net: Transmit zero UDP checksum as 0xFFFF

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 because 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 and TCP checksums
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.)

Signed-off-by: Ed Swierk <eswierk@skyportsystems.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

1 files changed, 6 insertions, 0 deletions

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));