Re: [PATCH 1/2] io_uring: fix big-endian compat signal mask handling

[Arnd Bergmann <arnd@arndb.de>]

On Tue, Mar 26, 2019 at 1:13 AM James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
> On Mon, 2019-03-25 at 17:24 +0100, Arnd Bergmann wrote:
> > On Mon, Mar 25, 2019 at 5:19 PM James Bottomley
> > <James.Bottomley@hansenpartnership.com> wrote:
> > > This looks a bit suboptimal: shouldn't in_compat_syscall() be hard
> > > coded to return 0 if CONFIG_COMPAT isn't defined?  That way the
> > > compiler can do the correct optimization and we don't have to
> > > litter #ifdefs and worry about undefined variables and other
> > > things.
> >
> > The check can be outside of the #ifdef, but set_compat_user_sigmask
> > is not declared then.
>
> Right, but shouldn't it be declared?  I thought BUILD_BUG_ON had nice
> magic that allowed it to work here (meaning if the compiler doesn't
> eliminate the branch we get a build bug).

My y2038 series originally went in that direction by allowing much more
of the compat code to be compiled and then discarded without the
#ifdefs (and combine it with the 32-bit time_t handling on 32-bit
architectures). I went away from that after Christoph and others found
the reuse of the compat interfaces too confusing.

The current state now is that most compat_* interfaces cannot be
compiled unless CONFIG_COMPAT is set, and making that work
in general is a lot of work, so I followed the usual precedent here
and used that #ifdef. This also matches what is done elsewhere
in the same file (see io_import_iovec).

> > I think for the future we can consider just moving the compat logic
> > into set_user_sigmask(), which would simplify most of the callers,
> > but that seemed to invasive as a bugfix for 5.1.
>
> Well, that too.  I've just been on a recent bender to stop #ifdefs
> after I saw what some people were doing with them.

I absolutely agree in general, and have sent many patches to
remove #ifdefs in other code when there was a good alternative
and the #ifdefs are wrong (which they are at least 30% of the time
in my experience).

The problems for doing this in general for compat code are

- some structures have a conditional compat_ioctl() callback
  pointer, and need an #ifdef around the assignment until
  we change the struct as well.
- Most compat handlers require the use of the compat_ptr()
  wrapper, I have a patch to move this to common code, but
  that was rejected previously
- many compat handlers rely on types from asm/compat.h
  that does not exist on architectures without compat support.

In this specific case, compat_sigset_t is required for declaring
set_compat_user_sigmask(), and the former is not easy to
define on non-compat architectures. I still think that the best
way forward here is to move it into set_user_sigmask()
next merge window, rather than doing a larger scale rewrite
of linux/compat.h to get this bug fixed now.

      Arnd