zdtm: add mnt_ro_root test

It makes root mount readonly and checks that it is still readonly after
migration.

Make zdtm/static writable for logs via "bind" desc option.

v2: explain why we don't have explicit rw/ro flag check
v3: use new zdtm "bind" desc option

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>

test/zdtm/lib/ns.c

@@ -57,6 +57,9 @@ static int prepare_mntns(void)
 	if (zdtm_bind) {
 		/*
 		 * Bindmount the directory to itself.
+		 * e.g.: The mnt_ro_root test makes "/" mount readonly, but we
+		 * still want to write logs to /zdtm/static/ so let's make it
+		 * separate writable bind mount.
 		 */
 		snprintf(bind_path, sizeof(bind_path),  "%s/%s", root, zdtm_bind);
 		if (mount(bind_path, bind_path, NULL, MS_BIND, NULL)) {

test/zdtm/static/Makefile

@@ -423,6 +423,7 @@ TST_DIR		=				\
 		mntns_ghost			\
 		mntns_ghost01			\
 		mntns_ro_root			\
+		mnt_ro_root			\
 		mntns_link_ghost		\
 		mntns_shared_bind		\
 		mntns_shared_bind02		\

test/zdtm/static/mnt_ro_root.c

@@ -0,0 +1,32 @@
+#include <sys/mount.h>
+
+#include "zdtmtst.h"
+
+const char *test_doc = "Check if root mount remains read-only after c/r";
+const char *test_author = "Pavel Tikhomirov <ptikhomirov@virtuozzo.com>";
+
+char *dirname;
+TEST_OPTION(dirname, string, "directory name", 1);
+
+int main(int argc, char **argv)
+{
+	test_init(argc, argv);
+
+	if (mount(NULL, "/", NULL, MS_REMOUNT | MS_RDONLY | MS_BIND, NULL)) {
+		pr_perror("mount");
+		return 1;
+	}
+
+	test_daemon();
+	test_waitsig();
+
+	/*
+	 * Note: In zdtm.py:check_visible_state() we already check for all
+	 * tests, that all mounts in the test's mount namespace remain the
+	 * same, by comparing mountinfo before and after c/r. So rw/ro mount
+	 * option inconsistency will be detected there and we don't need to
+	 * check it in the test itself.
+	 */
+	pass();
+	return 0;
+}

test/zdtm/static/mnt_ro_root.desc

@@ -0,0 +1,6 @@
+{
+	'flavor': 'ns uns',
+	'flags': 'suid',
+	'feature': 'mnt_id',
+	'bind': 'zdtm/static',
+}