linux-kernel - Re: [PATCH] f2fs: Perform sanity check before unlinking directory inode

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Message-ID: <9d0fef9d-4093-4a80-af3f-6dc45ffe886c@kernel.org>
Date: Mon, 13 Oct 2025 20:53:04 +0800
From: Chao Yu <chao@...nel.org>
To: "Nikola Z. Ivanov" <zlatistiv@...il.com>
Cc: chao@...nel.org, jaegeuk@...nel.org,
 linux-f2fs-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
 skhan@...uxfoundation.org, david.hunter.linux@...il.com,
 linux-kernel-mentees@...ts.linuxfoundation.org, khalid@...nel.org,
 syzbot+c07d47c7bc68f47b9083@...kaller.appspotmail.com
Subject: Re: [PATCH] f2fs: Perform sanity check before unlinking directory
 inode

On 10/13/25 05:19, Nikola Z. Ivanov wrote:
> On Thu, Oct 09, 2025 at 10:54:40AM +0800, Chao Yu wrote:
>> On 10/3/2025 9:47 PM, Nikola Z. Ivanov wrote:
>>> Current i_nlink corruption check does not take into account
>>> directory inodes which have one additional i_nlink for their "." entry.
>>>
>>> Add additional check and a common corruption path.
>>>
>>> Reported-by: syzbot+c07d47c7bc68f47b9083@...kaller.appspotmail.com
>>> Closes: https://syzkaller.appspot.com/bug?extid=c07d47c7bc68f47b9083
>>> Fixes: 81edb983b3f5 ("f2fs: add check for deleted inode")
>>> Signed-off-by: Nikola Z. Ivanov <zlatistiv@...il.com>
>>> ---
>>>   fs/f2fs/namei.c | 28 ++++++++++++++++++++--------
>>>   1 file changed, 20 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/fs/f2fs/namei.c b/fs/f2fs/namei.c
>>> index b882771e4699..68b33e8089b0 100644
>>> --- a/fs/f2fs/namei.c
>>> +++ b/fs/f2fs/namei.c
>>> @@ -502,12 +502,14 @@ static struct dentry *f2fs_lookup(struct inode *dir, struct dentry *dentry,
>>>   		goto out;
>>>   	}
>>> -	if (inode->i_nlink == 0) {
>>> +	if (unlikely(inode->i_nlink == 0)) {
>>>   		f2fs_warn(F2FS_I_SB(inode), "%s: inode (ino=%lx) has zero i_nlink",
>>>   			  __func__, inode->i_ino);
>>> -		err = -EFSCORRUPTED;
>>> -		set_sbi_flag(F2FS_I_SB(inode), SBI_NEED_FSCK);
>>> -		goto out_iput;
>>> +		goto corrupted;
>>> +	} else if (unlikely(S_ISDIR(inode->i_mode) && inode->i_nlink == 1)) {
>>> +		f2fs_warn(F2FS_I_SB(inode), "%s: directory inode (ino=%lx) has a single i_nlink",
>>> +			  __func__, inode->i_ino);
>>> +		goto corrupted;
>>
>> Can we detect such corruption in sanity_check_inode() as well? So that if
>> f2fs internal flow calls f2fs_iget() on corrupted inode, we can set SBI_NEED_FSCK
>> flag and then triggering fsck repairment later.
>>
>> Thanks,
>>
>>>   	}
>>>   	if (IS_ENCRYPTED(dir) &&
>>> @@ -533,6 +535,9 @@ static struct dentry *f2fs_lookup(struct inode *dir, struct dentry *dentry,
>>>   	trace_f2fs_lookup_end(dir, !IS_ERR_OR_NULL(new) ? new : dentry,
>>>   				ino, IS_ERR(new) ? PTR_ERR(new) : err);
>>>   	return new;
>>> +corrupted:
>>> +	err = -EFSCORRUPTED;
>>> +	set_sbi_flag(F2FS_I_SB(inode), SBI_NEED_FSCK);
>>>   out_iput:
>>>   	iput(inode);
>>>   out:
>>> @@ -572,10 +577,11 @@ static int f2fs_unlink(struct inode *dir, struct dentry *dentry)
>>>   	if (unlikely(inode->i_nlink == 0)) {
>>>   		f2fs_warn(F2FS_I_SB(inode), "%s: inode (ino=%lx) has zero i_nlink",
>>>   			  __func__, inode->i_ino);
>>> -		err = -EFSCORRUPTED;
>>> -		set_sbi_flag(F2FS_I_SB(inode), SBI_NEED_FSCK);
>>> -		f2fs_folio_put(folio, false);
>>> -		goto fail;
>>> +		goto corrupted;
>>> +	} else if (unlikely(S_ISDIR(inode->i_mode) && inode->i_nlink == 1)) {
>>> +		f2fs_warn(F2FS_I_SB(inode), "%s: directory inode (ino=%lx) has a single i_nlink",
>>> +			  __func__, inode->i_ino);
>>> +		goto corrupted;
>>>   	}
>>>   	f2fs_balance_fs(sbi, true);
>>> @@ -601,6 +607,12 @@ static int f2fs_unlink(struct inode *dir, struct dentry *dentry)
>>>   	if (IS_DIRSYNC(dir))
>>>   		f2fs_sync_fs(sbi->sb, 1);
>>> +
>>> +	goto fail;
>>> +corrupted:
>>> +	err = -EFSCORRUPTED;
>>> +	set_sbi_flag(F2FS_I_SB(inode), SBI_NEED_FSCK);
>>> +	f2fs_folio_put(folio, false);
>>>   fail:
>>>   	trace_f2fs_unlink_exit(inode, err);
>>>   	return err;
>>
> 
> Hi Chao,
> 
> Thank you for the suggestion.
> I will add this to sanity_check_inode and remove it
> from f2fs_lookup as it becomes redundant since f2fs_lookup
> obtains the inode through f2fs_iget. For f2fs_unlink I will
> move the i_nlink == 1 check to f2fs_rmdir.

Hi Nikola,

I meant we can move the i_nlink == 1 check from both f2fs_lookup() and
f2fs_unlink() to sanity_check_inode(), because before we create in-memory
inode, we will always call sanity_check_inode().

Let me know if you have other concerns.

Thanks,

> 
> I will send v2 as soon as I do some more testing.