linux-kernel - Re: [PATCH] fat: avoid parent link count underflow in rmdir

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Message-ID: <87ms2idcph.fsf@mail.parknet.co.jp>
Date: Tue, 13 Jan 2026 01:20:58 +0900
From: OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Zhiyu Zhang <zhiyuzhang999@...il.com>, viro@...iv.linux.org.uk,
        brauner@...nel.org, jack@...e.cz, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fat: avoid parent link count underflow in rmdir

Ping?

OGAWA Hirofumi <hirofumi@...l.parknet.co.jp> writes:

> Zhiyu Zhang <zhiyuzhang999@...il.com> writes:
>
>> Corrupted FAT images can leave a directory inode with an incorrect
>> i_nlink (e.g. 2 even though subdirectories exist). rmdir then
>> unconditionally calls drop_nlink(dir) and can drive i_nlink to 0,
>> triggering the WARN_ON in drop_nlink().
>>
>> Add a sanity check in vfat_rmdir() and msdos_rmdir(): only drop the
>> parent link count when it is at least 3, otherwise report a filesystem
>> error.
>>
>> Fixes: 9a53c3a783c2 ("[PATCH] r/o bind mounts: unlink: monitor i_nlink")
>> Reported-by: Zhiyu Zhang <zhiyuzhang999@...il.com>
>> Closes: https://lore.kernel.org/linux-fsdevel/aVN06OKsKxZe6-Kv@casper.infradead.org/T/#t
>> Tested-by: Zhiyu Zhang <zhiyuzhang999@...il.com>
>> Signed-off-by: Zhiyu Zhang <zhiyuzhang999@...il.com>
>
> Looks good. Thanks.
>
> Acked-by: OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
>
>> ---
>>  fs/fat/namei_msdos.c | 7 ++++++-
>>  fs/fat/namei_vfat.c  | 7 ++++++-
>>  2 files changed, 12 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/fat/namei_msdos.c b/fs/fat/namei_msdos.c
>> index 0b920ee40a7f..262ec1b790b5 100644
>> --- a/fs/fat/namei_msdos.c
>> +++ b/fs/fat/namei_msdos.c
>> @@ -325,7 +325,12 @@ static int msdos_rmdir(struct inode *dir, struct dentry *dentry)
>>  	err = fat_remove_entries(dir, &sinfo);	/* and releases bh */
>>  	if (err)
>>  		goto out;
>> -	drop_nlink(dir);
>> +	if (dir->i_nlink >= 3)
>> +		drop_nlink(dir);
>> +	else {
>> +		fat_fs_error(sb, "parent dir link count too low (%u)",
>> +			dir->i_nlink);
>> +	}
>>  
>>  	clear_nlink(inode);
>>  	fat_truncate_time(inode, NULL, S_CTIME);
>> diff --git a/fs/fat/namei_vfat.c b/fs/fat/namei_vfat.c
>> index 5dbc4cbb8fce..47ff083cfc7e 100644
>> --- a/fs/fat/namei_vfat.c
>> +++ b/fs/fat/namei_vfat.c
>> @@ -803,7 +803,12 @@ static int vfat_rmdir(struct inode *dir, struct dentry *dentry)
>>  	err = fat_remove_entries(dir, &sinfo);	/* and releases bh */
>>  	if (err)
>>  		goto out;
>> -	drop_nlink(dir);
>> +	if (dir->i_nlink >= 3)
>> +		drop_nlink(dir);
>> +	else {
>> +		fat_fs_error(sb, "parent dir link count too low (%u)",
>> +			dir->i_nlink);
>> +	}
>>  
>>  	clear_nlink(inode);
>>  	fat_truncate_time(inode, NULL, S_ATIME|S_MTIME);

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