reproducer for "do_move_mount(): don't leak MNTNS_PROPAGATING on failures"

[Al Viro <viro@xxxxxxxxxxxxxxxxxx>]

#include <string.h>
#include <unistd.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <stdio.h>

static int tmpfs(const char *name)
{
	return mount("none", name, "tmpfs", 0, NULL);
}
static int change(const char *name, int how)
{
	return mount(NULL, name, NULL, how, NULL);
}
static int bind(const char *from, const char *to)
{
	return mount(from, to, NULL, MS_BIND, NULL);
}
static _Bool exists(const char *name)
{
	return access(name, F_OK) != -1;
}

void playground(void)
{
	mkdir("/tmp/foo", 0700);
	tmpfs("/tmp/foo");
	change("/tmp/foo", MS_PRIVATE);
	chdir("/tmp/foo");
}

void cleanup(int fd)
{
	close(fd);
	chdir("/tmp");
	umount2("/tmp/foo", MNT_DETACH);
	rmdir("/tmp/foo");
}

main()
{
	playground();

	mkdir("A", 0700);
        mkdir("A/subdir", 0700);
	mkdir("B", 0700);
	bind("A", "A");
	change("A", MS_SHARED);

        int fd = open_tree(AT_FDCWD, "A", OPEN_TREE_CLONE);

	// this move_mount should fail (directory on top of non-directory)
        if (move_mount(fd, "", AT_FDCWD, "/dev/null", MOVE_MOUNT_F_EMPTY_PATH) == 0) {
		printf("unexpected success of first move_mount()\n");
		cleanup(fd);
		return -1;
	}
	// this should propagate into detached tree
	tmpfs("A/subdir");
	mkdir("A/subdir/foo", 0700);
	// move detached tree in, so we could check it
	if (move_mount(fd, "", AT_FDCWD, "B", MOVE_MOUNT_F_EMPTY_PATH) != 0) {
		printf("unexpected failure of the second move_mount()\n");
		cleanup(fd);
	}
	if (!exists("B/subdir/foo")) {
		printf("failed to propagate into detached tree\n");
		cleanup(fd);
		return -1;
	}

	printf("success\n");
	cleanup(fd);
	return 0;
}