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Working checksum computation

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This commit is contained in:
Quentin 2021-04-25 16:56:35 +02:00
parent 1ee8439ed8
commit 4b9bd48053
Signed by: quentin
GPG key ID: A98E9B769E4FF428
2 changed files with 35 additions and 52 deletions
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57
xdp/test.c
View file

@ -20,40 +20,30 @@ char pkt3[] = { 0x50,0x9a,0x4c,0x24,0x1a,0x72,0x04,0xf0,0x21,0x65,0x18,0x12,0x08
char csDesired[] = { 0x38, 0xf1 };
// Checksum utilities
__attribute__((__always_inline__))
static inline __u16 csum_fold_helper(__u64 csum) {
int i;
#pragma unroll
for (i = 0; i < 4; i ++) {
if (csum >> 16)
csum = (csum & 0xffff) + (csum >> 16);
}
return ~csum;
static __always_inline __u16 csum_fold_helper(__u32 csum)
{
return ~((csum & 0xffff) + (csum >> 16));
}
// Update checksum following RFC 1624 (Eqn. 3): https://tools.ietf.org/html/rfc1624
// HC' = ~(~HC + ~m + m')
// Where :
// HC - old checksum in header
// HC' - new checksum in header
// m - old value
// m' - new value
__attribute__((__always_inline__))
static inline void update_csum(__u64 *csum, __be32 old_flag,__be32 new_flag ) {
// ~HC
*csum = ~*csum;
*csum = *csum & 0xffff;
// + ~m
__u32 tmp;
tmp = ~old_addr;
*csum += tmp;
// + m
*csum += new_addr;
// then fold and complement result !
static __always_inline void ipv4_csum(void *data_start, int data_size, __u32 *csum) {
*csum = bpf_csum_diff(0, 0, data_start, data_size, *csum);
*csum = csum_fold_helper(*csum);
}
int checksum(__u16* ipbuf, __u32 len) {
// https://tools.ietf.org/html/rfc1071
__u32 sum = 0;
for (int i = 0; i < sizeof(struct iphdr)/2; i++) {
sum += ipbuf[i];
}
sum = (sum & 0xffff) + (sum >> 16);
sum = (sum & 0xffff) + (sum >> 16);
return ~sum;
}
// @TODO: Implement https://tools.ietf.org/html/rfc1624 instead
int main(void) {
void *data = pkt3;
@ -62,10 +52,9 @@ int main(void) {
struct ethhdr *eth = data; //structure ethernet from if_ether.h
struct iphdr *ip = (struct iphdr*)(eth + 1);
printf("%#06x\n", bpf_ntohs(ip->check));
__u64 cs = iph->check;
update_csum(&cs, old_addr, new_addr);
iph->check = cs;
printf("new cs: %x\n", ip->check);
ip->check = 0;
ip->check = checksum((__u16*)ip, sizeof(*ip));
printf("%#06x\n", bpf_ntohs(ip->check));
}
int main2(void) {

22
xdp/xdp_udp.c
View file

@ -32,24 +32,18 @@
##__VA_ARGS__); \
})
static inline unsigned short checksum(unsigned short *buf, int bufsz) {
unsigned long sum = 0;
while (bufsz > 1) {
sum += *buf;
buf++;
bufsz -= 2;
static __always_inline int checksum_ipv4(__u16* ipbuf) {
// https://tools.ietf.org/html/rfc1071
__u32 sum = 0;
for (int i = 0; i < sizeof(struct iphdr)/2; i++) {
sum += ipbuf[i];
}
if (bufsz == 1) {
sum += *(unsigned char *)buf;
}
sum = (sum & 0xffff) + (sum >> 16);
sum = (sum & 0xffff) + (sum >> 16);
return ~sum;
}
// @TODO: Implement https://tools.ietf.org/html/rfc1624 instead
SEC("xdp_udp")
int xdp_udp_func(struct xdp_md *ctx)
@ -74,8 +68,8 @@ int xdp_udp_func(struct xdp_md *ctx)
if(ip->protocol == IPPROTO_UDP) {
ip->frag_off |= bpf_htons(0x4000); //Set the DF flag to 1 -> 0b 0100 0000 0000 0000
ip->check = 0x0000;
ip->check = checksum((unsigned short*)ip, sizeof(ip));
ip->check = 0;
ip->check = checksum_ipv4((__u16*)ip);
}
}
return XDP_PASS;