[PATCH v2 13/15] ip: Define a bound for the string returned by ipproto_name()

[David Gibson <david@gibson.dropbear.id.au>]

ipproto_name() returns a static string of theoretically unbounded length.
That's going to be inconvenient in future, so introduce IPPROTO_STRLEN
giving an explicit bound on the length.  Use static_assert() and some
macros to ensure nothing we return can exceed this.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 ip.c | 18 ++++++++++++------
 ip.h |  1 +
 2 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/ip.c b/ip.c
index 4e4e0bf5..f2506bb1 100644
--- a/ip.c
+++ b/ip.c
@@ -15,6 +15,7 @@
  * Author: Stefano Brivio <sbrivio@redhat.com>
  */
 
+#include <assert.h>
 #include <stddef.h>
 #include <netinet/in.h>
 
@@ -24,7 +25,7 @@
  * ipproto_name() - Get IP protocol name from number
  * @proto:	IP protocol number
  *
- * Return: pointer to name of protocol @proto
+ * Return: pointer to name of protocol @proto (<= IPPROTO_STRLEN bytes)
  *
  * Usually this would be done with getprotobynumber(3) but that reads
  * /etc/protocols and might allocate, which isn't possible for us once
@@ -33,16 +34,21 @@
 const char *ipproto_name(uint8_t proto)
 {
 	switch (proto) {
+#define CASE(s)								\
+		static_assert(sizeof(s) <= IPPROTO_STRLEN,		\
+			      "Increase IPPROTO_STRLEN to contain " #s); \
+		return s;
 	case IPPROTO_ICMP:
-		return "ICMP";
+		CASE("ICMP");
 	case IPPROTO_TCP:
-		return "TCP";
+		CASE("TCP");
 	case IPPROTO_UDP:
-		return "UDP";
+		CASE("UDP");
 	case IPPROTO_ICMPV6:
-		return "ICMPv6";
+		CASE("ICMPv6");
 	default:
-		return "<unknown protocol>";
+		CASE("<unknown protocol>");
+#undef CASE
 	}
 }
 
diff --git a/ip.h b/ip.h
index f6c29e00..aab9b86a 100644
--- a/ip.h
+++ b/ip.h
@@ -117,6 +117,7 @@ static inline uint32_t ip6_get_flow_lbl(const struct ipv6hdr *ip6h)
 		ip6h->flow_lbl[2];
 }
 
+#define IPPROTO_STRLEN		(sizeof("<unknown protocol>"))
 const char *ipproto_name(uint8_t proto);
 
 /* IPv6 link-local all-nodes multicast address, ff02::1 */
-- 
2.53.0