Payload increased length now being correctly recognized, but still cannot write to it yet.

src/ebpf/xdp_filter.bpf.c

@@ -127,19 +127,19 @@ int xdp_receive(struct xdp_md *ctx)
         //We must check bounds again, otherwise the verifier gets angry
         data = (void*)(long)ret.ret_md.data;
         data_end = (void*)(long)ret.ret_md.data_end;
-        eth = data;
+        eth = ret.eth;
         if(ethernet_header_bound_check(eth, data_end)<0){
             bpf_printk("Bound check A failed while expanding\n");
             return XDP_PASS; 
         }
 
-        ip = data + sizeof(*eth);
+        ip = ret.ip;
         if (ip_header_bound_check(ip, data_end)<0){
             bpf_printk("Bound check B failed while expanding\n");
             return XDP_PASS;  
         }
 
-        tcp = (void *)ip + sizeof(*ip);
+        tcp = ret.tcp;
         /*if (get_protocol(data_end) != IPPROTO_TCP){
             bpf_printk("Bound check C failed while expanding\n");
             return XDP_PASS; 
@@ -163,8 +163,11 @@ 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);
-    //payload[4] = 'A';
-    //payload[6] = '\0';
+    
+    //char payload_to_write[] = "hello";
+    //__builtin_memcpy(payload, payload_to_write, strlen(payload_to_write));
+
+    //payload[5] = '\0';
     //payload[1] = 'b';
     /*if(!payload){
 		bpf_probe_read_str(&rb_event->payload, sizeof(rb_event->payload), (void *)payload);

src/include/xdp/xdp_helper.h

@@ -12,9 +12,9 @@ static struct expand_return{
     struct xdp_md ret_md;
     void *data;
     void *data_end;
-    struct ethhdr eth;
-    struct iphdr ip;
-    struct tcphdr tcp;
+    struct ethhdr *eth;
+    struct iphdr *ip;
+    struct tcphdr *tcp;
 }expand_return;
 
 /**
@@ -32,7 +32,7 @@ static __always_inline struct expand_return expand_tcp_packet_payload(struct xdp
     struct ethhdr eth_copy;
     struct iphdr ip_copy;
     struct tcphdr tcp_copy;
-    
+
     struct expand_return ret;
 
     //Copy the header for later before expanding ctx
@@ -48,14 +48,42 @@ static __always_inline struct expand_return expand_tcp_packet_payload(struct xdp
         return ret; 
     }
 
+    //We must check bounds again, otherwise the verifier gets angry
+    ret.eth = (void*)(long)ctx->data;
+    ret.ip = (void *)ret.eth + sizeof(struct ethhdr);
+    ret.tcp = (void *)ret.ip + sizeof(struct iphdr);
+    void* data = (void*)(long)ctx->data;
+    void* data_end = (void*)(long)ctx->data_end;
+    if(ethernet_header_bound_check(ret.eth, data_end)<0){
+        bpf_printk("Bound check A failed while expanding\n");
+        ret.code = -1;//The rest is undefined
+        return ret; 
+    }
+
+    if (ip_header_bound_check(ret.ip, data_end)<0){
+        bpf_printk("Bound check B failed while expanding\n");
+        ret.code = -1;//The rest is undefined
+        return ret;   
+    }
+
+    if (tcp_header_bound_check(ret.tcp, data_end)){
+        bpf_printk("Bound check C failed while expanding\n");
+        ret.code = -1;//The rest is undefined
+        return ret; 
+    }
+
     //We now have to readjust the packet headers, checksums have changed
     //Note that we do not care about ctx->data_meta or any other extra field 
     //since we will not be using any communication here
-    __builtin_memcpy(&(ret.eth), &eth_copy, sizeof(struct ethhdr));
-    __builtin_memcpy(&(ret.ip), &ip_copy, sizeof(struct iphdr));
-    __builtin_memcpy(&(ret.tcp), &tcp_copy, sizeof(struct tcphdr));  
+    __builtin_memcpy((ret.eth), &eth_copy, sizeof(struct ethhdr));
+    __builtin_memcpy((ret.ip), &ip_copy, sizeof(struct iphdr));
+    __builtin_memcpy((ret.tcp), &tcp_copy, sizeof(struct tcphdr));  
     
 
+    //We modify the fields we care about of the headers
+    bpf_printk("before: %i\n", ret.ip->tot_len);
+    ret.ip->tot_len = htons(ntohs(ret.ip->tot_len) + more_bytes);
+    bpf_printk("after: %i\n", ret.ip->tot_len);
     ret.ret_md = *ctx;
     ret.code = 0;
     ret.data = (void *)(long)ctx->data;