From: Alexander Bluhm Subject: Re: dhcpleased(8): fix pseudo csum from vio(4) offloading To: Jan Klemkow Cc: tech@openbsd.org Date: Thu, 25 Jan 2024 15:07:13 +0100 On Thu, Jan 25, 2024 at 02:49:30PM +0100, Jan Klemkow wrote: > On Sun, Jan 07, 2024 at 09:46:22PM +0100, Alexander Bluhm wrote: > > On Sun, Jan 07, 2024 at 01:38:23PM +0100, Jan Klemkow wrote: > > > we get packets with pseudo checksums from vio(4), since we use checksum > > > offloading. Thus, programs which are dealing with raw packets like > > > dhcpleased(8) also need to check for this kind of checksums or return > > > errors. > > > > I think kernel should fix this. If we have devices like vio(4) > > that report checksum OK, but we know that the ckecksum field is > > wrong, and we pass the packet header to user land, kernel should > > calculate correct checksum. At least divert_packet() does a trick > > like this. For raw sockets I would prefer something like this. > > > > Another possibility would be to pass a flag in bpf header that > > checksum is OK. Then dhcpleased could skip the checksum check. > > That would also help tcpdump complaining about bad checksums in bpf > > packets. > > > > As dhcpleased uses bpf, I think the second option is the way to go. > > Sounds like a better approach. > > ok? OK bluhm@ > Index: sys/net/bpf.c > =================================================================== > RCS file: /cvs/src/sys/net/bpf.c,v > diff -u -p -r1.221 bpf.c > --- sys/net/bpf.c 9 Mar 2023 05:56:58 -0000 1.221 > +++ sys/net/bpf.c 25 Jan 2024 13:34:17 -0000 > @@ -1397,6 +1397,8 @@ _bpf_mtap(caddr_t arg, const struct mbuf > if (ISSET(mp->m_pkthdr.csum_flags, > M_FLOWID)) > SET(tbh.bh_flags, BPF_F_FLOWID); > + tbh.bh_csumflags = > + mp->m_pkthdr.csum_flags; > > m_microtime(mp, &tv); > } else > Index: sys/net/bpf.h > =================================================================== > RCS file: /cvs/src/sys/net/bpf.h,v > diff -u -p -r1.71 bpf.h > --- sys/net/bpf.h 9 Mar 2023 05:56:58 -0000 1.71 > +++ sys/net/bpf.h 25 Jan 2024 13:34:17 -0000 > @@ -161,6 +161,7 @@ struct bpf_hdr { > #define BPF_F_DIR_IN (BPF_DIRECTION_IN << BPF_F_DIR_SHIFT) > #define BPF_F_DIR_OUT (BPF_DIRECTION_OUT << BPF_F_DIR_SHIFT) > u_int8_t bh_drops; > + u_int16_t bh_csumflags; /* checksum flags */ > }; > > #ifdef _KERNEL > Index: sbin/dhcpleased/dhcpleased.h > =================================================================== > RCS file: /cvs/src/sbin/dhcpleased/dhcpleased.h,v > diff -u -p -r1.15 dhcpleased.h > --- sbin/dhcpleased/dhcpleased.h 25 Nov 2023 12:00:39 -0000 1.15 > +++ sbin/dhcpleased/dhcpleased.h 25 Jan 2024 13:34:18 -0000 > @@ -287,6 +287,7 @@ struct imsg_propose_rdns { > struct imsg_dhcp { > uint32_t if_index; > ssize_t len; > + uint16_t csumflags; > uint8_t ether_align[2]; > uint8_t packet[1500]; > }; > Index: sbin/dhcpleased/engine.c > =================================================================== > RCS file: /cvs/src/sbin/dhcpleased/engine.c,v > diff -u -p -r1.41 engine.c > --- sbin/dhcpleased/engine.c 14 Dec 2023 09:58:37 -0000 1.41 > +++ sbin/dhcpleased/engine.c 25 Jan 2024 13:34:18 -0000 > @@ -24,6 +24,7 @@ > #include > #include > #include > +#include > > #include > #include > @@ -788,7 +789,8 @@ parse_dhcp(struct dhcpleased_iface *ifac > if (rem < (size_t)ip->ip_hl << 2) > goto too_short; > > - if (wrapsum(checksum((uint8_t *)ip, ip->ip_hl << 2, 0)) != 0) { > + if ((dhcp->csumflags & M_IPV4_CSUM_IN_OK) == 0 && > + wrapsum(checksum((uint8_t *)ip, ip->ip_hl << 2, 0)) != 0) { > log_warnx("%s: bad IP checksum", __func__); > return; > } > @@ -834,16 +836,19 @@ parse_dhcp(struct dhcpleased_iface *ifac > p += sizeof(*udp); > rem -= sizeof(*udp); > > - usum = udp->uh_sum; > - udp->uh_sum = 0; > - > - sum = wrapsum(checksum((uint8_t *)udp, sizeof(*udp), checksum(p, rem, > - checksum((uint8_t *)&ip->ip_src, 2 * sizeof(ip->ip_src), > - IPPROTO_UDP + ntohs(udp->uh_ulen))))); > - > - if (usum != 0 && usum != sum) { > - log_warnx("%s: bad UDP checksum", __func__); > - return; > + if ((dhcp->csumflags & M_UDP_CSUM_IN_OK) == 0) { > + usum = udp->uh_sum; > + udp->uh_sum = 0; > + > + sum = wrapsum(checksum((uint8_t *)udp, sizeof(*udp), > + checksum(p, rem, > + checksum((uint8_t *)&ip->ip_src, 2 * sizeof(ip->ip_src), > + IPPROTO_UDP + ntohs(udp->uh_ulen))))); > + > + if (usum != 0 && usum != sum) { > + log_warnx("%s: bad UDP checksum", __func__); > + return; > + } > } > > if (log_getverbose() > 1) { > Index: sbin/dhcpleased/frontend.c > =================================================================== > RCS file: /cvs/src/sbin/dhcpleased/frontend.c,v > diff -u -p -r1.32 frontend.c > --- sbin/dhcpleased/frontend.c 14 Dec 2023 09:58:37 -0000 1.32 > +++ sbin/dhcpleased/frontend.c 25 Jan 2024 13:34:18 -0000 > @@ -890,6 +890,7 @@ bpf_receive(int fd, short events, void * > } > memcpy(&imsg_dhcp.packet, p + hdr->bh_hdrlen, hdr->bh_caplen); > imsg_dhcp.len = hdr->bh_caplen; > + imsg_dhcp.csumflags = hdr->bh_csumflags; > frontend_imsg_compose_engine(IMSG_DHCP, 0, 0, &imsg_dhcp, > sizeof(imsg_dhcp)); > cont: