Re: [PATCH 2/2] dev_ioctl: split out SIOC?IFMAP ioctls

[Christoph Hellwig]

On Fri, Sep 25, 2020 at 02:28:29PM +0200, Arnd Bergmann wrote:
> > > +++ b/include/uapi/linux/if.h
> > > @@ -247,7 +247,13 @@ struct ifreq {
> > >               short   ifru_flags;
> > >               int     ifru_ivalue;
> > >               int     ifru_mtu;
> > > +#ifndef __KERNEL__
> > > +             /*
> > > +              * ifru_map is rarely used but causes the incompatibility
> > > +              * between native and compat mode.
> > > +              */
> > >               struct  ifmap ifru_map;
> > > +#endif
> >
> > Do we need a way to verify that this never changes the struct size?
> 
> Not sure which way you would want to check. The point of the patch
> is that it does change the struct size inside of the kernel but not
> in user space.
>
> Do you mean we should check that the (larger) user space size
> remains what it is for future changes, or that the (smaller)
> kernel size remains the same on all kernels, or maybe both?

I had something like:

	BUILD_BUG_ON(sizeof(struct ifmap) >
		     sizeof(struct ifreq) - IFNAMSIZ);

plus a suitable comment in mind.