Re: 32bit userland cannot work with DVB drivers in 64bit kernel, design issue

[Alan Cox <gnomes@lxorguk.ukuu.org.uk>]

On Thu, 2 Nov 2017 12:16:39 +0100
Menion <menion@gmail.com> wrote:

> Hi all
> I am investigating for Armbian, the feasability of running 32bit
> userland on single board computers based on arm64 SoC, where only 64
> bit kernel is available, for reducing the memory footprint.
> I have discovered that there is a flaw in the DVB frontend ioctl (at
> least) that prevents to do so.
> in frontend.h the biggest problem is:
> 
> struct dtv_properties {
> __u32 num;
> struct dtv_property *props;
> };
> 
> The master userland-kernel ioctl structure is based on an array set by
> pointer, so the 32bit userland will allocate 32bit pointer (and the
> resulting structure size) while the 64bit kernel will expect the 64bit
> pointers

And in some cases the pointer might end up aligned to the next 64bit
boundary.

> The void *reserved2 field will also give problem when crossing the
> 32-64bits boundaries
> As today the endire dvb linux infrastructure can only work in 32-32
> and 64-64 bit mode

If this isn't handled by the existing media compat_ioctl support then you
can send patches to use compat_ioctl handlers to fix this.

Alan