reproducer for "fix IS_MNT_PROPAGATING uses"

[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;
}

int 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");
}

int setup(void)
{
	int fd;

	playground();

	mkdir("A", 0700);
	mkdir("B", 0700);
	mkdir("C", 0700);

	tmpfs("A");
	change("A", MS_SHARED);
	bind("A", "B");
	change("B", MS_SLAVE);
	change("B", MS_SHARED);
	bind("B", "C");
	change("C", MS_SLAVE);

	mkdir("A/foo", 0700);
	mkdir("A/subdir", 0700);
	fd = open_tree(AT_FDCWD, "B", OPEN_TREE_CLONE);
	change("B", MS_PRIVATE);
	return fd;
}

main()
{
	int fd;

	fd = setup();
	bind("B", "A/subdir");
	printf("bind propagated to C: %s\n",
		exists("C/subdir/foo") ? "yes" : "no");
	cleanup(fd);

	fd = setup();
	move_mount(AT_FDCWD, "B", AT_FDCWD, "A/subdir", MOVE_MOUNT_F_EMPTY_PATH);
	printf("move_mount propagated to C: %s\n",
		exists("C/subdir/foo") ? "yes" : "no");
	cleanup(fd);

	fd = setup();
	move_mount(fd, "", AT_FDCWD, "A/subdir", MOVE_MOUNT_F_EMPTY_PATH);
	printf("move_mount (fd) propagated to C: %s\n",
		exists("C/subdir/foo") ? "yes" : "no");
	cleanup(fd);

	return 0;
}