This long email asks for no one's close attention but Florian's.  Other
readers can skim the email or skip it, at their discretion.

On Wed, Oct 20, 2021 at 10:12:02AM +0200, Florian Weimer wrote:

quoted

quoted

What does this mean?  I think only byte 0x2f is reserved.  The UTF-8
comment is misleading.  A historic/overlong encoding of / in multiple
UTF-8 bytes is *not* reserved.

I had not known that UTF-8 had an alternate encoding for any ASCII
character.  Does it indeed have an alternate encoding?  If so, where
can I learn more?

See the Security Considerations section in the RFC:

  <https://datatracker.ietf.org/doc/html/rfc3629#section-10>

Most file systems do not treat file names as UTF-8, so they do not
perform any validation.

I see.  That RFC explains it well:  there exists no legal alternate
encoding, but rather several illegal encodings that, were they not
illegal, *would be* alternate encodings.  In the case of the solidus,
the legal encoding is 2F but the illegal encodings are

    C0 AF
    E0 80 AF
    F0 80 80 AF
    F8 80 80 80 AF
    FC 80 80 80 80 AF

This problem has nothing to do with Unicode but is merely an artifact
of UTF-8 -- and that's your point, isn't it?  Most filesystems do not
care about UTF-8, so they do not perform any validation.

In view of your advice, I should think about how to rewrite the relevant
prose so that it is neither [i] confusing to inexperienced users
nor [ii] inaccurate.

Question:  the filename(7) manual page ought to emphasize the
requirements of filesystems widely deployed for general-purpose use on
standard Linux installations.  As far as I know, exactly three such
filesystems exist:

    * ext4;
    * xfs;
    * btrfs.

Do any other such filesystems exist?

Comments:

1.  I have heard of reiserfs and reiser4 but have not heard of anyone
that actually uses them since about 15 years ago.

2.  There are also nfs, iso9660/joliet/rockridge, vfat, ntfs, cifs
and a few others.  These are network-oriented, archive-oriented,
special-purpose, foreign and/or compatibility-oriented filesystems.  If
the filename(7) manual page mentions the requirements of such
filesystems at all, it should mention them only briefly, in passing.
Otherwise, the page would become too confusing and grow too long.
(Also, I know too little about most of these extra filesystems to write
about them.)

3.  Happily, the three main filesystems -- ext4, xfs and btrfs -- all
have similar filename requirements as far as I know.