Re: [e2fsprogs] ext2_dir_entry To ext2_dir_entry_2 Casting

[Wade Cline <clinew@linux.vnet.ibm.com>]

On 10/02/2012 02:08 PM, Theodore Ts'o wrote:

> On Tue, Oct 02, 2012 at 12:02:22PM -0700, Wade Cline wrote:
>> Hello Theodore Ts'o,
>>
>> Is there a function similar to ext2fs_dir_iterate2() that will call a hook
>> function on an ext2_dir_entry_2 structure and not an ext2_dir_entry
>> structure?
>>
>> The reason I ask is because btrfs-convert currently tries to do a cast
>> between the two structures as such:
>>
>> 	static int dir_iterate_proc(..., struct ext2_dir_entry *old, ...)
>> 	{
>> 	...
>> 		struct ext2_dir_entry_2 *dirent = (struct ext2_dir_entry_2 *)old;
>>
>> which works fine on little-endian machines, but breaks on big-endian machines.
> The reason why we don't have that function today is because what most
> programs (especially inside e2fsprogs) have done is to use the
> ext2_dir_entry structure and just simply used (name_len&  0xff) to get
> the actual name length, and in the case of e2fsck (which is the only
> program which in practice has cared about the file type) to get the
> file type by doing (name_len>>  8).
>
I see. That should work fine, too.

>> If there isn't, would you be interested in a patch that adds a
>> function, say, ext2_dir_entry_upgrade(struct ext2_dir_entry *old,
>> struct ext2_dir_entry_2 *new) that will convert one structure to the
>> other and take into account the endianness of the machine? This
>> would be better than just ad-hoc fixing the code in btrfs.
> We could do that, but that would mean needing to copy the data
> structure somewhere else, which means you would have to allocate
> memory for the new data structure, and then you'd have to release that
> memory later, etc.
>
> I would think that using (name_len&  0xFF) is a much simpler solution,
> and my suggestion is to not depend on the file type in the directory
> entry (since there might be some very old ext2 file systems that don't
> set the file type), and to use the inode's mode bits as authoratative
> for the file type of the inode.
>
> Regards,
>
> 							- Ted
Interesting compatibility issue. Will keep it in mind.

Thank you for the help.

Sincerely,
Wade