[PATCH] Adds a new ioctl32 syscall for backwards compatibility layers

[Andy Lutomirski]

On Fri, Jan 15, 2021 at 1:03 AM Arnd Bergmann <arnd at kernel.org> wrote:
>
> On Fri, Jan 15, 2021 at 3:06 AM Ryan Houdek <sonicadvance1 at gmail.com> wrote:
> > On Wed, Jan 6, 2021 at 12:49 AM Arnd Bergmann <arnd at kernel.org> wrote:
> >> On Wed, Jan 6, 2021 at 7:48 AM <sonicadvance1 at gmail.com> wrote:
> >> > From: Ryan Houdek <Sonicadvance1 at gmail.com>
> >> ...
> >>
> >> For x86, this has another complication, as some ioctls also need to
> >> check whether they are in an ia32 task (with packed u64 and 32-bit
> >> __kernel_old_time_t) or an x32 task (with aligned u64 and 64-bit
> >> __kernel_old_time_t). If the new syscall gets wired up on x86 as well,
> >> you'd need to decide which of the two behaviors you want.
> >
> >
> > I can have a follow-up patch that makes this do ni-syscall on x86_64 since
> > we can go through the int 0x80 handler, or x32 handler path and choose
> > whichever one there.
>
> I'd say for consistency
>

We need to make it crystal clear on x86 what this ioctl does.  We have
a silly selection of options:

 - ioctl32() via SYSCALL, x32 bit clear -- presumably does an i386 ioctl?
 - ioctl32() via SYSCALL, x32 bit set -- this needs to do something
clearly documented.
 - ioctl32() via int80 -- presumably you're not wiring this up

In any case, the compat alloc thing should just go away.  It's a hack
and serves no real purpose.

Finally, I'm not convinced that this patch works correctly.  We have
in_compat_syscall(), and code that uses it may well be reachable from
ioctl.  I personally would like to see in_compat_syscall() go away,
but some other people (Hi, Christoph!) disagree, and usage seems to be
increasing, not decreasing.