Re: [PATCH] xen/debugfs: Check debugfs initialization before using it

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Dec 09, 2013 at 04:44:05PM +0800, Ethan Zhao wrote:
> On Mon, Dec 9, 2013 at 4:25 PM, Greg KH <gregkh@linuxfoundation.org> wrote:
> > On Mon, Dec 09, 2013 at 09:43:16AM +0800, Ethan Zhao wrote:
> >> On Sun, Dec 8, 2013 at 10:01 PM, Greg KH <gregkh@linuxfoundation.org> wrote:
> >> > On Sun, Dec 08, 2013 at 07:31:02PM +0800, ethan.zhao wrote:
> >> >> Should check debugfs initialization with debugfs_initialized() before using it,
> >> >> Because if it isn't initialized, the return value of fake debugfs_create_dir() etc
> >> >> functions would be ERR_PTR(-ENODEV), checking with NULL will not work.
> >> >
> >> > So?  It should "just work" without this check, right?  What happens if
> >> > your patch isn't applied and debugfs isn't enabled?
> >>
> >> If debugfs isn't  configured, debugfs_initialized() and other
> >> functions are defined as following,
> >>
> >> static inline bool debugfs_initialized(void)
> >> {
> >> return false;
> >> }
> >>
> >> static inline struct dentry *debugfs_create_file(const char *name, umode_t mode,
> >> struct dentry *parent, void *data,
> >> const struct file_operations *fops)
> >> {
> >> return ERR_PTR(-ENODEV);
> >> }
> >>
> >> static inline struct dentry *debugfs_create_dir(const char *name,
> >> struct dentry *parent)
> >> {
> >> return ERR_PTR(-ENODEV);
> >> }
> >>
> >> And the checking code in xen\debugfs.c xen_init_debugfs() will not
> >> work, the return value is not NULL.
> >> d_xen_debug = debugfs_create_dir("xen", NULL);
> >>
> >> if (!d_xen_debug)
> >> pr_warning("Could not create 'xen' debugfs directory\n");
> >
> > Which is just fine, what is wrong with this?
> 
> If we no check with debugfs_initialized(),  the above code should be
> if  (!d_xen_debug || IS_ERR (d_xen_debug))
>     pr_warning("Could not create 'xen' debugfs directory\n");

No, you don't want to print out that message if debugfs is not enabled.

You really don't want to print anything out, as what can a user do about
this?

I still think the original code is correct, have you tried it with
debugfs disabled?

greg k-h