Add check for inode->i_nlink == 1 for directories during unlink,
as their value is decremented twice, which can trigger a warning in
drop_nlink. In such case mark the filesystem as corrupted and return
from the function call with the relevant failure return value.

Additionally add the check for i_nlink == 1 in
sanity_check_inode in order to detect on-disk corruption early.

Reported-by: syzbot+c07d47c7bc68f47b9083@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=c07d47c7bc68f47b9083
Tested-by: syzbot+c07d47c7bc68f47b9083@syzkaller.appspotmail.com
Signed-off-by: Nikola Z. Ivanov <zlatistiv@gmail.com>
---
 fs/f2fs/inode.c |  6 ++++++
 fs/f2fs/namei.c | 17 ++++++++++++-----
 2 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/fs/f2fs/inode.c b/fs/f2fs/inode.c
index 8c4eafe9ffac..e2405b79b3cc 100644
--- a/fs/f2fs/inode.c
+++ b/fs/f2fs/inode.c
@@ -294,6 +294,12 @@ static bool sanity_check_inode(struct inode *inode, struct folio *node_folio)
 		return false;
 	}
 
+	if (S_ISDIR(inode->i_mode) && unlikely(inode->i_nlink == 1)) {
+		f2fs_warn(sbi, "%s: directory inode (ino=%lx) has a single i_nlink",
+			  __func__, inode->i_ino);
+		return false;
+	}
+
 	if (f2fs_has_extra_attr(inode)) {
 		if (!f2fs_sb_has_extra_attr(sbi)) {
 			f2fs_warn(sbi, "%s: inode (ino=%lx) is with extra_attr, but extra_attr feature is off",
diff --git a/fs/f2fs/namei.c b/fs/f2fs/namei.c
index 40cf80fd9d9a..849cfdc57b30 100644
--- a/fs/f2fs/namei.c
+++ b/fs/f2fs/namei.c
@@ -570,12 +570,13 @@ 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",
+		f2fs_warn(sbi, "%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 out;
+		goto corrupted;
+	} else if (S_ISDIR(inode->i_mode) && unlikely(inode->i_nlink == 1)) {
+		f2fs_warn(sbi, "%s: directory inode (ino=%lx) has a single i_nlink",
+			  __func__, inode->i_ino);
+		goto corrupted;
 	}
 
 	f2fs_balance_fs(sbi, true);
@@ -601,6 +602,12 @@ static int f2fs_unlink(struct inode *dir, struct dentry *dentry)
 
 	if (IS_DIRSYNC(dir))
 		f2fs_sync_fs(sbi->sb, 1);
+
+	goto out;
+corrupted:
+	err = -EFSCORRUPTED;
+	set_sbi_flag(sbi, SBI_NEED_FSCK);
+	f2fs_folio_put(folio, false);
 out:
 	trace_f2fs_unlink_exit(inode, err);
 	return err;
-- 
2.51.0

On 11/5/25 19:09, Nikola Z. Ivanov wrote:
> Add check for inode->i_nlink == 1 for directories during unlink,
> as their value is decremented twice, which can trigger a warning in
> drop_nlink. In such case mark the filesystem as corrupted and return
> from the function call with the relevant failure return value.
> 
> Additionally add the check for i_nlink == 1 in
> sanity_check_inode in order to detect on-disk corruption early.
> 
> Reported-by: syzbot+c07d47c7bc68f47b9083@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=c07d47c7bc68f47b9083
> Tested-by: syzbot+c07d47c7bc68f47b9083@syzkaller.appspotmail.com
> Signed-off-by: Nikola Z. Ivanov <zlatistiv@gmail.com>

Reviewed-by: Chao Yu <chao@kernel.org>

Thanks,