linux-ext4 - Re: Inquiry: Possible built-in support for longer filenames in ext4 (beyond 256 bytes)

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Message-ID: <20251210090536.GB42106@macsyma.local>
Date: Wed, 10 Dec 2025 18:05:36 +0900
From: "Theodore Tso" <tytso@....edu>
To: Winston Wen <wentao@...ontech.com>
Cc: linux-ext4@...r.kernel.org
Subject: Re: Inquiry: Possible built-in support for longer filenames in ext4
 (beyond 256 bytes)

On Wed, Dec 10, 2025 at 03:02:11PM +0800, Winston Wen wrote:
> We are aware that workarounds like wrapfs can be used to support longer
> filenames, but in practice, this approach is not ideal for seamless
> user experience. We are therefore curious whether it would be feasible
> to implement built-in support for longer filenames in ext4 itself.

I don't think wrapfs can be used to support logner file names, because
the limitation is quite fundamental.  For example, the glibc
definition of struct dirent (which is returned by the readdir() system
call) is as follows (from the man readdir page):

           struct dirent {
               ino_t          d_ino;       /* Inode number */
               off_t          d_off;       /* Not an offset; see below */
               unsigned short d_reclen;    /* Length of this record */
               unsigned char  d_type;      /* Type of file; not supported
                                              by all filesystem types */
               char           d_name[256]; /* Null-terminated filename */
           };

So how you might store the longer file name isn't really going to
help, the problem goes far beyond the question of where this might be
stored on the file system.

					- Ted