Re: [PATCH 1/4] container_of: add container_of_const() that preserves const-ness of the pointer

[Greg Kroah-Hartman]

On Fri, Dec 02, 2022 at 12:48:57PM +0000, Sakari Ailus wrote:
> Hi Greg,
> 
> On Thu, Dec 01, 2022 at 08:30:54PM +0100, Greg Kroah-Hartman wrote:
> > container_of does not preserve the const-ness of a pointer that is
> > passed into it, which can cause C code that passes in a const pointer to
> > get a pointer back that is not const and then scribble all over the data
> > in it.  To prevent this, container_of_const() will preserve the const
> > status of the pointer passed into it using the newly available _Generic()
> > method.
> > 
> > Co-developed-by: Jason Gunthorpe <jgg@xxxxxxxx>
> > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > 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>
> > ---
> >  include/linux/container_of.h | 14 ++++++++++++++
> >  1 file changed, 14 insertions(+)
> > 
> > diff --git a/include/linux/container_of.h b/include/linux/container_of.h
> > index 2008e9f4058c..3c290e865151 100644
> > --- a/include/linux/container_of.h
> > +++ b/include/linux/container_of.h
> > @@ -22,4 +22,18 @@
> >  		      "pointer type mismatch in container_of()");	\
> >  	((type *)(__mptr - offsetof(type, member))); })
> >  
> > +/**
> > + * container_of_const - cast a member of a structure out to the containing
> > + *			structure and preserve the const-ness of the pointer
> > + * @ptr_type:		the type of the pointer @ptr
> > + * @ptr:		the pointer to the member
> > + * @member_type:	the type of the container struct this is embedded in.
> > + * @member:		the name of the member within the struct.
> > + */
> > +#define container_of_const(ptr_type, ptr, member_type, member)		\
> 
> I missed earlier you had four arguments for the macro instead of three.
> 
> With default: this can be done with just three:
> 
> #define container_of_const(ptr, member_type, member)		\
> 	_Generic(ptr,							\
> 		 const typeof(*(ptr)) *: ((const member_type *)container_of(ptr, member_type, member)),\
> 		 default: ((member_type *)container_of(ptr, member_type, member))	\
> 	)
> 
> The const typeof(*(ptr)) * will match if ptr is const, otherwise default
> matches. 

I had tried to use typeof before, your way is easier, thanks, I couldn't
get it to work myself...

> But you can't have typeof(*(ptr)) * instead of default as the two
> types are still the same, hence default.

Ah.  Ok, I really didn't want to use default, and that's probably why it
wasn't working for me.

> I've tested this on GCC 10.2.1 and clang 11.0.1.
> 
> This should also make it a bit easier for existing users to switch to the
> new macro and hopefully eventual rename back to container_of() once all
> users have been converted.

True.  Ok, I'll try this later tomorrow and send out a new version if
all goes well, thanks!

greg k-h