linux-kernel - Re: newlines in filenames; POSIX.1-2024

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Message-ID: <20250422222131.GE569616@mit.edu>
Date: Tue, 22 Apr 2025 17:21:31 -0500
From: "Theodore Ts'o" <tytso@....edu>
To: Alejandro Colomar <alx@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-man@...r.kernel.org
Subject: Re: newlines in filenames; POSIX.1-2024

On Wed, Apr 16, 2025 at 06:50:00PM +0200, Alejandro Colomar wrote:
> 
> I'm updating the manual pages for POSIX.1-2024.  One of the changes
> in this revision is that POSIX now encourages implementations to
> disallow using new-line characters in file names.
> 
> Historically, Linux (and maybe all existing POSIX systems?) has
> allowed new-line characters in file names.

Do we have any information of which implementations (if any) might
decide to disallow new-line characters?

If the Austin Group is going to add these sorts of "encouragements"
without engaging with us dirctly, it seems to be much like King Canute
commanding that the tide not come in....

Personally, I'm not convinced a newline is any different from any
number of weird-sh*t characters, such as zero-width space Unicode
characters, ASCII ETX or EOF characters, etc.

I suppose we could add a new mount option which disallows the
weird-sh*t characters, but I bet it will break some userspace
programs, and it also begs the question of *which* weird-sh*t
characters should be disallowed by the kernel.

> I guess there's no intention to change that behavior.  But I should
> ask.  I thought of adding this paragraph to all pages that create
> file names:
> 
> 	+.SH CAVEATS
> 	+POSIX.1-2024 encourages implementations to
> 	+disallow creation of filenames containing new-line characters.
> 	+Linux doesn't follow this,
> 	+and allows using new-line characters.
> 
> Are there any comments?

I think this is giving the Austin Group way more attention/respect
than they deserve, especially when it's an optional "encourage", but
whatever...

						- Ted