Re: [PATCH] f2fs: Add check for deleted inode

[Chao Yu <chao@kernel.org>]

On 2024/11/24 9:04, 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.
> 
> #syz test
> 
> Reported-by: syzbot+b01a36acd7007e273a83@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=b01a36acd7007e273a83
> Fixes: 4c8ff7095bef ("f2fs: support data compression")

Hello Leo,

I'm fine w/ this change, but I didn't get it, how did above commit introduce
this bug?

Thanks,

> Signed-off-by: Leo Stone <leocstone@gmail.com>
> ---
>   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;
>   }
>