MAC/do: parse_and_set_conf(): Obey empty parameters; Add doc

parse_and_set_conf() is meant to be used in all situations when there is
a need to set or modify some jail's MAC/do configuration.  This entails
passing the information of whether some parameter was explicitly
specified.  For example, an administrator setting/modifying jail
parameters may not specify executable paths but only rules, in which
case the executable paths value is copied from the currently-applicable
configuration.  The sysctl(8) knobs case always leverages this feature,
since setting a knob changes one parameter at a time.

Currently, a NULL or empty string argument is treated as a non-specified
parameter.  This causes a bug where disabling MAC/do in a jail does not
actually work because, to this end, parse_and_set_conf() is passed an
empty string which it then interprets as a request to copy the currently
applicable configuration's value, which may well not be empty.

Fix this problem by only treating NULL as a marker for a non-specified
parameter, in accordance with the original design for this function.

While here, write some documentation to explain the interface.  While
here, remove the original herald comment for parse_and_set_rules(),
which was inadvertently pushed apart from the replacing
parse_and_set_conf().

Reviewed by:    bapt
Fixes:          9818224174 ("MAC/do: Executable paths feature (GSoC 2025's final state)")
MFC after:      1 month
Sponsored by:   The FreeBSD Foundation
Pull Request:   https://ron-dev.freebsd.org/FreeBSD/src/pulls/38

sys/security/mac_do/mac_do.c

@@ -1361,12 +1361,6 @@ set_default_conf(struct prison *const pr)
 	drop_conf(conf);
 }
 
-/*
- * Parse a rules specification and assign them to a jail.
- *
- * Returns the same error code as parse_rules() (which see).
- */
-
 static void
 clone_rules(struct rules *const dst, const struct rules *const src)
 {
@@ -1413,7 +1407,17 @@ clone_exec_paths(struct exec_paths *const dst,
 	    sizeof(dst->exec_paths_str));
 }
 
-/* Must be called with '*parse_error' set to NULL. */
+/*
+ * Sets/modifies the MAC/do configuration for a jail.
+ *
+ * Must be called with '*parse_error' set to NULL.
+ *
+ * Supports explicitly setting all parameters or only some of them, in which
+ * case the implicit ones are copied from the currently applicable configuration
+ * (that of the closest ancestor jail that has one).
+ *
+ * An unspecified parameter must be passed as NULL.
+ */
 static int
 parse_and_set_conf(struct prison *pr, const char *rules_string,
     const char *exec_paths_string, struct parse_error **parse_error)
@@ -1421,17 +1425,13 @@ parse_and_set_conf(struct prison *pr, const char *rules_string,
 	struct conf *applicable_conf = NULL;
 	struct conf *conf;
 	int error = 0;
-	bool need_applicable_conf;
 
-	need_applicable_conf = (rules_string == NULL || rules_string[0] == '\0' ||
-	    exec_paths_string == NULL || exec_paths_string[0] == '\0');
-
-	if (need_applicable_conf)
+	if (rules_string == NULL || exec_paths_string == NULL)
 		applicable_conf = find_conf(pr, NULL);
 
 	conf = new_conf();
 
-	if (rules_string != NULL && rules_string[0] != '\0') {
+	if (rules_string != NULL) {
 		error = parse_rules(rules_string, &conf->rules, parse_error);
 		if (error != 0)
 			goto error;
@@ -1439,7 +1439,7 @@ parse_and_set_conf(struct prison *pr, const char *rules_string,
 	else if (applicable_conf != NULL)
 		clone_rules(&conf->rules, &applicable_conf->rules);
 
-	if (exec_paths_string != NULL && exec_paths_string[0] != '\0') {
+	if (exec_paths_string != NULL) {
 		error = parse_exec_paths(exec_paths_string, &conf->exec_paths,
 		    parse_error);
 		if (error != 0)