Arbitrary payload modification fully works now. Absolutely ridicous the time it took me (2 days) to realize I was computing the ip checksum with the old checksum values, which made it not to work. Finally can keep going

src/ebpf/xdp_filter.bpf.c

@@ -122,6 +122,7 @@ int xdp_receive(struct xdp_md *ctx)
 
     bpf_printk("OLD data_end: %i, payload: %i\n", data_end, payload);
     struct expand_return ret = expand_tcp_packet_payload(ctx, eth, ip, tcp, 2);
+    bpf_printk("Control back to main program with retcode %i\n", ret.code);
     if(ret.code == 0){
         //We must check bounds again, otherwise the verifier gets angry
         ctx = ret.ret_md;
@@ -156,14 +157,14 @@ int xdp_receive(struct xdp_md *ctx)
         //Quite a trick to avoid the verifier complaining when it's clear we are OK with the payload
         //Line 6367 https://lxr.missinglinkelectronics.com/linux/kernel/bpf/verifier.c
         if(payload_size < 0|| payload_size>88888){
-            bpf_printk("exploding heavily\n");
+            bpf_printk("Unlikely you are here, but OK\n");
             return XDP_PASS;
         }
-        if(payload_size -1 < data_end - (void*)payload ){
+        /*if(payload_size -1 < data_end - (void*)payload ){
             return XDP_PASS;
-        }
+        }*/
 
-        //Revise this, the idea is to use payload_size, but th everifier keeps thinking it will go out of bounds
+        //Revise this, the idea is to use payload_size, but the verifier keeps thinking it will go out of bounds
         //Also, note that sizeof(..) is returning strlen +1, but it's ok because
         //we do not want to write at payload[6]
         if((void*)payload + sizeof(SECRET_PACKET_PAYLOAD) +1 > data_end){
@@ -176,7 +177,8 @@ int xdp_receive(struct xdp_md *ctx)
             return XDP_PASS;
         }
         char* temp_data = (char*)payload;
-        payload[5] = 'a';
+        payload[4] = 'a';
+        payload[5] = '\0';
 
         bpf_printk("BPF finished with ret %i and payload %s of size %i\n ", ret.code, payload, payload_size);
     }else{
@@ -185,7 +187,7 @@ int xdp_receive(struct xdp_md *ctx)
     data_len_next = data_end-data;
     bpf_printk("Previous length: %i, current length: %i\n", data_len_prev, data_len_next);
     bpf_printk("NEW data_end: %i, payload: %i\n", data_end, payload);
-    bpf_printk("And on NEW CTX data_end: %i, payload: %i\n", ctx->data_end);
+    bpf_printk("And on NEW CTX data_end: %i, payload: %i\n", ctx->data_end, payload);
     char payload_to_write[] = "hello";
 
     /*if (tcp_payload_bound_check(payload, payload_size, data_end)){

src/include/packet/protocol/ip_helper.h

@@ -24,7 +24,7 @@ static __always_inline unsigned short checksum(unsigned short *addr, int nbytes)
     if(nbytes>0){
         sum +=htons((unsigned char)*addr);
     }
-            
+        
     while (sum>>16){
         sum = (sum & 0xffff) + (sum >> 16);
     }
@@ -34,30 +34,34 @@ static __always_inline unsigned short checksum(unsigned short *addr, int nbytes)
 }
 
 
-static __always_inline uint16_t csum_fold_helper(uint32_t csum)
+static __always_inline __u16 csum_fold_helper(__u32 csum)
 {
-    bpf_printk("csumA: %u\n", csum & 0xffff);
-    bpf_printk("csumB: %u\n", csum >> 16);
-    bpf_printk("csumA+B: %u\n", (csum & 0xffff) + (csum >> 16));
-    bpf_printk("csumNEG(A+B): %u\n", ~((csum & 0xffff) + (csum >> 16)));
+    //return ~((csum & 0xffff) + (csum >> 16));
+    //The following solves some errors where the last summatory overflows
     #pragma unroll
-    for (int ii = 0; ii < 4; ii++) {
-    if (csum >> 16)
-        csum = (csum & 0xffff) + (csum >> 16);
+    for (int i = 0; i < 4; i ++) {
+        if (csum >> 16){
+            csum = (csum & 0xffff) + (csum >> 16);
+        }
     }
-    return ~csum;
+    return ~csum;                 
 }
 /**
 * IP checksum calculation.
 * Following RFC 1071, using BPFs.*
 */
-static __always_inline void ipv4_csum(void *data_start, int data_size, uint32_t *csum)
+static __always_inline void ipv4_csum(void *data_start, int data_size, __u32 *csum)
 {
-    bpf_printk("csum: %u\n", *csum);
-	*csum = bpf_csum_diff(0, 0, data_start, data_size, *csum);
-	bpf_printk("csum: %u\n", *csum);
+    //WITH EBPF HELPERS
+    bpf_printk("csum: %u for data_start %u, data_size %i\n", *csum, data_start, data_size);
+	
+    unsigned char* p = (unsigned char*) data_start;
+    for(int ii = 0; ii<20; ii++){
+        bpf_printk("B%i: %x\n", ii, p[ii]);
+    }
+
+    *csum = bpf_csum_diff(0, 0, data_start, data_size, *csum);
     *csum = csum_fold_helper(*csum);
-    bpf_printk("csum: %u\n", *csum);
 }
 
 #endif

src/include/xdp/xdp_helper.h

@@ -85,7 +85,8 @@ static __always_inline struct expand_return expand_tcp_packet_payload(struct xdp
     //We modify the fields we care about of the headers
     bpf_printk("before: %i, checksum %u\n", ret.ip->tot_len, ret.ip->check);
     ret.ip->tot_len = htons(ntohs(ret.ip->tot_len) + more_bytes);
-    uint32_t csum = 0;
+    __u32 csum = 0;
+    ret.ip->check = 0;
 	ipv4_csum(ret.ip, sizeof(struct iphdr), &csum);
 	ret.ip->check = csum;
     bpf_printk("after: %i, checksum %u\n", ret.ip->tot_len, ret.ip->check);
@@ -96,4 +97,8 @@ static __always_inline struct expand_return expand_tcp_packet_payload(struct xdp
     return ret;
 }
 
+
+
+
+
 #endif