Re: Fw: [Intel-gfx] [PATCH 1/5] dri: cleanup debugfs error handling

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Oct 11, 2021 at 07:38:22PM +0300, Jani Nikula wrote:
> On Mon, 11 Oct 2021, Greg KH <gregkh@linuxfoundation.org> wrote:
> > On Mon, Oct 11, 2021 at 04:19:58PM +0200, Christian König wrote:
> >> > > > > And then throw it away, later, when you want to remove the directory,
> >> > > > > look it up with a call to debugfs_lookup() and pass that to
> >> > > > > debugfs_remove() (which does so recursively).
> >> > > > > 
> >> > > > > There should never be a need to save, or check, the result of any
> >> > > > > debugfs call.  If so, odds are it is being used incorrectly.
> >> > > Yeah, exactly that's the problem I see here.
> >> > > 
> >> > > We save the return value because the DRM subsystem is creating a debugfs
> >> > > directory for the drivers to use.
> >> > That's fine for now, not a big deal.  And even if there is an error,
> >> > again, you can always feed that error back into the debugfs subsystem on
> >> > another call and it will handle it correctly.
> >> 
> >> Problem is it isn't, we have a crash because the member isn't a pointer but
> >> an ERR_PTR instead.
> >
> > Again, that is fine, you can feed that into debugfs and it will "just
> > work".  Treat it as an opaque pointer, not a *dentry and you will be
> > fine.
> 
> Hmm, some of the patches add things like:
> 
> +
> +	if (!root)
> +		goto error;
> +
> 	minor->debugfs_root = debugfs_create_dir(name, root);
> 
> Superficially this seems okay, as it looks like debugfs_create_dir()
> doesn't actually cope with NULL values.

Yes it does, it puts things at the root of debugfs.

But why are you checking for NULL here, as the return value of a debugfs
call can never be NULL?

> However, since ->debugfs_root
> comes from debugfs_create_dir() I presume it'll never be NULL on errors
> anyway but rather an error pointer!

That is correct.

> So I think we probably need to go through the drm subsystem and look for
> existing similar patterns in fix them.

Please do.  I know I made one pass at it a while ago but I think someone
else went through and cleaned them up again.

thanks,

greg k-h