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.

Reported-by: syzbot+b01a36acd7007e273a83@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=b01a36acd7007e273a83
Fixes: 39a53e0ce0df ("f2fs: add superblock and major in-memory structure")
Signed-off-by: Leo Stone <leocstone@gmail.com>
---
v3: Correct "Fixes" line again. Sorry for my confusion.
v2: https://lore.kernel.org/linux-f2fs-devel/c0998223-6141-40e1-be08-d79bfc28eb0a@kernel.org/T/
v1: https://lore.kernel.org/all/20241124010459.23283-1-leocstone@gmail.com/T/
---
 fs/f2fs/inode.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/f2fs/inode.c b/fs/f2fs/inode.c
index 1ed86df343a5..65f1dc32f173 100644
--- a/fs/f2fs/inode.c
+++ b/fs/f2fs/inode.c
@@ -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

On 2024/12/3 1:12, Leo Stone wrote:
> 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.
> 
> Reported-by: syzbot+b01a36acd7007e273a83@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=b01a36acd7007e273a83
> Fixes: 39a53e0ce0df ("f2fs: add superblock and major in-memory structure")
> Signed-off-by: Leo Stone <leocstone@gmail.com>

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

Thanks,