Re: [PATCH net-next v3] ipv6: add IFLA_INET6_RA_MTU to expose mtu value in the RA message

David Ahern
2021-08-09T22:44:06Z

On 8/9/21 8:01 AM, Rocco Yue wrote:
> @@ -6129,6 +6136,66 @@ static void ipv6_ifa_notify(int event, struct inet6_ifaddr *ifp)
> __ipv6_ifa_notify(event, ifp);
> }
>
> +static inline size_t inet6_iframtu_msgsize(void)
> +{
> + return NLMSG_ALIGN(sizeof(struct ifinfomsg))
> + + nla_total_size(IFNAMSIZ) /* IFLA_IFNAME */
> + + nla_total_size(4); /* IFLA_INET6_RA_MTU */
> +}
> +
> +static int inet6_fill_iframtu(struct sk_buff *skb, struct inet6_dev *idev)
> +{
> + struct net_device *dev = idev->dev;
> + struct ifinfomsg *hdr;
> + struct nlmsghdr *nlh;
> +
> + nlh = nlmsg_put(skb, 0, 0, RTM_NEWLINK, sizeof(*hdr), 0);
> + if (!nlh)
> + return -EMSGSIZE;
> +
> + hdr = nlmsg_data(nlh);
> + hdr->ifi_family = AF_INET6;
> + hdr->__ifi_pad = 0;
> + hdr->ifi_index = dev->ifindex;
> + hdr->ifi_flags = dev_get_flags(dev);
> + hdr->ifi_change = 0;
> +
> + if (nla_put_string(skb, IFLA_IFNAME, dev->name) ||
> + nla_put_u32(skb, IFLA_INET6_RA_MTU, idev->ra_mtu))
> + goto nla_put_failure;
> +
> + nlmsg_end(skb, nlh);
> + return 0;
> +
> +nla_put_failure:
> + nlmsg_cancel(skb, nlh);
> + return -EMSGSIZE;
> +}
> +
> +void inet6_iframtu_notify(struct inet6_dev *idev)
> +{
> + struct sk_buff *skb;
> + struct net *net = dev_net(idev->dev);
> + int err = -ENOBUFS;
> +
> + skb = nlmsg_new(inet6_iframtu_msgsize(), GFP_ATOMIC);
> + if (!skb)
> + goto errout;
> +
> + err = inet6_fill_iframtu(skb, idev);
> + if (err < 0) {
> + /* -EMSGSIZE implies BUG in inet6_iframtu_msgsize() */
> + WARN_ON(err == -EMSGSIZE);
> + kfree_skb(skb);
> + goto errout;
> + }
> + rtnl_notify(skb, net, 0, RTNLGRP_IPV6_IFINFO, NULL, GFP_ATOMIC);
> + return;
> +errout:
> + if (err < 0)
> + rtnl_set_sk_err(net, RTNLGRP_IPV6_IFINFO, err);
> +}

pretty sure you don't need to build a new notify function.

> +
> #ifdef CONFIG_SYSCTL
>
> static int addrconf_sysctl_forward(struct ctl_table *ctl, int write,
> diff --git a/net/ipv6/ndisc.c b/net/ipv6/ndisc.c
> index c467c6419893..a04164cbd77f 100644
> --- a/net/ipv6/ndisc.c
> +++ b/net/ipv6/ndisc.c
> @@ -1496,6 +1496,12 @@ static void ndisc_router_discovery(struct sk_buff *skb)
> memcpy(&n, ((u8 *)(ndopts.nd_opts_mtu+1))+2, sizeof(mtu));
> mtu = ntohl(n);
>
> + if (in6_dev->ra_mtu != mtu) {
> + in6_dev->ra_mtu = mtu;
> + inet6_iframtu_notify(in6_dev);
> + ND_PRINTK(2, info, "update ra_mtu to %d\n", in6_dev->ra_mtu);
> + }
> +
> if (mtu < IPV6_MIN_MTU || mtu > skb->dev->mtu) {
> ND_PRINTK(2, warn, "RA: invalid mtu: %d\n", mtu);
> } else if (in6_dev->cnf.mtu6 != mtu) {

Since this MTU is getting reported via af_info infrastructure,
rtmsg_ifinfo should be sufficient.

From there use 'ip monitor' to make sure you are not generating multiple
notifications; you may only need this on the error path.