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From: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
To: Thadeu Lima de Souza Cascardo <cascardo@igalia.com>
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Gwendal
 Grignou <gwendal@chromium.org>, dlunev@chromium.org
Subject: Re: [PATCH] fat: ignore .. subdir and always add a link to dirs
In-Reply-To: <ZfFmvGRlNR4ZiMMC@quatroqueijos.cascardo.eti.br> (Thadeu Lima de
	Souza Cascardo's message of "Wed, 13 Mar 2024 05:41:32 -0300")
References: <ZdhsYAUCe9GVMnYE@quatroqueijos.cascardo.eti.br>
	<87v86fnz2o.fsf@mail.parknet.co.jp>
	<Zd6PdxOC8Gs+rX+j@quatroqueijos.cascardo.eti.br>
	<87le75s1fg.fsf@mail.parknet.co.jp>
	<Zd74fjlVJZic8UxI@quatroqueijos.cascardo.eti.br>
	<87h6hek50l.fsf@mail.parknet.co.jp>
	<Ze2IAnSX7lr1fZML@quatroqueijos.cascardo.eti.br>
	<87cys2jfop.fsf@mail.parknet.co.jp>
	<ZfFcpWRWdnWmtebd@quatroqueijos.cascardo.eti.br>
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	<ZfFmvGRlNR4ZiMMC@quatroqueijos.cascardo.eti.br>
Date: Wed, 13 Mar 2024 20:06:41 +0900
Message-ID: <874jdajsqm.fsf@mail.parknet.co.jp>
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Thadeu Lima de Souza Cascardo <cascardo@igalia.com> writes:

>> So you break the mkdir/rmdir link counting, isn't it?
>> 
>
> It is off by one on those images with directories without ".." subdir.
> Otherwise, everything else works fine. mkdir/rmdir inside such directories work
> without any issues as rmdir that same directory.

mkdir() increase link count, rmdir decrease link count. Your change set
a dir link count always 2? So if there are 3 normal subdirs, and rmdir
all those normal dirs, link count underflow.

Thanks.

> If, on the other hand, we left everything as is and only skipped the
> validation, such directories would be created with a link count of 0. Then,
> doing a mkdir inside them would crash the kernel with a BUG as we cannot
> increment the link count of an inode with 0 links.
>
> So the idea of the fix here is that, independently of the existence of "..",
> the link count will always be at least 1.

-- 
OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>