Re: [PATCH 05/19] VFS: add common error checks to lookup_one_qstr()

[Al Viro]

On Thu, Feb 06, 2025 at 04:42:42PM +1100, NeilBrown wrote:

> @@ -1700,6 +1702,15 @@ struct dentry *lookup_one_qstr(const struct qstr *name,
>  	if ((flags & LOOKUP_INTENT_FLAGS) == 0)
>  		/* ->lookup must have given final answer */
>  		d_lookup_done(dentry);
> +found:
> +	if (d_is_negative(dentry) && !(flags & LOOKUP_CREATE)) {
> +		dput(dentry);
> +		return ERR_PTR(-ENOENT);
> +	}
> +	if (d_is_positive(dentry) && (flags & LOOKUP_EXCL)) {
> +		dput(dentry);
> +		return ERR_PTR(-EEXIST);
> +	}

Final dput() on an in-lookup dentry would blow up.  What happens if we get
there without LOOKUP_CREATE, but with something else from LOOKUP_INTENT_FLAGS?

That, BTW, is another lovely example of the reasons why making state (in-lookup
in this case, locking elsewhere) transitions dependent upon the function arguments
is a bad idea.