NetLabel: cleanup and document CIPSO constants

This patch collects all of the CIPSO constants and puts them in one place; it
also documents each value explaining how the value is derived.
Signed-off-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: James Morris <jmorris@namei.org>

net/ipv4/cipso_ipv4.c

@@ -91,6 +91,33 @@ static struct cipso_v4_map_cache_bkt *cipso_v4_cache = NULL;
 int cipso_v4_rbm_optfmt = 0;
 int cipso_v4_rbm_strictvalid = 1;
 
+/*
+ * Protocol Constants
+ */
+
+/* Maximum size of the CIPSO IP option, derived from the fact that the maximum
+ * IPv4 header size is 60 bytes and the base IPv4 header is 20 bytes long. */
+#define CIPSO_V4_OPT_LEN_MAX          40
+
+/* Length of the base CIPSO option, this includes the option type (1 byte), the
+ * option length (1 byte), and the DOI (4 bytes). */
+#define CIPSO_V4_HDR_LEN              6
+
+/* Base length of the restrictive category bitmap tag (tag #1). */
+#define CIPSO_V4_TAG_RBM_BLEN         4
+
+/* Base length of the enumerated category tag (tag #2). */
+#define CIPSO_V4_TAG_ENUM_BLEN        4
+
+/* Base length of the ranged categories bitmap tag (tag #5). */
+#define CIPSO_V4_TAG_RNG_BLEN         4
+/* The maximum number of category ranges permitted in the ranged category tag
+ * (tag #5).  You may note that the IETF draft states that the maximum number
+ * of category ranges is 7, but if the low end of the last category range is
+ * zero then it is possibile to fit 8 category ranges because the zero should
+ * be omitted. */
+#define CIPSO_V4_TAG_RNG_CAT_MAX      8
+
 /*
  * Helper Functions
  */
@@ -1109,15 +1136,12 @@ static int cipso_v4_map_cat_rng_hton(const struct cipso_v4_doi *doi_def,
 				     unsigned char *net_cat,
 				     u32 net_cat_len)
 {
-	/* The constant '16' is not random, it is the maximum number of
-	 * high/low category range pairs as permitted by the CIPSO draft based
-	 * on a maximum IPv4 header length of 60 bytes - the BUG_ON() assertion
-	 * does a sanity check to make sure we don't overflow the array. */
 	int iter = -1;
-	u16 array[16];
+	u16 array[CIPSO_V4_TAG_RNG_CAT_MAX * 2];
 	u32 array_cnt = 0;
 	u32 cat_size = 0;
 
+	/* make sure we don't overflow the 'array[]' variable */
 	BUG_ON(net_cat_len > 30);
 
 	for (;;) {
@@ -1196,9 +1220,6 @@ static int cipso_v4_map_cat_rng_ntoh(const struct cipso_v4_doi *doi_def,
  * Protocol Handling Functions
  */
 
-#define CIPSO_V4_OPT_LEN_MAX          40
-#define CIPSO_V4_HDR_LEN              6
-
 /**
  * cipso_v4_gentag_hdr - Generate a CIPSO option header
  * @doi_def: the DOI definition