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

[Chao Yu <chao@kernel.org>]

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@syzkaller.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@gmail.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;