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net: qualcomm: rmnet: avoid unnecessary byte-swapping

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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 <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Alex Elder
2021-06-11 14:05:28 -05:00
committed by David S. Miller
parent e5adbbdfa2
commit a2918a169f
1 changed files with 4 additions and 4 deletions
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8
drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
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@@ -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 {