Re: [PATCH v2 15/18] ACPI: platform_profile: Remove platform_profile_handler from exported symbols

[Kurt Borja]

On Tue, Jan 14, 2025 at 07:55:53PM +0200, Ilpo Järvinen wrote:
> On Tue, 14 Jan 2025, Kurt Borja wrote:
> > On Tue, Jan 14, 2025 at 06:55:34PM +0200, Ilpo Järvinen wrote:
> > > On Tue, 14 Jan 2025, Kurt Borja wrote:
> > > 
> > > > In order to protect the platform_profile_handler from API consumers,
> > > > allocate it in platform_profile_register() and modify it's signature
> > > > accordingly.
> > > > 
> > > > Remove the platform_profile_handler from all consumer drivers and
> > > > replace them with a pointer to the class device, which is
> > > > now returned from platform_profile_register().
> > > > 
> > > > Replace *pprof with a pointer to the class device in the rest of
> > > > exported symbols.
> > > > 
> > > > Signed-off-by: Kurt Borja <kuurtb@xxxxxxxxx>
> > > > ---
> 
> > > > -int platform_profile_register(struct platform_profile_handler *pprof, void *drvdata)
> > > > +struct device *platform_profile_register(struct device *dev, const char *name,
> > > > +					 void *drvdata,
> > > > +					 const struct platform_profile_ops *ops)
> > > >  {
> > > > +	int minor;
> > > >  	int err;
> > > >  
> > > > -	/* Sanity check the profile handler */
> > > > -	if (!pprof || !pprof->ops->profile_set || !pprof->ops->profile_get ||
> > > > -	    !pprof->ops->probe) {
> > > > +	/* Sanity check */
> > > > +	if (!dev || !name || !ops || !ops->profile_get ||
> > > > +	    !ops->profile_set || !ops->probe) {
> > > >  		pr_err("platform_profile: handler is invalid\n");
> > > > -		return -EINVAL;
> > > > +		return ERR_PTR(-EINVAL);
> > > >  	}
> > > >  
> > > > -	err = pprof->ops->probe(drvdata, pprof->choices);
> > > > +	struct platform_profile_handler *pprof __free(kfree) = kzalloc(
> > > > +		sizeof(*pprof), GFP_KERNEL);
> > > > +	if (!pprof)
> > > > +		return ERR_PTR(-ENOMEM);
> > > > +
> > > > +	err = ops->probe(drvdata, pprof->choices);
> > > >  	if (err < 0)
> > > > -		return err;
> > > > +		return ERR_PTR(err);
> > > >  
> > > >  	if (bitmap_empty(pprof->choices, PLATFORM_PROFILE_LAST)) {
> > > >  		pr_err("platform_profile: no available profiles\n");
> > > > -		return -EINVAL;
> > > > +		return ERR_PTR(-EINVAL);
> > > >  	}
> > > >  
> > > >  	guard(mutex)(&profile_lock);
> > > >  
> > > >  	/* create class interface for individual handler */
> > > > -	pprof->minor = ida_alloc(&platform_profile_ida, GFP_KERNEL);
> > > > -	if (pprof->minor < 0)
> > > > -		return pprof->minor;
> > > > +	minor = ida_alloc(&platform_profile_ida, GFP_KERNEL);
> > > > +	if (minor < 0)
> > > > +		return ERR_PTR(minor);
> > > >  
> > > > +	pprof->name = name;
> > > > +	pprof->ops = ops;
> > > > +	pprof->minor = minor;
> > > >  	pprof->class_dev.class = &platform_profile_class;
> > > > -	pprof->class_dev.parent = pprof->dev;
> > > > +	pprof->class_dev.parent = dev;
> > > >  	dev_set_drvdata(&pprof->class_dev, drvdata);
> > > >  	dev_set_name(&pprof->class_dev, "platform-profile-%d", pprof->minor);
> > > >  	err = device_register(&pprof->class_dev);
> > > >  	if (err) {
> > > > -		put_device(&pprof->class_dev);
> > > > +		put_device(&no_free_ptr(pprof)->class_dev);
> > > >  		goto cleanup_ida;
> > > >  	}
> > > >  
> > > > @@ -504,20 +524,21 @@ int platform_profile_register(struct platform_profile_handler *pprof, void *drvd
> > > >  	if (err)
> > > >  		goto cleanup_cur;
> > > >  
> > > > -	return 0;
> > > > +	return &no_free_ptr(pprof)->class_dev;
> > > >  
> > > >  cleanup_cur:
> > > > -	device_unregister(&pprof->class_dev);
> > > > +	device_unregister(&no_free_ptr(pprof)->class_dev);
> > > 
> > > I don't like how this is architected.
> > > 
> > > IMO, no_free_ptr() should not appear on error/rollback paths. The pointer 
> > > is going to be freed despite the code just told it's not going to be 
> > > freed, which sends conflicting signals. Obviously, it is because this 
> > > function has relinquished its ownership of the pointer but as is it seems 
> > > a dangerous/confusing pattern.
> > 
> > Makes sense.
> > 
> > Quick fix would be to replace `goto cleanup_cur` with
> > 
> > 	device_unregister(&no_free_ptr(pprof)->class_dev);
> > 	goto cleanup_ida;
> > 
> > and add a comment about ownership. Similar to the put_device() call
> > above. Is this ok? If not I will think of a better way of writing this.
> 
> I think it would still be on the error path which is undesirable. While a 
> comment would make it understandable, it would be more logical to call 
> no_free_ptr() near device_register() which is when the ownership 
> gets transferred.
> 
> The trouble with that approach then is that no_free_ptr(pprof) will set 
> the pprof to NULL because of how the internal cleanup magic prevents 
> automatic freeing of pprof (Don't ask me how I know about that trap :-D).

:')

> 
> I suppose you could take pointer of the pprof->class_dev into a local 
> variable before making the device_register() call since that is all you 
> need after that point?
> 
> So my suggestion is along the lines of:
> 
> 	/* device_register() takes the ownership of the pointer */
> 	class_dev = &no_free_ptr(pprof)->class_dev;
> 	err = device_register(class_dev);
> 	...

Yes, this makes a lot of sense. I will do it this way.

Thanks Ilpo.

~ Kurt

> 	
> 
> > > >  cleanup_ida:
> > > > -	ida_free(&platform_profile_ida, pprof->minor);
> > > > +	ida_free(&platform_profile_ida, minor);
> > > >  
> > > > -	return err;
> > > > +	return ERR_PTR(err);
> > > >  }
> > > >  EXPORT_SYMBOL_GPL(platform_profile_register);
> 
> --
>  i.
>