examples:dns_matching: fix key length and buffer overrun

Changed key length to 255, maximum length of DNS domain name.
Also fixed double increment of loop variable. These both changes
fix buffer overrun.
Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>

examples/networking/dns_matching/dns_matching.c

@@ -43,7 +43,7 @@ struct dns_char_t
 } BPF_PACKET_HEADER;
 
 struct Key {
-  unsigned char p[32];
+  unsigned char p[255];
 };
 
 struct Leaf {
@@ -85,8 +85,8 @@ int dns_matching(struct __sk_buff *skb)
         u16 i = 0;
         struct dns_char_t *c;
         // This unroll worked not in latest BCC version.
-        for(u8 j = 0; i<255;i++){
-          if (cursor == sentinel) goto end; c = cursor_advance(cursor, 1); key.p[i++] = c->c;
+        for(i = 0; i<255;i++){
+          if (cursor == sentinel) goto end; c = cursor_advance(cursor, 1); key.p[i] = c->c;
         }
         end:
         {}

examples/networking/dns_matching/dns_matching.py

@@ -11,8 +11,8 @@ import struct
 
 
 def encode_dns(name):
-  size = 32
-  if len(name) > 253:
+  size = 255
+  if len(name) > 255:
     raise Exception("DNS Name too long.")
   b = bytearray(size)
   i = 0;