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

[Nikola Z. Ivanov]

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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://syzkaller.appspot.com/bug?extid=c07d47c7bc68f47b9083
> > Fixes: 81edb983b3f5 ("f2fs: add check for deleted inode")
> > Signed-off-by: Nikola Z. Ivanov <zlatistiv@xxxxxxxxx>
> > ---
> >   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.

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