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.)

v3:

Leave net_tx_pkt_update_ip_checksums() alone since it's only computing
a partial sum of the IP pseudo-header.

Rename wrapper to net_checksum_finish_nozero() for clarity.

v2:

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

Signed-off-by: Ed Swierk <eswierk@skyportsystems.com>
---
 hw/net/e1000.c         | 2 +-
 hw/net/net_rx_pkt.c    | 2 +-
 hw/net/net_tx_pkt.c    | 2 +-
 hw/net/vmxnet3.c       | 3 ++-
 include/net/checksum.h | 6 ++++++
 5 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/hw/net/e1000.c b/hw/net/e1000.c
index 72a92be..804ec08 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_nozero(sum));
     }
 }
 
diff --git a/hw/net/net_rx_pkt.c b/hw/net/net_rx_pkt.c
index cef1c2e..98a5030 100644
--- a/hw/net/net_rx_pkt.c
+++ b/hw/net/net_rx_pkt.c
@@ -518,7 +518,7 @@ _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_nozero(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..e29c881 100644
--- a/hw/net/net_tx_pkt.c
+++ b/hw/net/net_tx_pkt.c
@@ -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_nozero(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 8c4bae5..cdc307d 100644
--- a/hw/net/vmxnet3.c
+++ b/hw/net/vmxnet3.c
@@ -972,7 +972,8 @@ 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_nozero(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..05a0d27 100644
--- a/include/net/checksum.h
+++ b/include/net/checksum.h
@@ -34,6 +34,12 @@ net_checksum_add(int len, uint8_t *buf)
 }
 
 static inline uint16_t
+net_checksum_finish_nozero(uint32_t sum)
+{
+    return net_checksum_finish(sum) ?: 0xFFFF;
+}
+
+static inline uint16_t
 net_raw_checksum(uint8_t *data, int length)
 {
     return net_checksum_finish(net_checksum_add(length, data));
-- 
1.9.1

On 11/15/2017 09:07 PM, Ed Swierk via Qemu-devel wrote:

[meta-comment]

> 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.)

This part below...

> 
> v3:
> 
> Leave net_tx_pkt_update_ip_checksums() alone since it's only computing
> a partial sum of the IP pseudo-header.
> 
> Rename wrapper to net_checksum_finish_nozero() for clarity.
> 
> v2:
> 
> Add a wrapper net_checksum_finish_hdr() rather than duplicating the
> logic at every caller.

...through here, belongs...

> 
> Signed-off-by: Ed Swierk <eswierk@skyportsystems.com>
> ---

...here, after the --- separator (you can have more than one ---; 'git
am' stops parsing at the first).  It is data that is useful to
reviewers, but will be meaningless a year from now when browsing git
history (at which point we won't care how many revisions the patch went
through on the list, but only what was checked in).

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

On Wed, Nov 15, 2017 at 7:55 PM, Eric Blake <eblake@redhat.com> wrote:
>
> On 11/15/2017 09:07 PM, Ed Swierk via Qemu-devel wrote:
>
> This part below...
>
> >
> > v3:
> >
> > Leave net_tx_pkt_update_ip_checksums() alone since it's only computing
> > a partial sum of the IP pseudo-header.
> >
> > Rename wrapper to net_checksum_finish_nozero() for clarity.
> >
> > v2:
> >
> > Add a wrapper net_checksum_finish_hdr() rather than duplicating the
> > logic at every caller.
>
> ...through here, belongs...
>
> >
> > Signed-off-by: Ed Swierk <eswierk@skyportsystems.com>
> > ---
>
> ...here, after the --- separator (you can have more than one ---; 'git
> am' stops parsing at the first).  It is data that is useful to
> reviewers, but will be meaningless a year from now when browsing git
> history (at which point we won't care how many revisions the patch went
> through on the list, but only what was checked in).

Thanks for the clue. I'll repost a corrected version.

Maybe checkpatch.pl should squawk if it sees ^v[0-9]\+: before the first ---?

--Ed