Add support for mounting single files in nullfs

My main use-case for this is to support mounting config files and secrets
into OCI containers. My current workaround copies the files into the
container which is messy and risks secrets leaking into container images
if the cleanup fails.

Reviewed by:	mjg, kib
Tested by:	pho
Differential Revision: https://reviews.freebsd.org/D37478

sbin/mount/getmntopts.c

@@ -139,6 +139,20 @@ checkpath(const char *path, char *resolved)
 	return (0);
 }
 
+int
+checkpath_allow_file(const char *path, char *resolved)
+{
+	struct stat sb;
+
+	if (realpath(path, resolved) == NULL || stat(resolved, &sb) != 0)
+		return (1);
+	if (!S_ISDIR(sb.st_mode) && !S_ISREG(sb.st_mode)) {
+		errno = ENOTDIR;
+		return (1);
+	}
+	return (0);
+}
+
 void
 build_iovec(struct iovec **iov, int *iovlen, const char *name, void *val,
 	    size_t len)

sbin/mount/mntopts.h

@@ -103,6 +103,7 @@ struct mntopt {
 void getmntopts(const char *, const struct mntopt *, int *, int *);
 void rmslashes(char *, char *);
 int checkpath(const char *, char resolved_path[]);
+int checkpath_allow_file(const char *, char resolved_path[]);
 extern int getmnt_silent;
 void build_iovec(struct iovec **iov, int *iovlen, const char *name, void *val, size_t len);
 void build_iovec_argf(struct iovec **iov, int *iovlen, const char *name, const char *fmt, ...);

sbin/mount/mount.c

@@ -89,6 +89,7 @@ struct statfs *getmntpt(const char *);
 int	hasopt(const char *, const char *);
 int	ismounted(struct fstab *, struct statfs *, int);
 int	isremountable(const char *);
+int	allow_file_mount(const char *);
 void	mangle(char *, struct cpa *);
 char   *update_options(char *, char *, int);
 int	mountfs(const char *, const char *, const char *,
@@ -502,6 +503,15 @@ isremountable(const char *vfsname)
 	return (0);
 }
 
+int
+allow_file_mount(const char *vfsname)
+{
+
+	if (strcmp(vfsname, "nullfs") == 0)
+		return (1);
+	return (0);
+}
+
 int
 hasopt(const char *mntopts, const char *option)
 {
@@ -548,9 +558,16 @@ mountfs(const char *vfstype, const char *spec, const char *name, int flags,
 	static struct cpa mnt_argv;
 
 	/* resolve the mountpoint with realpath(3) */
-	if (checkpath(name, mntpath) != 0) {
-		xo_warn("%s", mntpath);
-		return (1);
+	if (allow_file_mount(vfstype)) {
+		if (checkpath_allow_file(name, mntpath) != 0) {
+			xo_warn("%s", mntpath);
+			return (1);
+		}
+	} else {
+		if (checkpath(name, mntpath) != 0) {
+			xo_warn("%s", mntpath);
+			return (1);
+		}
 	}
 	name = mntpath;
 

sbin/mount_nullfs/mount_nullfs.8

@@ -64,6 +64,17 @@ but in other respects it is indistinguishable from the original.
 .Pp
 The
 .Nm
+utility supports mounting both directories and single files.
+Both
+.Ar target
+and
+.Ar mount_point
+must be the same type.
+Mounting directories to files or files to
+directories is not supported.
+.Pp
+The
+.Nm
 file system differs from a traditional
 loopback file system in two respects: it is implemented using
 a stackable layers techniques, and its

sbin/mount_nullfs/mount_nullfs.c

@@ -48,6 +48,7 @@ static const char rcsid[] =
 
 #include <sys/param.h>
 #include <sys/mount.h>
+#include <sys/stat.h>
 #include <sys/uio.h>
 
 #include <err.h>
@@ -61,6 +62,14 @@ static const char rcsid[] =
 
 static void	usage(void) __dead2;
 
+static int
+stat_realpath(const char *path, char *resolved, struct stat *sbp)
+{
+	if (realpath(path, resolved) == NULL || stat(resolved, sbp) != 0)
+		return (1);
+	return (0);
+}
+
 int
 main(int argc, char *argv[])
 {
@@ -71,6 +80,8 @@ main(int argc, char *argv[])
 	char errmsg[255];
 	int ch, iovlen;
 	char nullfs[] = "nullfs";
+	struct stat target_stat;
+	struct stat mountpoint_stat;
 
 	iov = NULL;
 	iovlen = 0;
@@ -98,10 +109,18 @@ main(int argc, char *argv[])
 		usage();
 
 	/* resolve target and mountpoint with realpath(3) */
-	if (checkpath(argv[0], target) != 0)
+	if (stat_realpath(argv[0], target, &target_stat) != 0)
 		err(EX_USAGE, "%s", target);
-	if (checkpath(argv[1], mountpoint) != 0)
+	if (stat_realpath(argv[1], mountpoint, &mountpoint_stat) != 0)
 		err(EX_USAGE, "%s", mountpoint);
+	if (!S_ISDIR(target_stat.st_mode) && !S_ISREG(target_stat.st_mode))
+		errx(EX_USAGE, "%s: must be either a file or directory",
+		    target);
+	if ((target_stat.st_mode & S_IFMT) !=
+	    (mountpoint_stat.st_mode & S_IFMT))
+		errx(EX_USAGE,
+		    "%s: must be same type as %s (file or directory)",
+		    mountpoint, target);
 
 	build_iovec(&iov, &iovlen, "fstype", nullfs, (size_t)-1);
 	build_iovec(&iov, &iovlen, "fspath", mountpoint, (size_t)-1);