[v2] net: Transmit zero UDP checksum as 0xFFFF

Commit Message

Denis V. Lunev" via Nov. 15, 2017, 2:57 a.m. UTC

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 when
modifying a UDP datagram header. Doing this on IPv4 packets and TCP
segments is unnecessary but legal. Add a wrapper for
net_checksum_finish() that makes the substitution.

(We can't just change net_checksum_finish(), as that function is also
used by receivers to verify checksums, and in that case the expected
value is always 0x0000.)

v2:

Add a wrapper net_checksum_finish_hdr() rather than duplicating the
logic at every caller.

Signed-off-by: Ed Swierk <[e-mail]>
---
 hw/net/e1000.c         | 2 +-
 hw/net/net_rx_pkt.c    | 2 +-
 hw/net/net_tx_pkt.c    | 6 +++---
 hw/net/vmxnet3.c       | 3 ++-
 include/net/checksum.h | 7 +++++++
 5 files changed, 14 insertions(+), 6 deletions(-)

Comments

Denis V. Lunev" via Nov. 15, 2017, 11:42 p.m. UTC | #1

On Tue, Nov 14, 2017 at 6:57 PM, Ed Swierk <[e-mail]> wrote:
> diff --git a/hw/net/net_tx_pkt.c b/hw/net/net_tx_pkt.c
> index 20b2549..dc95f12 100644
> --- a/hw/net/net_tx_pkt.c
> +++ b/hw/net/net_tx_pkt.c
> @@ -126,12 +126,12 @@ void net_tx_pkt_update_ip_checksums(struct NetTxPkt *pkt)
>
>          /* Calculate IP pseudo header checksum */
>          cntr = eth_calc_ip4_pseudo_hdr_csum(ip_hdr, pkt->payload_len, &cso);
> -        csum = cpu_to_be16(~net_checksum_finish(cntr));
> +        csum = cpu_to_be16(~net_checksum_finish_hdr(cntr));
>      } else if (gso_type == VIRTIO_NET_HDR_GSO_TCPV6) {
>          /* Calculate IP pseudo header checksum */
>          cntr = eth_calc_ip6_pseudo_hdr_csum(ip_hdr, pkt->payload_len,
>                                              IP_PROTO_TCP, &cso);
> -        csum = cpu_to_be16(~net_checksum_finish(cntr));
> +        csum = cpu_to_be16(~net_checksum_finish_hdr(cntr));
>      } else {
>          return;
>      }

Actually this change looks wrong. The checksum here is just the
partial sum of the IP pseudo-header fields, to be incorporated into
the actual UDP or TCP checksum (including the packet payload) at some
later stage. The partial sum should never be zero (if
net_rx_pkt_fix_l4_csum() is to be believed), but changing 0x0000 to
0xFFFF and then storing the complement ensures just that.

Unless someone with more of a clue says otherwise, I'll drop this change in v3.

--Ed

Patch

diff --git a/hw/net/e1000.c b/hw/net/e1000.c
index d642314..4e33667 100644
--- a/hw/net/e1000.c
+++ b/hw/net/e1000.c
@@ -506,7 +506,7 @@  putsum(uint8_t *data, uint32_t n, uint32_t sloc, uint32_t css, uint32_t cse)
         n = cse + 1;
     if (sloc < n-1) {
         sum = net_checksum_add(n-css, data+css);
-        stw_be_p(data + sloc, net_checksum_finish(sum));
+        stw_be_p(data + sloc, net_checksum_finish_hdr(sum));
     }
 }
 
diff --git a/hw/net/net_rx_pkt.c b/hw/net/net_rx_pkt.c
index 1019b50..a2b2c2d 100644
--- a/hw/net/net_rx_pkt.c
+++ b/hw/net/net_rx_pkt.c
@@ -517,7 +517,7 @@  _net_rx_pkt_calc_l4_csum(struct NetRxPkt *pkt)
     cntr += net_checksum_add_iov(pkt->vec, pkt->vec_len,
                                  pkt->l4hdr_off, csl, cso);
 
-    csum = net_checksum_finish(cntr);
+    csum = net_checksum_finish_hdr(cntr);
 
     trace_net_rx_pkt_l4_csum_calc_csum(pkt->l4hdr_off, csl, cntr, csum);
 
diff --git a/hw/net/net_tx_pkt.c b/hw/net/net_tx_pkt.c
index 20b2549..dc95f12 100644
--- a/hw/net/net_tx_pkt.c
+++ b/hw/net/net_tx_pkt.c
@@ -126,12 +126,12 @@  void net_tx_pkt_update_ip_checksums(struct NetTxPkt *pkt)
 
         /* Calculate IP pseudo header checksum */
         cntr = eth_calc_ip4_pseudo_hdr_csum(ip_hdr, pkt->payload_len, &cso);
-        csum = cpu_to_be16(~net_checksum_finish(cntr));
+        csum = cpu_to_be16(~net_checksum_finish_hdr(cntr));
     } else if (gso_type == VIRTIO_NET_HDR_GSO_TCPV6) {
         /* Calculate IP pseudo header checksum */
         cntr = eth_calc_ip6_pseudo_hdr_csum(ip_hdr, pkt->payload_len,
                                             IP_PROTO_TCP, &cso);
-        csum = cpu_to_be16(~net_checksum_finish(cntr));
+        csum = cpu_to_be16(~net_checksum_finish_hdr(cntr));
     } else {
         return;
     }
@@ -486,7 +486,7 @@  static void net_tx_pkt_do_sw_csum(struct NetTxPkt *pkt)
         net_checksum_add_iov(iov, iov_len, pkt->virt_hdr.csum_start, csl, cso);
 
     /* Put the checksum obtained into the packet */
-    csum = cpu_to_be16(net_checksum_finish(csum_cntr));
+    csum = cpu_to_be16(net_checksum_finish_hdr(csum_cntr));
     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..ee7cdeb 100644
--- a/hw/net/vmxnet3.c
+++ b/hw/net/vmxnet3.c
@@ -970,7 +970,8 @@  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));
+    stw_be_p(data + vhdr->csum_offset,
+             net_checksum_finish_hdr(net_checksum_add(len, data)));
 
     vhdr->flags &= ~VIRTIO_NET_HDR_F_NEEDS_CSUM;
     vhdr->flags |= VIRTIO_NET_HDR_F_DATA_VALID;
diff --git a/include/net/checksum.h b/include/net/checksum.h
index 7df472c..9878c8b 100644
--- a/include/net/checksum.h
+++ b/include/net/checksum.h
@@ -34,6 +34,13 @@  net_checksum_add(int len, uint8_t *buf)
 }
 
 static inline uint16_t
+net_checksum_finish_hdr(uint32_t sum)
+{
+    uint16_t s = net_checksum_finish(sum);
+    return s ? s : 0xFFFF;
+}
+
+static inline uint16_t
 net_raw_checksum(uint8_t *data, int length)
 {
     return net_checksum_finish(net_checksum_add(length, data));