Checksum forwarding issue on XEN
Roger Pau Monné
roger.pau at citrix.com
Thu Nov 5 15:16:15 UTC 2015
Hello,
Adding the persons that contributed that code in case they can shed some
light.
El 03/11/15 a les 21.12, Larry Baird ha escrit:
> Has anybody made any progress on "Bug 188261 - [xen] FreeBSD DomU PVHVM
> guests cannot 'route' traffic for other Xen PV guests on same Dom0 Host."
> (https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=188261)?
>
> The code for checksum calculation in the function xnb_add_mbuf_cksum() looks
> suspect.
>
> switch (iph->ip_p) {
> case IPPROTO_TCP:
> if (mbufc->m_pkthdr.csum_flags & CSUM_IP_VALID) {
> size_t tcplen = ntohs(iph->ip_len) - sizeof(struct ip);
> struct tcphdr *th = (struct tcphdr*)(iph + 1);
> th->th_sum = in_pseudo(iph->ip_src.s_addr,
> iph->ip_dst.s_addr, htons(IPPROTO_TCP + tcplen));
> th->th_sum = in_cksum_skip(mbufc,
> sizeof(struct ether_header) + ntohs(iph->ip_len),
> sizeof(struct ether_header) + (iph->ip_hl << 2));
> }
> break;
> case IPPROTO_UDP:
> if (mbufc->m_pkthdr.csum_flags & CSUM_IP_VALID) {
> size_t udplen = ntohs(iph->ip_len) - sizeof(struct ip);
> struct udphdr *uh = (struct udphdr*)(iph + 1);
> uh->uh_sum = in_pseudo(iph->ip_src.s_addr,
> iph->ip_dst.s_addr, htons(IPPROTO_UDP + udplen));
> uh->uh_sum = in_cksum_skip(mbufc,
> sizeof(struct ether_header) + ntohs(iph->ip_len),
> sizeof(struct ether_header) + (iph->ip_hl << 2));
> }
> break;
> default:
> break;
> }
>
>
> Both in_pseudo() and in_cksum_skip() set the same checksum. Does this
> make since to anybody?
The bug you are referring to affects FreeBSD when running as a guest
using xen-netfront, but the code snipped above and the function
referenced (xnb_add_mbuf_cksum) is only used on FreeBSD when running as
a host (AKA Dom0) by xen-netback.
TBH, I don't know that much about FreeBSD network subsystem to have an
opinion, but it certainly looks weird. Patches are welcome :).
Roger.
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