[edk2-devel] [PATCH 1/1] Ext4Pkg: Add documentation surrounding ext4 directory entries

[Pedro Falcato]

On Wed, Jan 11, 2023 at 11:34 PM Marvin Häuser <mhaeuser at posteo.de> wrote:

>
> > On 12. Jan 2023, at 00:21, Pedro Falcato <pedro.falcato at gmail.com>
> wrote:
> >
> > 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 at gmail.com>
> > Cc: Marvin Häuser <mhaeuser at 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
>
> laid out
>
> > 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.
>
> Why not document all that right above the member names? Would remove
> redundant info and be more Doxygen-friendly.
>
> > +  //
> > +  // 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
> >
>

ACK, sending v2.


-- 
Pedro


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