netdev - Re: [PATCH net-next v4 3/5] net: core: add MPLS update core helper and use in OvS

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Message-ID: <CA+FuTSfOwv_Oj2q5T=0ergA32f1akR0mBiG-qRcr6z0F_FWnVA@mail.gmail.com>
Date:   Mon, 1 Jul 2019 11:28:39 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     John Hurley <john.hurley@...ronome.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>, jiri@...lanox.com,
        Cong Wang <xiyou.wangcong@...il.com>,
        David Ahern <dsahern@...il.com>, simon.horman@...ronome.com,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        oss-drivers@...ronome.com
Subject: Re: [PATCH net-next v4 3/5] net: core: add MPLS update core helper
 and use in OvS

On Mon, Jul 1, 2019 at 8:31 AM John Hurley <john.hurley@...ronome.com> wrote:
>
> Open vSwitch allows the updating of an existing MPLS header on a packet.
> In preparation for supporting similar functionality in TC, move this to a
> common skb helper function.
>
> Signed-off-by: John Hurley <john.hurley@...ronome.com>
> Reviewed-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
> Reviewed-by: Simon Horman <simon.horman@...ronome.com>
> ---
>  /**
> + * skb_mpls_update_lse() - modify outermost MPLS header and update csum
> + *
> + * @skb: buffer
> + * @mpls_lse: new MPLS label stack entry to update to
> + *
> + * Expects skb->data at mac header.
> + *
> + * Returns 0 on success, -errno otherwise.
> + */
> +int skb_mpls_update_lse(struct sk_buff *skb, __be32 mpls_lse)
> +{
> +       struct mpls_shim_hdr *old_lse = mpls_hdr(skb);
> +       int err;
> +
> +       if (unlikely(!eth_p_mpls(skb->protocol)))
> +               return -EINVAL;
> +
> +       err = skb_ensure_writable(skb, skb->mac_len + MPLS_HLEN);
> +       if (unlikely(err))
> +               return err;
> +
> +       if (skb->ip_summed == CHECKSUM_COMPLETE) {
> +               __be32 diff[] = { ~old_lse->label_stack_entry, mpls_lse };
> +
> +               skb->csum = csum_partial((char *)diff, sizeof(diff), skb->csum);
> +       }
> +
> +       old_lse->label_stack_entry = mpls_lse;

skb_ensure_writable may have reallocated the skb linear. old_lse needs
to be loaded after. Or, safer:

  mpls_hdr(skb)->label_stack_entry = mpls_lse;