Change /etc/mtab to /proc/self/mounts

Fix misleading error message:

 "The /dev/zfs device is missing and must be created.", if /etc/mtab is missing.

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Eric Desrochers <eric.desrochers@canonical.com>
Closes #4680 
Closes #5029

lib/libspl/include/sys/mnttab.h

@@ -38,7 +38,7 @@
 #undef MNTTAB
 #endif /* MNTTAB */
 
-#define	MNTTAB		"/etc/mtab"
+#define	MNTTAB		"/proc/self/mounts"
 #define	MNT_LINE_MAX	4096
 
 #define	MNT_TOOLONG	1	/* entry exceeds MNT_LINE_MAX */

lib/libzfs/libzfs_dataset.c

@@ -1908,9 +1908,9 @@ zfs_unset_recvd_props_mode(zfs_handle_t *zhp, uint64_t *cookie)
  * zfs_prop_get_int() are built using this interface.
  *
  * Certain properties can be overridden using 'mount -o'.  In this case, scan
- * the contents of the /etc/mtab entry, searching for the appropriate options.
- * If they differ from the on-disk values, report the current values and mark
- * the source "temporary".
+ * the contents of the /proc/self/mounts entry, searching for the
+ * appropriate options. If they differ from the on-disk values, report the
+ * current values and mark the source "temporary".
  */
 static int
 get_numeric_property(zfs_handle_t *zhp, zfs_prop_t prop, zprop_source_t *src,
@@ -1981,8 +1981,9 @@ get_numeric_property(zfs_handle_t *zhp, zfs_prop_t prop, zprop_source_t *src,
 
 	/*
 	 * Because looking up the mount options is potentially expensive
-	 * (iterating over all of /etc/mtab), we defer its calculation until
-	 * we're looking up a property which requires its presence.
+	 * (iterating over all of /proc/self/mounts), we defer its
+	 * calculation until we're looking up a property which requires
+	 * its presence.
 	 */
 	if (!zhp->zfs_mntcheck &&
 	    (mntopt_on != NULL || prop == ZFS_PROP_MOUNTED)) {

lib/libzfs/libzfs_mount.c

@@ -347,8 +347,8 @@ zfs_add_option(zfs_handle_t *zhp, char *options, int len,
 		return (0);
 
 	/*
-	 * zfs_prop_get_int() to not used to ensure our mount options
-	 * are not influenced by the current /etc/mtab contents.
+	 * zfs_prop_get_int() is not used to ensure our mount options
+	 * are not influenced by the current /proc/self/mounts contents.
 	 */
 	value = getprop_uint64(zhp, prop, &source);
 
@@ -1184,8 +1184,8 @@ mountpoint_compare(const void *a, const void *b)
  * Unshare and unmount all datasets within the given pool.  We don't want to
  * rely on traversing the DSL to discover the filesystems within the pool,
  * because this may be expensive (if not all of them are mounted), and can fail
- * arbitrarily (on I/O error, for example).  Instead, we walk /etc/mtab and
- * gather all the filesystems that are currently mounted.
+ * arbitrarily (on I/O error, for example).  Instead, we walk /proc/self/mounts
+ * and gather all the filesystems that are currently mounted.
  */
 int
 zpool_disable_datasets(zpool_handle_t *zhp, boolean_t force)

lib/libzfs/libzfs_util.c

@@ -67,9 +67,9 @@ libzfs_error_init(int error)
 		    "loaded.\nTry running '/sbin/modprobe zfs' as root "
 		    "to load them.\n"));
 	case ENOENT:
-		return (dgettext(TEXT_DOMAIN, "The /dev/zfs device is "
-		    "missing and must be created.\nTry running 'udevadm "
-		    "trigger' as root to create it.\n"));
+		return (dgettext(TEXT_DOMAIN, "/dev/zfs and /proc/self/mounts "
+		    "are required.\nTry running 'udevadm trigger' and 'mount "
+		    "-t proc proc /proc' as root.\n"));
 	case ENOEXEC:
 		return (dgettext(TEXT_DOMAIN, "The ZFS modules cannot be "
 		    "auto-loaded.\nTry running '/sbin/modprobe zfs' as "