linux-ext4 - Re: [f2fs-dev] [PATCH v6 0/9] Support negative dentries on case-insensitive ext4 and f2fs

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Date: Mon, 20 Nov 2023 21:02:54 -0500
From: "Theodore Ts'o" <tytso@....edu>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Christian Brauner <brauner@...nel.org>,
        Gabriel Krisman Bertazi <krisman@...e.de>, viro@...iv.linux.org.uk,
        linux-f2fs-devel@...ts.sourceforge.net, ebiggers@...nel.org,
        linux-fsdevel@...r.kernel.org, jaegeuk@...nel.org,
        linux-ext4@...r.kernel.org
Subject: Re: [f2fs-dev] [PATCH v6 0/9] Support negative dentries on
 case-insensitive ext4 and f2fs

On Mon, Nov 20, 2023 at 10:07:51AM -0800, Linus Torvalds wrote:
> Of course, "do it in shared generic code" doesn't tend to really fix
> the braindamage, but at least it's now shared braindamage and not
> spread out all over. I'm looking at things like
> generic_ci_d_compare(), and it hurts to see the mindless "let's do
> lookups and compares one utf8 character at a time". What a disgrace.
> Somebody either *really* didn't care, or was a Unicode person who
> didn't understand the point of UTF-8.

This isn't because of case-folding brain damage, but rather Unicode
brain damage.  We compare one character at a time because it's
possible for some character like é to either be encoded as 0x0089 (aka
"Latin Small Letter E with Acute") OR as 0x0065 0x0301 ("Latin Small
Letter E" plus "Combining Acute Accent").

Typically, we pretend that UTF-8 means that we can just encode é, or
0x0089 as 0xC3 0xA9 and then call it a day and just use strcmp(3) on
the sucker.  But Unicode is a lot more insane than that.  Technically,
0x65 0xCC 0x81 is the same character as 0xC3 0xA9.

> Oh well. I guess people went "this is going to suck anyway, so let's
> make sure it *really* sucks".

It's more like, "this is going to suck, but if it's going to suck
anyway, let's implement the full Unicode spec in all its gory^H^H^H^H
glory, whether or not it's sane".

					- Ted