linux-kernel - Re: why do we need utf8 normalization when compare name?

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Message-ID: <20200302104741.b5lypijqlbpq5lgz@yavin>
Date:   Mon, 2 Mar 2020 21:47:41 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     lampahome <pahome.chen@...lab.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: why do we need utf8 normalization when compare name?

On 2020-03-02, Aleksa Sarai <cyphar@...har.com> wrote:
> On 2020-03-02, lampahome <pahome.chen@...lab.org> wrote:
> > According to case insensitive since kernel 5.2, d_compare will
> > transform string into normalized form and then compare.
> >
> > But why do we need this normalization function? Could we just compare
> > by utf8 string?
> 
> The problem is that there are multiple ways to represent the same glyph
> in Unicode -- for instance, you can represent Å (the symbol for
> angstrom) as both U+212B and U+0041 U+030A (the latin letter "A"
> followed by the ring-above symbol "°"). Different software may choose to
> represent the same glyphs in different Unicode forms, hence the need for
> normalisation.

Sorry, a better example would've been "ñ" (U+00F1). You can also
represent it as "n" (U+006E) followed by "◌̃" (U+0303 -- "combining
tilde"). Both forms are defined by Unicode to be canonically equivalent
so it would be incorrect to treat the two Unicode strings differently
(that isn't quite the case for "Å").

> [1] is the Wikipedia article that describes this problem and what the
> different kinds of Unicode normalisation are.
> 
> [1]: https://en.wikipedia.org/wiki/Unicode_equivalence

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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