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Date:   Mon, 26 Apr 2021 01:34:30 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Shreeya Patel <shreeya.patel@collabora.com>
Cc:     fstests@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        krisman@collabora.com, preichl@redhat.com, kernel@collabora.com
Subject: Re: [PATCH] generic/453: Exclude filenames that are not supported by
 exfat
Message-ID: <20210426003430.GH235567@casper.infradead.org>
References: <20210425223105.1855098-1-shreeya.patel@collabora.com>
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On Mon, Apr 26, 2021 at 04:01:05AM +0530, Shreeya Patel wrote:
> exFAT filesystem does not support the following character codes
> 0x0000 - 0x001F ( Control Codes ), /, ?, :, ", \, *, <, |, >

ummm ...

> -# Fake slash?
> -setf "urk\xc0\xafmoo" "FAKESLASH"

That doesn't use any of the explained banned characters.  It uses 0xc0,
0xaf.

Now, in utf-8, that's an nonconforming sequence.  "The Unicode and UCS
standards require that producers of UTF-8 shall use the shortest form
possible, for example, producing a two-byte sequence with first byte 0xc0
is nonconforming.  Unicode 3.1 has added the requirement that conforming
programs must not accept non-shortest forms in their input."

So is it that exfat is rejecting nonconforming sequences?  Or is it
converting the nonconforming sequence from 0xc0 0xaf to the conforming
sequence 0x2f, and then rejecting it (because it's '/')?