[ovs-dev] [PATCH v3 1/2] skbuff: Add skb_network_trim

[Pravin Shelar]

On Mon, Jan 22, 2018 at 6:42 PM, Ed Swierk <eswierk at skyportsystems.com> wrote:
> IPv4 and IPv6 packets may arrive with lower-layer padding that is not
> included in the L3 length. For example, a short IPv4 packet may have
> up to 6 bytes of padding following the IP payload when received on an
> Ethernet device with a minimum packet length of 64 bytes.
>
> Higher-layer processing functions in netfilter (e.g. nf_ip_checksum(),
> and help() in nf_conntrack_ftp) assume skb->len reflects the length of
> the L3 header and payload, rather than referring back to
> ip_hdr->tot_len or ipv6_hdr->payload_len, and get confused by
> lower-layer padding.
>
> In the normal IPv4 receive path, ip_rcv() trims the packet to
> ip_hdr->tot_len before invoking netfilter hooks. In the IPv6 receive
> path, ip6_rcv() does the same using ipv6_hdr->payload_len. Similarly
> in the br_netfilter receive path, br_validate_ipv4() and
> br_validate_ipv6() trim the packet to the L3 length before invoking
> netfilter hooks.
>
> Currently the openvswitch receive path does not perform such trimming,
> which will be fixed by the next patch. In preparation, this patch adds
> a generic skb_network_trim() function.
>
> Signed-off-by: Ed Swierk <eswierk at skyportsystems.com>
> ---
>  include/linux/skbuff.h |  2 ++
>  net/core/skbuff.c      | 44 ++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 46 insertions(+)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 051e093..0478645 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -4018,6 +4018,8 @@ struct sk_buff *skb_checksum_trimmed(struct sk_buff *skb,
>                                      unsigned int transport_len,
>                                      __sum16(*skb_chkf)(struct sk_buff *skb));
>
> +int skb_network_trim(struct sk_buff *skb);
> +
>  /**
>   * skb_head_is_locked - Determine if the skb->head is locked down
>   * @skb: skb to check
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 15fa5ba..cef3d1e 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -4743,6 +4743,50 @@ struct sk_buff *skb_checksum_trimmed(struct sk_buff *skb,
>  }
>  EXPORT_SYMBOL(skb_checksum_trimmed);
>
> +/**
> + * skb_network_trim - trim skb to length specified by the network header
> + * @skb: the skb to trim
> + *
> + * Trims the skb to the length specified by the IP/IPv6 header,
> + * removing any trailing lower-layer padding. This prepares the skb
> + * for higher-layer processing that assumes skb->len excludes padding.
> + *
> + * Leaves the skb alone if the protocol is not IP or IPv6. Frees the
> + * skb on error.
> + *
> + * Caller needs to pull the skb to the network header.
> + */
> +int skb_network_trim(struct sk_buff *skb)
> +{
> +       unsigned int len;
> +       int err = -ENOMEM;
> +
> +       switch (skb->protocol) {
> +       case htons(ETH_P_IP):
> +               if (!pskb_may_pull(skb, sizeof(struct iphdr)))
> +                       goto out;
Since you are going to move this to OVS specific code, can you remove
this skb-pull, which is not required in OVS code path.

> +               len = ntohs(ip_hdr(skb)->tot_len);
> +               break;
> +       case htons(ETH_P_IPV6):
> +               if (!pskb_may_pull(skb, sizeof(struct ipv6hdr)))
> +                       goto out;
> +               len = sizeof(struct ipv6hdr)
> +                       + ntohs(ipv6_hdr(skb)->payload_len);