On Tue, Nov 14, 2017 at 6:10 PM, Jason Wang <jasow...@redhat.com> wrote: > > > On 2017年11月15日 07:25, Ed Swierk wrote: >> >> The checksum algorithm used by IPv4, TCP and UDP allows a zero value >> to be represented by either 0x0000 and 0xFFFF. But per RFC 768, a zero >> UDP checksum must be transmitted as 0xFFFF, as 0x0000 is a special >> value meaning no checksum. >> >> Substitute 0xFFFF whenever a checksum is computed as zero on a UDP >> datagram. Doing this on IPv4 packets and TCP segments is unnecessary >> but legal. >> >> (While it is tempting to make the substitution in >> net_checksum_finish(), that function is also used by receivers to >> verify checksums, and in that case the expected value is always >> 0x0000.) > > > Then looks like you'd better have an wrapper for net_checksum_finish() and > do things there.
I'll do that in v2. >> index 1019b50..e820132 100644 >> --- a/hw/net/net_rx_pkt.c >> +++ b/hw/net/net_rx_pkt.c >> @@ -588,6 +588,9 @@ bool net_rx_pkt_fix_l4_csum(struct NetRxPkt *pkt) >> /* Calculate L4 checksum */ >> csum = cpu_to_be16(_net_rx_pkt_calc_l4_csum(pkt)); >> + if (!csum) { >> + csum = 0xFFFF; /* For UDP, zero checksum must be sent as 0xFFFF >> */ >> + } > > > I thought we should only do this for tx? We need to do this any time we modify the checksum field in a UDP datagram header for someone else to verify. Normally this happens on the tx path, and that someone is a remote system. But here net_rx_pkt_fix_l4_csum() is used to fill in the checksum on packets received with the NEEDS_CSUM vhdr flag before passing them along to the guest. --Ed