9bind.c

/* © 2008 sqweek <sqweek@gmail.com>
 * See COPYING for details.
 */
#define _GNU_SOURCE //for O_PATH
#include <err.h>
#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>

#include <sys/stat.h>
#include <sys/types.h>
#include <sys/mount.h>

int
main(int argc, char **argv)
{
	char *old = NULL, *new = NULL;
	int mountfd;
	char mountbuf[64];
	struct stat stbuf;

	while (*++argv) {
		if (!old) {
			old = *argv;
		} else if (!new) {
			new = *argv;
		} else {
			errx(1, "%s: too many arguments", *argv);
		}
	}

	if (!old || !new) {
		errx(1, "usage: 9bind old new");
	}

	/* Make sure mount exists, is writable, and not sticky */
	mountfd = open(new, O_PATH | O_DIRECTORY | O_NOFOLLOW);
	if (mountfd == -1 || fstat(mountfd, &stbuf) || faccessat(mountfd, ".", W_OK, 0)) {
		err(1, "%s", new);
	}
	if (stbuf.st_mode & S_ISVTX) {
		errx(1, "%s: refusing to bind over sticky directory", new);
	}

	snprintf(mountbuf, sizeof(mountbuf), "/proc/self/fd/%d", mountfd);
	if (mount(old, mountbuf, NULL, MS_BIND, NULL)) {
		err(1, "mount");
	}

	return 0;
}