From a2918a169f57f965e4e5949822c2602c90e388ab Mon Sep 17 00:00:00 2001 From: Alex Elder Date: Fri, 11 Jun 2021 14:05:28 -0500 Subject: [PATCH] net: qualcomm: rmnet: avoid unnecessary byte-swapping 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 Signed-off-by: David S. Miller --- 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 4f93355..39f198d 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 { -- 2.7.4