Re: [PATCH v1 1/2] gpiolib: Fix line event handling in compatible mode

[Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>]

On Tue, Dec 10, 2019 at 10:06:04AM +0100, Bartosz Golaszewski wrote:
> śr., 4 gru 2019 o 20:42 Andy Shevchenko
> <andriy.shevchenko@xxxxxxxxxxxxxxx> napisał(a):
> >
> > The introduced line even handling ABI in the commit
> >
> >   61f922db7221 ("gpio: userspace ABI for reading GPIO line events")
> >
> > missed the fact that 64-bit kernel may serve for 32-bit applications.
> > In such case the very first check in the lineevent_read() will fail
> > due to alignment differences.
> >
> > To workaround this we do several things here:
> > - put warning comment to UAPI header near to the structure description
> > - derive the size of the structure in the compatible mode from its members
> > - check for the size of this structure in the ->read() callback
> > - return only one event in the compatible mode at a time
> >
> > Above mitigation will work at least with libgpiod which does one event
> > at a time.
> >
> > Since the bug hasn't been reported earlier we assume that there is close
> > to zero actual users of the compatible mode to monitor GPIO events and thus
> > we might consider to rework this ABI in the future.
> >
> 
> How come this only affects the read operation but not the structures
> passed as arguments to ioctl() calls?

On x86 the rest of the structures is naturally aligned by 8 bytes.

But you are right, the ABI is broken more widely than simple ->read().

-- 
With Best Regards,
Andy Shevchenko