Fix .. cluster number for first-level dirs

On FAT32, the cluster number of / is 0 in dir entries (and not the actually
used cluster)

CHANGELOG.md

@@ -43,6 +43,7 @@ Bug fixes:
 * Always respect `fat_type` from `FormatVolumeOptions`
 * Fill FAT32 root directory clusters with zeros after allocation to avoid interpreting old data as directory entries
 * Put '.' and '..' in the first two directory entries. (fixes "Expected a valid '.' entry in this slot." fsck error)
+* Set the cluster number to 0 in the ".." directory entry if it points to the root dir
 
 0.3.4 (2020-07-20)
 ------------------

src/dir.rs

@@ -38,6 +38,13 @@ impl<IO: ReadWriteSeek, TP, OCC> DirRawStream<'_, IO, TP, OCC> {
             DirRawStream::Root(_) => None,
         }
     }
+
+    pub(crate) fn is_root_dir(&self) -> bool {
+        match self {
+            DirRawStream::File(file) => file.is_root_dir(),
+            DirRawStream::Root(_) => true,
+        }
+    }
 }
 
 // Note: derive cannot be used because of invalid bounds. See: https://github.com/rust-lang/rust/issues/26925
@@ -304,8 +311,13 @@ impl<'a, IO: ReadWriteSeek, TP: TimeProvider, OCC: OemCpConverter> Dir<'a, IO, T
                 let sfn_entry = self.create_sfn_entry(dot_sfn, FileAttributes::DIRECTORY, entry.first_cluster());
                 dir.write_entry(".", sfn_entry)?;
                 let dotdot_sfn = ShortNameGenerator::generate_dotdot();
-                let sfn_entry =
-                    self.create_sfn_entry(dotdot_sfn, FileAttributes::DIRECTORY, self.stream.first_cluster());
+                // cluster of the root dir shall be set to 0 in directory entries.
+                let dotdot_cluster = if self.stream.is_root_dir() {
+                    None
+                } else {
+                    self.stream.first_cluster()
+                };
+                let sfn_entry = self.create_sfn_entry(dotdot_sfn, FileAttributes::DIRECTORY, dotdot_cluster);
                 dir.write_entry("..", sfn_entry)?;
                 Ok(dir)
             }

src/file.rs

@@ -214,6 +214,10 @@ impl<'a, IO: ReadWriteSeek, TP, OCC> File<'a, IO, TP, OCC> {
         disk.flush()?;
         Ok(())
     }
+
+    pub(crate) fn is_root_dir(&self) -> bool {
+        self.entry.is_none()
+    }
 }
 
 impl<IO: ReadWriteSeek, TP: TimeProvider, OCC> File<'_, IO, TP, OCC> {