Re: [PATCH v2] driver core: allow kobj_to_dev() to take a const pointer

[Sakari Ailus]

Hi Greg,

On Mon, Oct 17, 2022 at 10:04:14AM +0200, Greg Kroah-Hartman wrote:
> On Mon, Oct 17, 2022 at 07:54:52AM +0000, Sakari Ailus wrote:
> > Hi Greg,
> > 
> > On Sun, Oct 16, 2022 at 12:41:26PM +0200, Greg Kroah-Hartman wrote:
> > > If a const * to a kobject is passed to kobj_to_dev(), we want to return
> > > back a const * to a device as the driver core shouldn't be modifying a
> > > constant structure.  But when dealing with container_of() the pointer
> > > const attribute is cast away, so we need to manually handle this by
> > > determining the type of the pointer passed in to know the type of the
> > > pointer to pass out.
> > 
> > Alternatively container_of() could be fixed, but that will likely produce
> > lots of warnings currently.
> 
> Yeah, we can not do that because, as you found out, there's just too
> many warnings that it would cause.  Let's work on the individual
> subsystems to clean them all up first before worrying about the core
> container_of() macro as that should fix the majority of the build
> warnings.

Sounds reasonable.

> 
> > > Luckily _Generic can do this type of magic, and as the kernel now
> > > supports C11 it is availble to us to handle this type of build-time type
> > > detection.
> > > 
> > > Cc: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx>
> > > Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> > > Cc: "Rafael J. Wysocki" <rafael@xxxxxxxxxx>
> > > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > ---
> > > v2 - use _Generic() to make this type safe as pointed out by Sakari
> > > 
> > >  include/linux/device.h | 18 +++++++++++++++++-
> > >  1 file changed, 17 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/include/linux/device.h b/include/linux/device.h
> > > index 424b55df0272..023ea50b1916 100644
> > > --- a/include/linux/device.h
> > > +++ b/include/linux/device.h
> > > @@ -680,11 +680,27 @@ struct device_link {
> > >  	bool supplier_preactivated; /* Owned by consumer probe. */
> > >  };
> > >  
> > > -static inline struct device *kobj_to_dev(struct kobject *kobj)
> > > +static inline struct device *__kobj_to_dev(struct kobject *kobj)
> > >  {
> > >  	return container_of(kobj, struct device, kobj);
> > >  }
> > >  
> > > +static inline const struct device *__kobj_to_dev_const(const struct kobject *kobj)
> > > +{
> > > +	return container_of(kobj, const struct device, kobj);
> > > +}
> > > +
> > > +/*
> > > + * container_of() will happily take a const * and spit back a non-const * as it
> > > + * is just doing pointer math.  But we want to be a bit more careful in the
> > > + * driver code, so manually force any const * of a kobject to also be a const *
> > > + * to a device.
> > > + */
> > 
> > container_of() documentation has (probably?) never warned about this.
> 
> We never thought of it before :(
> 
> > Wouldn't such a comment be more appropriate there? Albeit it wouldn't be
> > needed if container_of() were fixed.
> 
> Some comment added to container_of() would be great, but that does not
> remove the need to keep this one.

I can send a patch for that.

For this one:

Reviewed-by: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx>

-- 
Kind regards,

Sakari Ailus