Linux: Fix mount/unmount when dataset name has a space

The custom zpl_show_devname() helper should translate spaces in
to the octal escape sequence \040.  The getmntent(2) function
is aware of this convention and properly translates the escape
character back to a space when reading the fsname.

Without this change the `zfs mount` and `zfs unmount` commands
incorrectly detect when a dataset with a name containing spaces
is mounted.

Reviewed-by: Ryan Moeller <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes openzfs#11182 
Closes openzfs#11187

Author: behlendorf

module/os/linux/zfs/zpl_super.c

@@ -185,13 +185,26 @@ zpl_remount_fs(struct super_block *sb, int *flags, char *data)
 static int
 __zpl_show_devname(struct seq_file *seq, zfsvfs_t *zfsvfs)
 {
-	char *fsname;
-
 	ZFS_ENTER(zfsvfs);
-	fsname = kmem_alloc(ZFS_MAX_DATASET_NAME_LEN, KM_SLEEP);
+
+	char *fsname = kmem_alloc(ZFS_MAX_DATASET_NAME_LEN, KM_SLEEP);
 	dmu_objset_name(zfsvfs->z_os, fsname);
-	seq_puts(seq, fsname);
+
+	for (int i = 0; fsname[i] != 0; i++) {
+		/*
+		 * Spaces in the dataset name must be converted to their
+		 * octal escape sequence for getmntent(3) to correctly
+		 * parse then fsname portion of /proc/self/mounts.
+		 */
+		if (fsname[i] == ' ') {
+			seq_puts(seq, "\\040");
+		} else {
+			seq_putc(seq, fsname[i]);
+		}
+	}
+
 	kmem_free(fsname, ZFS_MAX_DATASET_NAME_LEN);
+
 	ZFS_EXIT(zfsvfs);
 
 	return (0);

tests/zfs-tests/tests/functional/cli_root/zfs_create/zfs_create_001_pos.ksh

@@ -51,6 +51,8 @@ function cleanup
 			log_must zfs destroy -f ${datasets[$i]}
 		((i = i + 1))
 	done
+
+	zfs destroy -f "$TESTPOOL/with a space"
 }
 
 log_onexit cleanup
@@ -68,4 +70,8 @@ while (( $i < ${#datasets[*]} )); do
 	((i = i + 1))
 done
 
+log_must zfs create "$TESTPOOL/with a space"
+log_must zfs unmount "$TESTPOOL/with a space"
+log_must zfs mount "$TESTPOOL/with a space"
+
 log_pass "'zfs create <filesystem>' works as expected."