Re: uhid: broken interface: 32/64-bit compatibility

[Kirill A. Shutemov]

Johan Hedberg wrote:
> Hi David,
> 
> On Fri, Feb 15, 2013, David Herrmann wrote:
> > On Fri, Feb 15, 2013 at 12:29 PM, Kirill A. Shutemov
> > <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> > > Hi David and all,
> > >
> > > There's claim in uhid.h that the interface is "compatible even between
> > > architectures". But it obviously is not true: struct uhid_create_req
> > > contains pointer which breaks everything.
> > >
> > > The easy way to demonstrate the issue is compile uhid-example.c with -m32
> > > and try to run it on 64 bit kernel. Creating of the device will fail.
> > 
> > Indeed, we missed that. We should probably also notify the HIDP
> > developers as "struct hidp_connadd_req" suffers from the same
> > problems. (CC'ed)
> > 
> > > I don't see an easy way to fix this. Few options:
> > >
> > > 1. Replace the pointer with u64. It will fix the issue, but it breaks ABI
> > >    which is never a good idea. Not sure how many users interface already
> > >    has.
> > 
> > The only users I am aware of is an HID debugging tool and experimental
> > HoG Bluetooth support (bluez). Maybe Marcel or Johan can comment
> > whether this is already used by bluez-5? If it is, then we shouldn't
> > break ABI and go with #2+#3. Otherwise, I think changing to u64 should
> > be ok.
> > On the other hand, it would break any future build for older stable
> > kernels so not breaking ABI is probably the best idea. Any comments? I
> > can add a COMPAT fix and a comment to fix this in the next version of
> > UHID_CREATE.
> 
> The HoG code in BlueZ 5 does indeed use this API and it's also not
> anymore behind any kind of experimental flag (i.e. it is an officially
> supported feature).
> 
> Johan

Here's my attempt to fix the issue.

Not sure if tricks with padding in a good idea. We can  just use __u64
instead of pointer, but it will require update of userspace to silence
cast warning and will cause warning if you will try to use updated
userspace with old kernel headers.

Any comments?