[PATCH 1/1] 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 | 18 +++++++++++++++++-
 Features/Ext4Pkg/Ext4Dxe/Ext4Dxe.h  |  3 +--
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/Features/Ext4Pkg/Ext4Dxe/Ext4Disk.h b/Features/Ext4Pkg/Ext4Dxe/Ext4Disk.h
index 4fd91a423324..2dad967e575d 100644
--- a/Features/Ext4Pkg/Ext4Dxe/Ext4Disk.h
+++ b/Features/Ext4Pkg/Ext4Dxe/Ext4Disk.h
@@ -397,12 +397,28 @@ typedef struct _Ext4Inode {
   UINT32       i_projid;
 } EXT4_INODE;
 
+#define EXT4_NAME_MAX  255
+
 typedef struct {
+  // ext4 directory entries are layed out in the following fashion:
+  // offset 0x0: UINT32 inode number (if 0, unused entry, should skip.)
+  // offset 0x4: UINT16 rec_len: Dir entry's length.
+  //             Note: rec_len >= name_len + EXT4_MIN_DIR_ENTRY_LEN and rec_len % 4 == 0.
+  // offset 0x6: UINT8 name_len: Dir entry's name's length
+  // offset 0x7: UINT8 file_type Dir entry's file type indicator
+  // offset 0x8: CHAR8 name[name_len]: Variable length character array; not null-terminated.
+  //
+  // Further note: 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.
   UINT32    inode;
   UINT16    rec_len;
   UINT8     name_len;
   UINT8     file_type;
-  CHAR8     name[255];
+  CHAR8     name[EXT4_NAME_MAX];
 } 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..81ba568c5947 100644
--- a/Features/Ext4Pkg/Ext4Dxe/Ext4Dxe.h
+++ b/Features/Ext4Pkg/Ext4Dxe/Ext4Dxe.h
@@ -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