Re: [PATCH -next] fs/ntfs3: Fix potential NULL/IS_ERR bug in ntfs_lookup()

[Al Viro]

On Thu, Jan 12, 2023 at 01:32:48AM +0000, Peng Zhang wrote:
> From: ZhangPeng <zhangpeng362@xxxxxxxxxx>
> 
> Dan Carpenter reported a Smatch static checker warning:
> 
> fs/ntfs3/namei.c:96 ntfs_lookup()
> error: potential NULL/IS_ERR bug 'inode'
> It will cause null-ptr-deref when dir_search_u() returns NULL if the
> file is not found.
> Fix this by replacing IS_ERR() with IS_ERR_OR_NULL() to add a check for
> NULL.

That's a bad approach - you are papering over bad calling conventions instead of
fixing them.

IS_ERR_OR_NULL is almost never the right tool.  Occasionally there are valid
cases for function possibly returning pointer/NULL/ERR_PTR(...); this is
almost certainly not one of those.

Incidentally, inodes with NULL ->i_op should never exist.  _Any_ place that
sets ->i_op to NULL is broken, plain and simple.  A new instance of struct
inode has ->i_op pointing to empty method table; it *is* initialized.