#define _GNU_SOURCE
#include <sys/types.h>
#include <sys/mount.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>

#define bail(msg) \
	do { printf("mount_to_symlink: %s: %m\n", msg); exit(1); } while (0)

int is_symlink(const char *path)
{
	struct stat stat = {};
	if (lstat(path, &stat) < 0)
		bail("lstat(<path>)");
	return S_ISLNK(stat.st_mode);
}

int main(int argc, char **argv)
{
	struct stat stat = {};
	char *src, *dst, *src_fdpath, *dst_fdpath;
	int src_fd, dst_fd;

	if (argc != 3)
		bail("usage: mount_to_symlink <src> <dst>");

	src_fdpath = src = argv[1];
	dst_fdpath = dst = argv[2];

	if (is_symlink(src)) {
		// open source fd
		src_fd = open(src, O_PATH | O_CLOEXEC | O_NOFOLLOW);
		if (src_fd < 0)
			bail("open(<src>, O_PATH|O_NOFOLLOW)");
		// construct fd path
		asprintf(&src_fdpath, "/proc/self/fd/%d", src_fd);
	}

	if (is_symlink(dst)) {
		// open target fd
		dst_fd = open(dst, O_PATH | O_CLOEXEC | O_NOFOLLOW);
		if (dst_fd < 0)
			bail("open(<dst>, O_PATH|O_NOFOLLOW)");
		// construct fd path
		asprintf(&dst_fdpath, "/proc/self/fd/%d", dst_fd);
	}

	// try to mount
	mount(src_fdpath, dst_fdpath, "", MS_BIND, "");
	printf("mount(%s, %s, MS_BIND) = %m (%d)\n", src, dst, -errno);
	return 0;
}