6lowpan: introduce LOWPAN_IPHC_MAX_HC_BUF_LEN

This patch introduces the LOWPAN_IPHC_MAX_HC_BUF_LEN define which
represent the worst-case supported IPHC buffer length. It's used to
allocate the stack buffer space for creating the IPHC header.
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

include/net/6lowpan.h

@@ -64,12 +64,20 @@
 #define EUI64_ADDR_LEN		8
 
 #define LOWPAN_NHC_MAX_ID_LEN	1
+/* Maximum next header compression length which we currently support inclusive
+ * possible inline data.
+ */
+#define LOWPAN_NHC_MAX_HDR_LEN	(sizeof(struct udphdr))
 /* Max IPHC Header len without IPv6 hdr specific inline data.
  * Useful for getting the "extra" bytes we need at worst case compression.
  *
  * LOWPAN_IPHC + CID + LOWPAN_NHC_MAX_ID_LEN
  */
 #define LOWPAN_IPHC_MAX_HEADER_LEN	(2 + 1 + LOWPAN_NHC_MAX_ID_LEN)
+/* Maximum worst case IPHC header buffer size */
+#define LOWPAN_IPHC_MAX_HC_BUF_LEN	(sizeof(struct ipv6hdr) +	\
+					 LOWPAN_IPHC_MAX_HEADER_LEN +	\
+					 LOWPAN_NHC_MAX_HDR_LEN)
 
 /*
  * ipv6 address based on mac

net/6lowpan/iphc.c

@@ -429,7 +429,7 @@ int lowpan_header_compress(struct sk_buff *skb, struct net_device *dev,
 {
 	u8 tmp, iphc0, iphc1, *hc_ptr;
 	struct ipv6hdr *hdr;
-	u8 head[100] = {};
+	u8 head[LOWPAN_IPHC_MAX_HC_BUF_LEN] = {};
 	int ret, addr_type;
 
 	if (type != ETH_P_IPV6)