f2fs: Add check for deleted inode

The syzbot reproducer mounts a f2fs image, then tries to unlink an existing file. However, the unlinked file already has a link count of 0 when it is read for the first time in do_read_inode().

Add a check to sanity_check_inode() for i_nlink == 0.

---
v3: Correct "Fixes" line again. Sorry for my confusion.
v2: https://lore.kernel.org/linux-f2fs-devel/[email protected]/T/
v1: https://lore.kernel.org/all/[email protected]/T/

---
 fs/f2fs/inode.c | 6 ++++++
 1 file changed, 6 insertions(+)

index 1ed86df343a5..65f1dc32f173 100644
@@ -372,6 +372,12 @@ static bool sanity_check_inode(struct inode *inode, struct page *node_page)
 		return false;
 	}
 
+	if (inode->i_nlink == 0) {
+		f2fs_warn(sbi, "%s: inode (ino=%lx) has a link count of 0",
+			  __func__, inode->i_ino);
+		return false;
+	}
+
 	return true;
 }
 

2.43.0

Thanks,