LSM: SafeSetID: add read handler

For debugging a running system, it is very helpful to be able to see what
policy the system is using. Add a read handler that can dump out a copy of
the loaded policy.

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Micah Morton <mortonm@chromium.org>

security/safesetid/lsm.h

@@ -41,6 +41,7 @@ struct setuid_rule {
 
 struct setuid_ruleset {
 	DECLARE_HASHTABLE(rules, SETID_HASH_BITS);
+	char *policy_str;
 	struct rcu_head rcu;
 };