Re: [Qemu-devel] [PATCH] net: Transmit zero UDP checksum as 0xFFFF

[Ed Swierk via Qemu-devel]

On Tue, Nov 14, 2017 at 6:10 PM, Jason Wang  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 0x and 0x. But per RFC 768, a zero
>> UDP checksum must be transmitted as 0x, as 0x is a special
>> value meaning no checksum.
>>
>> Substitute 0x 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
>> 0x.)
>
>
> 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 = 0x; /* For UDP, zero checksum must be sent as 0x
>> */
>> +}
>
>
> 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

Re: [Qemu-devel] [PATCH] net: Transmit zero UDP checksum as 0xFFFF

[Jason Wang]

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 0x and 0x. But per RFC 768, a zero
UDP checksum must be transmitted as 0x, as 0x is a special
value meaning no checksum.

Substitute 0x 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
0x.)


Then looks like you'd better have an wrapper for net_checksum_finish() 
and do things there.




Signed-off-by: Ed Swierk 
---
  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 0x */
+stw_be_p(data + sloc, sum ? sum : 0x);
  }
  }
  
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 = 0x; /* For UDP, zero checksum must be sent as 0x */
+}


I thought we should only do this for tx?

Thanks

[Qemu-devel] [PATCH] net: Transmit zero UDP checksum as 0xFFFF

[Ed Swierk via Qemu-devel]

The checksum algorithm used by IPv4, TCP and UDP allows a zero value
to be represented by either 0x and 0x. But per RFC 768, a zero
UDP checksum must be transmitted as 0x, as 0x is a special
value meaning no checksum.

Substitute 0x 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
0x.)

Signed-off-by: Ed Swierk 
---
 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 0x */
+stw_be_p(data + sloc, sum ? sum : 0x);
 }
 }
 
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 = 0x; /* For UDP, zero checksum must be sent as 0x */
+}
 
 /* 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 = 0x; /* For UDP, zero checksum must be sent as 0x */
+}
 iov_from_buf(>vec[NET_TX_PKT_PL_START_FRAG], pkt->payload_frags,
  pkt->virt_hdr.csum_offset, , 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 = 0x; /* For UDP, zero checksum must be sent as 0x */
+}
 iov_from_buf(iov, iov_len, csum_offset, , 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 0x */
+stw_be_p(data + vhdr->csum_offset, sum ? sum : 0x);
 
 vhdr->flags &= ~VIRTIO_NET_HDR_F_NEEDS_CSUM;
 vhdr->flags |= VIRTIO_NET_HDR_F_DATA_VALID;
-- 
1.9.1