linux-kernel - Re: [PATCH] fat: Prevent the race of read/write the FAT16 and FAT32 entry

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Message-ID: <20250728160445.GC222315@ZenIV>
Date: Mon, 28 Jul 2025 17:04:45 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Edward Adam Davis <eadavis@...com>
Cc: syzbot+d3c29ed63db6ddf8406e@...kaller.appspotmail.com,
	hirofumi@...l.parknet.co.jp, linkinjeon@...nel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	sj1557.seo@...sung.com, syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] fat: Prevent the race of read/write the FAT16 and FAT32
 entry

On Mon, Jul 28, 2025 at 07:37:02PM +0800, Edward Adam Davis wrote:
> The writer and reader access FAT32 entry without any lock, so the data
> obtained by the reader is incomplete.

Could you be more specific?  "Incomplete" in which sense?

> Add spin lock to solve the race condition that occurs when accessing
> FAT32 entry.

Which race condition would that be?

> FAT16 entry has the same issue and is handled together.

FWIW, I strongly suspect that
	* "issue" with FAT32 is a red herring coming from mindless parroting
of dumb tool output
	* issue with FAT16 just might be real, if architecture-specific.
If 16bit stores are done as 32bit read-modify-write, we might need some
serialization.  Assuming we still have such architectures, that is -
alpha used to be one, but support for pre-BWX models got dropped.
Sufficiently ancient ARM?