libsepol: add sepol_av_perm_to_string

Add a wrapper around the utility function sepol_av_to_string() on the
service internal policy.  This allows callers to convert a permission
bit set into a string representation without access to the internal
policy structure.

Acked-by: James Carter <jwcart2@gmail.com>
Signed-off-by: Christian Göttsche <cgzones@googlemail.com>

libsepol/include/sepol/policydb/services.h

@@ -103,6 +103,15 @@ extern int sepol_string_to_av_perm(sepol_security_class_t tclass,
 					const char *perm_name,
 					sepol_access_vector_t *av);
 
+/*
+ * Return a string representation of the permission av bit associated with
+ * tclass.
+ * Returns a pointer to an internal buffer, overridden by the next call to
+ * this function or sepol_av_to_string().
+ */
+ extern const char *sepol_av_perm_to_string(sepol_security_class_t tclass,
+					sepol_access_vector_t av);
+
 /*
  * Compute a SID to use for labeling a new object in the 
  * class `tclass' based on a SID pair.  

libsepol/src/services.c

@@ -1233,6 +1233,12 @@ out:
 	return STATUS_ERR;
 }
 
+ const char *sepol_av_perm_to_string(sepol_security_class_t tclass,
+					sepol_access_vector_t av)
+{
+	return sepol_av_to_string(policydb, tclass, av);
+}
+
 /*
  * Write the security context string representation of 
  * the context associated with `sid' into a dynamically