| Message ID | 1510701938-53743-1-git-send-email-eswierk@skyportsystems.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
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. > > Signed-off-by: Ed Swierk <eswierk@skyportsystems.com> > --- > hw/net/e1000.c | 5 +++-- > hw/net/net_rx_pkt.c | 3 +++ > hw/net/net_tx_pkt.c | 6 ++++++ > hw/net/vmxnet3.c | 7 +++++-- > 4 files changed, 17 insertions(+), 4 deletions(-) > > diff --git a/hw/net/e1000.c b/hw/net/e1000.c > index d642314..97242a1 100644 > --- a/hw/net/e1000.c > +++ b/hw/net/e1000.c > @@ -505,8 +505,9 @@ putsum(uint8_t *data, uint32_t n, uint32_t sloc, uint32_t css, uint32_t cse) > if (cse && cse < n) > n = cse + 1; > if (sloc < n-1) { > - sum = net_checksum_add(n-css, data+css); > - stw_be_p(data + sloc, net_checksum_finish(sum)); > + sum = net_raw_checksum(data + css, n - css); > + /* For UDP, zero checksum must be sent as 0xFFFF */ > + stw_be_p(data + sloc, sum ? sum : 0xffff); > } > } > > diff --git a/hw/net/net_rx_pkt.c b/hw/net/net_rx_pkt.c > 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? Thanks
On Tue, Nov 14, 2017 at 6:10 PM, Jason Wang <jasowang@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
diff --git a/hw/net/e1000.c b/hw/net/e1000.c index d642314..97242a1 100644 --- a/hw/net/e1000.c +++ b/hw/net/e1000.c @@ -505,8 +505,9 @@ putsum(uint8_t *data, uint32_t n, uint32_t sloc, uint32_t css, uint32_t cse) if (cse && cse < n) n = cse + 1; if (sloc < n-1) { - sum = net_checksum_add(n-css, data+css); - stw_be_p(data + sloc, net_checksum_finish(sum)); + sum = net_raw_checksum(data + css, n - css); + /* For UDP, zero checksum must be sent as 0xFFFF */ + stw_be_p(data + sloc, sum ? sum : 0xffff); } } diff --git a/hw/net/net_rx_pkt.c b/hw/net/net_rx_pkt.c 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 */ + } /* Set calculated checksum to checksum word */ iov_from_buf(pkt->vec, pkt->vec_len, diff --git a/hw/net/net_tx_pkt.c b/hw/net/net_tx_pkt.c index 20b2549..21194e7 100644 --- a/hw/net/net_tx_pkt.c +++ b/hw/net/net_tx_pkt.c @@ -136,6 +136,9 @@ void net_tx_pkt_update_ip_checksums(struct NetTxPkt *pkt) return; } + if (!csum) { + csum = 0xFFFF; /* For UDP, zero checksum must be sent as 0xFFFF */ + } iov_from_buf(&pkt->vec[NET_TX_PKT_PL_START_FRAG], pkt->payload_frags, pkt->virt_hdr.csum_offset, &csum, sizeof(csum)); } @@ -487,6 +490,9 @@ static void net_tx_pkt_do_sw_csum(struct NetTxPkt *pkt) /* Put the checksum obtained into the packet */ csum = cpu_to_be16(net_checksum_finish(csum_cntr)); + if (!csum) { + csum = 0xFFFF; /* For UDP, zero checksum must be sent as 0xFFFF */ + } iov_from_buf(iov, iov_len, csum_offset, &csum, sizeof csum); } diff --git a/hw/net/vmxnet3.c b/hw/net/vmxnet3.c index 90f6943..de9c40e 100644 --- a/hw/net/vmxnet3.c +++ b/hw/net/vmxnet3.c @@ -942,6 +942,7 @@ static void vmxnet3_rx_need_csum_calculate(struct NetRxPkt *pkt, bool isip4, isip6, istcp, isudp; uint8_t *data; int len; + uint16_t sum; if (!net_rx_pkt_has_virt_hdr(pkt)) { return; @@ -969,8 +970,10 @@ static void vmxnet3_rx_need_csum_calculate(struct NetRxPkt *pkt, data = (uint8_t *)pkt_data + vhdr->csum_start; len = pkt_len - vhdr->csum_start; - /* Put the checksum obtained into the packet */ - stw_be_p(data + vhdr->csum_offset, net_raw_checksum(data, len)); + sum = net_raw_checksum(data, len); + /* Put the checksum obtained into the packet; for UDP, zero checksum */ + /* must be sent as 0xFFFF */ + stw_be_p(data + vhdr->csum_offset, sum ? sum : 0xFFFF); vhdr->flags &= ~VIRTIO_NET_HDR_F_NEEDS_CSUM; vhdr->flags |= VIRTIO_NET_HDR_F_DATA_VALID;
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.) Signed-off-by: Ed Swierk <eswierk@skyportsystems.com> --- hw/net/e1000.c | 5 +++-- hw/net/net_rx_pkt.c | 3 +++ hw/net/net_tx_pkt.c | 6 ++++++ hw/net/vmxnet3.c | 7 +++++-- 4 files changed, 17 insertions(+), 4 deletions(-)