Add two options to allow mount to avoid covering up existing mount points.

The two options are

* nocover/cover:  Prevent/allow mounting over an existing root mountpoint.
E.g., "mount -t ufs -o nocover /dev/sd1a /usr/local" will fail if /usr/local
is already a mountpoint.
* emptydir/noemptydir:  Prevent/allow mounting on a non-empty directory.
E.g., "mount -t ufs -o emptydir /dev/sd1a /usr" will fail.

Neither of these options is intended to be a default, for historical and
compatibility reasons.

Reviewed by:	allanjude, kib
Differential Revision:	https://reviews.freebsd.org/D21458

3 files changed, 15 insertions, 3 deletions

diff --git a/sbin/mount/mntopts.h b/sbin/mount/mntopts.h
index 924ead253890..183d6d9e501d 100644
--- a/sbin/mount/mntopts.h
+++ b/sbin/mount/mntopts.h
@@ -65,7 +65,8 @@ struct mntopt {
 #define MOPT_UPDATE		{ "update",	0, MNT_UPDATE, 0 }
 #define MOPT_RO			{ "ro",		0, MNT_RDONLY, 0 }
 #define MOPT_RW			{ "rw",		1, MNT_RDONLY, 0 }
-
+#define	MOPT_NOCOVER		{ "cover",	1, MNT_NOCOVER, 0 }
+#define	MOPT_EMPTYDIR		{ "emptydir",	0, MNT_EMPTYDIR, 0 }
 /* This is parsed by mount(8), but is ignored by specific mount_*(8)s. */
 #define MOPT_AUTO		{ "auto",	0, 0, 0 }
 
@@ -95,7 +96,9 @@ struct mntopt {
 	MOPT_ACLS,							\
 	MOPT_NFS4ACLS,							\
 	MOPT_AUTOMOUNTED,						\
-	MOPT_UNTRUSTED
+	MOPT_UNTRUSTED,							\
+	MOPT_NOCOVER,							\
+	MOPT_EMPTYDIR
 
 void getmntopts(const char *, const struct mntopt *, int *, int *);
 void rmslashes(char *, char *);
diff --git a/sbin/mount/mount.8 b/sbin/mount/mount.8
index 7ff94fb7c165..3aee1bb86151 100644
--- a/sbin/mount/mount.8
+++ b/sbin/mount/mount.8
@@ -28,7 +28,7 @@
 .\"     @(#)mount.8	8.8 (Berkeley) 6/16/94
 .\" $FreeBSD$
 .\"
-.Dd March 22, 2017
+.Dd August 28, 2019
 .Dt MOUNT 8
 .Os
 .Sh NAME
@@ -162,6 +162,8 @@ When used with the
 .Fl u
 flag, this is the same as specifying the options currently in effect for
 the mounted file system.
+.It Cm emptydir
+Require that the mount point directory be empty.
 .It Cm force
 The same as
 .Fl f ;
@@ -237,6 +239,9 @@ flag.
 Disable read clustering.
 .It Cm noclusterw
 Disable write clustering.
+.It Cm nocover
+Do not mount if the requested mount point is already
+the root of a mount point.
 .It Cm noexec
 Do not allow execution of any binaries on the mounted file system.
 This option is useful for a server that has file systems containing
diff --git a/sbin/mount/mount.c b/sbin/mount/mount.c
index 102e87009b0f..938a220b41c3 100644
--- a/sbin/mount/mount.c
+++ b/sbin/mount/mount.c
@@ -119,6 +119,8 @@ static struct opt {
 	{ MNT_AUTOMOUNTED,	"automounted" },
 	{ MNT_VERIFIED,		"verified" },
 	{ MNT_UNTRUSTED,	"untrusted" },
+	{ MNT_NOCOVER,		"nocover" },
+	{ MNT_EMPTYDIR,		"emptydir" },
 	{ 0, NULL }
 };
 
@@ -975,6 +977,8 @@ flags2opts(int flags)
 	if (flags & MNT_ACLS)		res = catopt(res, "acls");
 	if (flags & MNT_NFS4ACLS)	res = catopt(res, "nfsv4acls");
 	if (flags & MNT_UNTRUSTED)	res = catopt(res, "untrusted");
+	if (flags & MNT_NOCOVER)	res = catopt(res, "nocover");
+	if (flags & MNT_EMPTYDIR)	res = catopt(res, "emptydir");
 
 	return (res);
 }