = CHECKSUM_NONE;
>         }
>
> diff --git a/net/sched/act_csum.c b/net/sched/act_csum.c
> index e978ccd4..85cb150 100644
> --- a/net/sched/act_csum.c
> +++ b/net/sched/act_csum.c
> @@ -337,6 +337,7 @@ static int tcf_csum_sctp(struct sk_buff *skb, unsigned int ihl,
>
>         sctph->checksum = sctp_compute_cksum(skb,
>                                              skb_network_offset(skb) + ihl);
> +       skb->csum_not_inet = 0;
>         skb->ip_summed = CHECKSUM_NONE;
>
>         return 1;
> diff --git a/net/sctp/output.c b/net/sctp/output.c
> index 814eac0..0dc227b 100644
> --- a/net/sctp/output.c
> +++ b/net/sctp/output.c
> @@ -528,6 +528,7 @@ static int sctp_packet_pack(struct sctp_packet *packet,
>         } else {
>  chksum:
>                 head->ip_summed = CHECKSUM_PARTIAL;
> +               head->csum_not_inet = 1;
>                 head->csum_start = skb_transport_header(head) - head->head;
>                 head->csum_offset = offsetof(struct sctphdr, checksum);
>         }
> --
> 2.7.4
>