linux-kernel - Re: [PATCH] drivers: example: fix memory leak

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Message-ID: <87tt1lowqd.fsf@mail.parknet.co.jp>
Date: Tue, 02 Sep 2025 21:38:34 +0900
From: OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To: YangWen <anmuxixixi@...il.com>
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drivers: example: fix memory leak

YangWen <anmuxixixi@...il.com> writes:

> On Tue, 02 Sep 2025 23:13:42 +0900, OGAWA Hirofumi wrote:
>> Hm, what is wrong with temporary inconsistent?
>> 
>> If it had the race with future modification, it can be temporary
>> inconsistent. However, future modification will fix it by updating with
>> latest blocks, right?
>> 
>> Or did you actually get the inconsistent state after clean unmount?
>
> Thanks for your comment.
>
> This is not only a temporary in-memory inconsistency.  KCSAN detected a
> race where fat12_ent_put() updates two bytes of a 12-bit FAT entry while
> fat_mirror_bhs() concurrently memcpy()’s the entire sector.  The mirror
> FAT may therefore receive a torn entry.
>
> Since fat_mirror_bhs() marks those buffers dirty, the corrupted mirror
> content can be flushed to disk.  In our syzkaller testing, this already
> resulted in runtime errors such as:
>
>     FAT-fs (loop4): error, clusters badly computed (421 != 418)
>     FAT-fs (loop4): error, fat_bmap_cluster: request beyond EOF (i_pos 2075)
>
> These errors occurred even after a clean unmount, which suggests that the
> inconsistent FAT entries were actually written to disk and not corrected
> later by "future modification".
>
> FAT16/32 do not suffer from this problem because their entries are
> naturally aligned 16/32-bit accesses, which are atomic on supported
> architectures.  FAT12 is special because of the 12-bit packing across
> two bytes.
>
> So I think it is necessary to protect memcpy() in fat_mirror_bhs() with
> fat12_entry_lock to avoid copying a torn FAT12 entry.

Sounds like strange. FAT driver never read the mirror FAT area in
runtime. Why did you think the mirror FAT affect to it?

Thanks.
-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>