Internet checksums are used for IPv4 header checksum, as well as TCP
segment and UDP datagram checksums. Such a checksum represents the
negated sum of adjacent pairs of bytes, using ones' complement
arithmetic.
One property of the Internet checkum is byte order independence [1].
Specifically, the sum of byte-swapped pairs is equal to the result
of byte swapping the sum of those same pairs when not byte-swapped.
So for example if a, b, c, d, y, and z are hexadecimal digits, and
PLUS represents ones' complement addition:
If: ab PLUS cd = yz
Then: ba PLUS dc = zy
For this reason, there is no need to swap the order of bytes in the
checksum value held in a message header, nor the one in the QMAPv4
trailer, in order to operate on them.
In other words, we can determine whether the hardware-computed
checksum matches the one in the message header without any byte
swaps.
(This patch leaves in place all existing type casts.)
[1] https://tools.ietf.org/html/rfc1071
Signed-off-by: Alex Elder <[email protected]>
---
drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)
diff --git a/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c b/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
index 4f93355e9a93a..39f198d7595bd 100644
--- a/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
+++ b/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
@@ -78,15 +78,15 @@ rmnet_map_ipv4_dl_csum_trailer(struct sk_buff *skb,
* trailer checksum. Therefore the checksum in the trailer is
* just the checksum computed over the IP payload.
*/
- ip_payload_csum = (__force __sum16)~ntohs(csum_trailer->csum_value);
+ ip_payload_csum = (__force __sum16)~csum_trailer->csum_value;
pseudo_csum = ~csum_tcpudp_magic(ip4h->saddr, ip4h->daddr,
ntohs(ip4h->tot_len) - ip4h->ihl * 4,
ip4h->protocol, 0);
- addend = (__force __be16)ntohs((__force __be16)pseudo_csum);
+ addend = (__force __be16)pseudo_csum;
pseudo_csum = csum16_add(ip_payload_csum, addend);
- addend = (__force __be16)ntohs((__force __be16)*csum_field);
+ addend = (__force __be16)*csum_field;
csum_temp = ~csum16_sub(pseudo_csum, addend);
csum_value_final = (__force u16)csum_temp;
@@ -105,7 +105,7 @@ rmnet_map_ipv4_dl_csum_trailer(struct sk_buff *skb,
}
}
- if (csum_value_final == ntohs((__force __be16)*csum_field)) {
+ if (csum_value_final == (__force u16)*csum_field) {
priv->stats.csum_ok++;
return 0;
} else {
--
2.27.0