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

[Arnd Bergmann]

On Tue, Sep 29, 2020 at 7:53 PM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> On Fri, Sep 25, 2020 at 02:28:29PM +0200, Arnd Bergmann wrote:

> > 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.

But that condition is true on all 64-bit architectures, which is the
fundamental issue I'm working around. I can try to capture that
better in the comment though.

My expectation here is that passing the smaller 'ifreq' structure
to ndo_do_ioctl() is safe as long as all drivers use only the
remaining members of ifr_ifru that all fit into the first 16 bytes.
Do you see a problem with that assumption?

      Arnd