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

[Jani Nikula <jani.nikula@linux.intel.com>]

On Mon, 11 Oct 2021, Greg KH <gregkh@linuxfoundation.org> wrote:
> 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.

Oh, thanks for the correction.

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

Just musing on what is going on in the patch, and why such changes
initially seem like good ideas and get through review.


Thanks,
Jani.


>
>> 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

-- 
Jani Nikula, Intel Open Source Graphics Center