1 files changed, 94 insertions, 0 deletions
diff --git a/xdp_checksum.h b/xdp_checksum.h
new file mode 100644
index 0000000..1d35565
--- /dev/null
+++ b/xdp_checksum.h
@@ -0,0 +1,94 @@
+#ifndef XDP_CHECKSUM_H
+#define XDP_CHECKSUM_H 1
+
+#include <linux/ip.h>
+#include <linux/udp.h>
+#include <stdlib.h>
+
+static __always_inline __u16 ip_checksum(struct iphdr *iph) {
+  __u32 sum = 0;
+  __u16 *ptr = (__u16 *)iph;
+
+// IP header is iph->ihl * 4 bytes in size --> iph->ihl * 2 16 bit works
+#pragma unroll
+  for (int i = 0; i < 10; i++) { // max 20 bytes (5 * 4)
+    if (i >= iph->ihl * 2)
+      break;
+    if (i == 5)
+      continue; // checksum field at offset 10–11, skip for calc
+    sum += (__u32)ptr[i];
+  }
+
+  while (sum >> 16)
+    sum = (sum & 0xFFFF) + (sum >> 16);
+
+  return ~sum;
+}
+
+static __always_inline __u16 csum_fold_helper(__u32 sum) {
+  while (sum >> 16)
+    sum = (sum & 0xffff) + (sum >> 16);
+  return ~sum;
+}
+
+static __always_inline __u16 csum_add(__u32 sum, __u32 value) {
+  sum += value;
+  return sum;
+}
+
+static __always_inline __u32 csum16_add(__u32 sum, __u16 val) {
+  sum += val;
+  if (sum > 0xffff)
+    sum -= 0xffff;
+  return sum;
+}
+
+static __always_inline __u16 udp_checksum(struct iphdr *iph,
+                                          struct udphdr *udph, void *data_end) {
+  __u32 sum = 0;
+
+  // IP header source/dest address + layer4 protocol
+  sum = csum16_add(sum, (__u16)(iph->saddr >> 16));
+  sum = csum16_add(sum, (__u16)(iph->saddr & 0xffff));
+  sum = csum16_add(sum, (__u16)(iph->daddr >> 16));
+  sum = csum16_add(sum, (__u16)(iph->daddr & 0xffff));
+  sum = csum16_add(sum, __constant_htons(IPPROTO_UDP));
+  // UDP header length
+  sum = csum16_add(sum, udph->len);
+  // UDP header checksum
+  __u16 *ptr = (__u16 *)udph;
+#pragma unroll
+  for (size_t i = 0; i < sizeof(struct udphdr) / 2; i++) {
+    if ((void *)(ptr + 1) > data_end)
+      break;
+    sum = csum16_add(sum, *ptr);
+    ptr++;
+  }
+
+  // UDP payload checksum
+  void *payload = (void *)udph + sizeof(struct udphdr);
+  int payload_len = __constant_ntohs(udph->len) - sizeof(struct udphdr);
+  ptr = (__u16 *)payload;
+
+  for (int i = 0; i < 256; i++) { // max 512 bytes
+    if (payload_len <= 0)
+      break;
+    if ((void *)(ptr + 1) > data_end)
+      break;
+    sum = csum16_add(sum, *ptr);
+    ptr++;
+    payload_len -= 2;
+  }
+
+  // if payload length off: pad wiuth null
+  if (payload_len == 1) {
+    __u8 *last = (__u8 *)ptr;
+    if ((void *)(last + 1) <= data_end) {
+      sum = csum16_add(sum, (*last) << 8);
+    }
+  }
+
+  return csum_fold_helper(sum);
+}
+
+#endif