Always allow '-o remount,ro'

Allow the mount(8) utility to always operate on all datasets when
remounting them read-only.  This critical for rc.sysinit/umountroot
which remounts the root filesystem read-only during shutdown to
ensure everything is correctly flushed to disk.

Fix minor typo, the check to set zfsutil should use the bitwise
'&'.  I must have accidentally hit the adjacent '*' and obviously
neither the compiler or my code review caught this.  Fix it now.

diff --git a/cmd/mount_zfs/mount_zfs.c b/cmd/mount_zfs/mount_zfs.c
index fbb954a..52e5992 100644 (file)
--- a/cmd/mount_zfs/mount_zfs.c
+++ b/cmd/mount_zfs/mount_zfs.c
@@ -298,7 +298,7 @@ main(int argc, char **argv)
        char mntopts[MNT_LINE_MAX] = { '\0' };
        char badopt[MNT_LINE_MAX] = { '\0' };
        char *dataset, *mntpoint;
-       unsigned long mntflags = 0, zfsflags = 0;
+       unsigned long mntflags = 0, zfsflags = 0, remount_ro = 0;
        int sloppy = 0, fake = 0, verbose = 0, nomtab = 0, zfsutil = 0;
        int error, c;
 
@@ -402,7 +402,10 @@ main(int argc, char **argv)
        if (mntflags & MS_REMOUNT)
                nomtab = 1;
 
-       if (zfsflags * ZS_ZFSUTIL)
+       if ((mntflags & MS_REMOUNT) && (mntflags & MS_RDONLY))
+               remount_ro = 1;
+
+       if (zfsflags & ZS_ZFSUTIL)
                zfsutil = 1;
 
        if ((g_zfs = libzfs_init()) == NULL)
@@ -427,6 +430,10 @@ main(int argc, char **argv)
         * 'zfs mount'.  However, since 'zfs mount' actually invokes 'mount'
         * we differentiate the two cases using the 'zfsutil' mount option.
         * This mount option should only be supplied by the 'zfs mount' util.
+        *
+        * The only exception to the above rule is '-o remount,ro'.  This is
+        * always allowed for non-legacy datasets for rc.sysinit/umountroot
+        * to safely remount the root filesystem and flush its cache.
         */
        if (zfsutil && !strcmp(legacy, ZFS_MOUNTPOINT_LEGACY)) {
                (void) fprintf(stderr, gettext(
@@ -437,7 +444,7 @@ main(int argc, char **argv)
                return (MOUNT_USAGE);
        }
 
-       if (!zfsutil && strcmp(legacy, ZFS_MOUNTPOINT_LEGACY)) {
+       if (!zfsutil && strcmp(legacy, ZFS_MOUNTPOINT_LEGACY) && !remount_ro) {
                (void) fprintf(stderr, gettext(
                    "filesystem '%s' cannot be mounted using 'mount'.\n"
                    "Use 'zfs set mountpoint=%s' or 'zfs mount %s'.\n"