gbproxy: Add gbprox_clear_patch_filter() (Coverity)

Add a separate function to clear the IMSI filter to be used instead of
gbprox_set_patch_filter(cfg, NULL, ...). Albeit it fixes a Coverity
issue (Unchecked return value), it is a false positive, since the
return value is always 0 in these cases. Nevertheless it is more
obvious what happens when an explicit clear function is called. Using
NULL as filter argument of gbprox_set_patch_filter still clears the
filter.

Fixes: Coverity CID 1231255
Sponsored-by: On-Waves ehf

openbsc/include/openbsc/gb_proxy.h

@@ -115,6 +115,7 @@ void gbprox_reset(struct gbproxy_config *cfg);
 
 int gbprox_set_patch_filter(struct gbproxy_config *cfg, const char *filter,
 		const char **err_msg);
+void gbprox_clear_patch_filter(struct gbproxy_config *cfg);
 
 void gbprox_delete_tlli(struct gbproxy_peer *peer,
 			       struct gbproxy_tlli_info *tlli_info);

openbsc/src/gprs/gb_proxy.c

@@ -481,16 +481,21 @@ static void gbprox_delete_tllis(struct gbproxy_peer *peer)
 	OSMO_ASSERT(llist_empty(&state->enabled_tllis));
 }
 
+void gbprox_clear_patch_filter(struct gbproxy_config *cfg)
+{
+	if (cfg->check_imsi) {
+		regfree(&cfg->imsi_re_comp);
+		cfg->check_imsi = 0;
+	}
+}
+
 int gbprox_set_patch_filter(struct gbproxy_config *cfg, const char *filter,
 		const char **err_msg)
 {
 	static char err_buf[300];
 	int rc;
 
-	if (cfg->check_imsi) {
-		regfree(&cfg->imsi_re_comp);
-		cfg->check_imsi = 0;
-	}
+	gbprox_clear_patch_filter(cfg);
 
 	if (!filter)
 		return 0;

openbsc/src/gprs/gb_proxy_vty.c

@@ -193,7 +193,7 @@ static int set_core_apn(struct vty *vty, const char *apn, const char *filter)
 		talloc_free(g_cfg->core_apn);
 		g_cfg->core_apn = NULL;
 		g_cfg->core_apn_size = 0;
-		gbprox_set_patch_filter(g_cfg, NULL, NULL);
+		gbprox_clear_patch_filter(g_cfg);
 		return CMD_SUCCESS;
 	}
 
@@ -206,7 +206,7 @@ static int set_core_apn(struct vty *vty, const char *apn, const char *filter)
 	}
 
 	if (!filter) {
-		gbprox_set_patch_filter(g_cfg, NULL, NULL);
+		gbprox_clear_patch_filter(g_cfg);
 	} else if (gbprox_set_patch_filter(g_cfg, filter, &err_msg) != 0) {
 		vty_out(vty, "Match expression invalid: %s%s",
 			err_msg, VTY_NEWLINE);

openbsc/tests/gbproxy/gbproxy_test.c

@@ -1571,6 +1571,12 @@ static void test_gbproxy_imsi_matching(void)
 	OSMO_ASSERT(gbprox_set_patch_filter(&cfg, NULL, &err_msg) == 0);
 	OSMO_ASSERT(cfg.check_imsi == 0);
 
+	OSMO_ASSERT(gbprox_set_patch_filter(&cfg, filter_re2, &err_msg) == 0);
+	OSMO_ASSERT(cfg.check_imsi == 1);
+
+	gbprox_clear_patch_filter(&cfg);
+	OSMO_ASSERT(cfg.check_imsi == 0);
+
 	peer = gbproxy_peer_alloc(&cfg, 20);
 
 	OSMO_ASSERT(gbprox_set_patch_filter(&cfg, filter_re2, &err_msg) == 0);