zdtm: add mntns_pivot_root_ro test

This checks that superblock readonly flag is applied to nested mntns
roots on restore.

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

test/zdtm/static/Makefile

@@ -402,6 +402,7 @@ TST_DIR		=				\
 		mnt_ext_root			\
 		mnt_ext_collision		\
 		mntns_pivot_root		\
+		mntns_pivot_root_ro		\
 		mnt_tracefs			\
 		mntns_deleted			\
 		unlink_regular00		\
@@ -580,6 +581,7 @@ socket-tcp-closed-last-ack:	CFLAGS += -D ZDTM_TCP_LAST_ACK
 socket-tcp-skip-in-flight:	CFLAGS += -D ZDTM_IPV4V6
 tun_ns:			CFLAGS += -DTUN_NS
 mnt_ext_manual:		CFLAGS += -D ZDTM_EXTMAP_MANUAL
+mntns_pivot_root_ro:	CFLAGS += -DMNTNS_PIVOT_ROOT_RO
 sigpending:		LDLIBS += -lrt
 vdso01:			LDLIBS += -lrt
 scm01:			CFLAGS += -DKEEP_SENT_FD

test/zdtm/static/mntns_pivot_root.c

@@ -8,6 +8,7 @@
 #include <linux/limits.h>
 #include <sys/syscall.h>
 #include <unistd.h>
+#include <errno.h>
 
 #include "zdtmtst.h"
 #include "lock.h"
@@ -86,6 +87,21 @@ static int child(void)
 	}
 	close(fd);
 
+#ifdef MNTNS_PIVOT_ROOT_RO
+	/*
+	 * Hack to make cr_pivot_root work on readonly mntns root,
+	 * normally nested containers have /tmp directory
+	 */
+	mkdir("tmp", 0755);
+	/*
+	 * Make superblock readonly
+	 */
+	if (mount(NULL, "/", NULL, MS_REMOUNT | MS_RDONLY, NULL)) {
+		pr_perror("remount_ro");
+		goto err;
+	}
+#endif
+
 	futex_set_and_wake(futex, TEST_CHILD);
 	futex_wait_while_lt(futex, TEST_CHECK);
 
@@ -94,6 +110,21 @@ static int child(void)
 		goto err;
 	}
 
+#ifdef MNTNS_PIVOT_ROOT_RO
+	/*
+	 * Check superblock readonly
+	 */
+	fd = open(testfile, O_WRONLY);
+	if (fd >= 0) {
+		pr_err("Open on readonly superblock should fail\n");
+		close(fd);
+		goto err;
+	} else if (errno != EROFS) {
+		pr_perror("open write");
+		goto err;
+	}
+#endif
+
 	futex_set_and_wake(futex, TEST_EXIT);
 	return 0;
 err:

test/zdtm/static/mntns_pivot_root_ro.c

@@ -0,0 +1 @@
+mntns_pivot_root.c

test/zdtm/static/mntns_pivot_root_ro.desc

@@ -0,0 +1 @@
+mntns_pivot_root.desc