[PATCH 2/3] Ext4Pkg: Add documentation surrounding ext4 directory entries

["Pedro Falcato" <pedro.falcato@gmail.com>]

Several questions have popped up regarding the ext4 directory entry
layout and contents off-list. Attempt to clarify these issues by adding
some explanatory comments.

Signed-off-by: Pedro Falcato <pedro.falcato@gmail.com>
Cc: Marvin Häuser <mhaeuser@posteo.de>
---
 Features/Ext4Pkg/Ext4Dxe/Ext4Disk.h | 21 +++++++++++++++++++--
 Features/Ext4Pkg/Ext4Dxe/Ext4Dxe.h  |  5 ++---
 2 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/Features/Ext4Pkg/Ext4Dxe/Ext4Disk.h b/Features/Ext4Pkg/Ext4Dxe/Ext4Disk.h
index 4fd91a423324..d0a455d0e572 100644
--- a/Features/Ext4Pkg/Ext4Dxe/Ext4Disk.h
+++ b/Features/Ext4Pkg/Ext4Dxe/Ext4Disk.h
@@ -1,7 +1,7 @@
 /** @file
   Raw filesystem data structures
 
-  Copyright (c) 2021 Pedro Falcato All rights reserved.
+  Copyright (c) 2021 - 2023 Pedro Falcato All rights reserved.
   SPDX-License-Identifier: BSD-2-Clause-Patent
 
   Layout of an EXT2/3/4 filesystem:
@@ -397,12 +397,29 @@ typedef struct _Ext4Inode {
   UINT32       i_projid;
 } EXT4_INODE;
 
+#define EXT4_NAME_MAX  255
+
 typedef struct {
+  // offset 0x0: inode number (if 0, unused entry, should skip.)
   UINT32    inode;
+  // offset 0x4: Directory entry's length.
+  //             Note: rec_len >= name_len + EXT4_MIN_DIR_ENTRY_LEN and rec_len % 4 == 0.
   UINT16    rec_len;
+  // offset 0x6: Directory entry's name's length
   UINT8     name_len;
+  // offset 0x7: Directory entry's file type indicator
   UINT8     file_type;
-  CHAR8     name[255];
+  // offset 0x8: name[name_len]: Variable length character array; not null-terminated.
+  CHAR8     name[EXT4_NAME_MAX];
+  // Further notes on names:
+  // 1) We use EXT4_NAME_MAX here instead of flexible arrays for ease of use around the driver.
+  //
+  // 2) ext4 directories are defined, as the documentation puts it, as:
+  // "a directory is more or less a flat file that maps an arbitrary byte string
+  // (usually ASCII) to an inode number on the filesystem". So, they are not
+  // necessarily encoded with ASCII, UTF-8, or any of the sort. We must treat it
+  // as a bag of bytes. When interacting with EFI interfaces themselves (which expect UCS-2)
+  // we skip any directory entry that is not valid UTF-8.
 } EXT4_DIR_ENTRY;
 
 #define EXT4_MIN_DIR_ENTRY_LEN  8
diff --git a/Features/Ext4Pkg/Ext4Dxe/Ext4Dxe.h b/Features/Ext4Pkg/Ext4Dxe/Ext4Dxe.h
index adf3c13f6ea9..466e49523030 100644
--- a/Features/Ext4Pkg/Ext4Dxe/Ext4Dxe.h
+++ b/Features/Ext4Pkg/Ext4Dxe/Ext4Dxe.h
@@ -1,7 +1,7 @@
 /** @file
   Common header for the driver
 
-  Copyright (c) 2021 - 2022 Pedro Falcato All rights reserved.
+  Copyright (c) 2021 - 2023 Pedro Falcato All rights reserved.
   SPDX-License-Identifier: BSD-2-Clause-Patent
 **/
 
@@ -31,8 +31,7 @@
 
 #include "Ext4Disk.h"
 
-#define SYMLOOP_MAX    8
-#define EXT4_NAME_MAX  255
+#define SYMLOOP_MAX  8
 //
 // We need to specify path length limit for security purposes, to prevent possible
 // overflows and dead-loop conditions. Originally this limit is absent in FS design,
-- 
2.39.0