Re: violating function pointer signature

Steven Rostedt
2020-11-18T17:17:58Z

On Wed, 18 Nov 2020 08:50:37 -0800
Nick Desaulniers <ndesaulniers@google.com> wrote:

> On Wed, Nov 18, 2020 at 5:23 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Tue, Nov 17, 2020 at 03:34:51PM -0500, Steven Rostedt wrote:
> >
> > > > > Since all tracepoints callbacks have at least one parameter (__data), we
> > > > > could declare tp_stub_func as:
> > > > >
> > > > > static void tp_stub_func(void *data, ...)
> > > > > {
> > > > > return;
> > > > > }
> > > > >
> > > > > And now C knows that tp_stub_func() can be called with one or more
> > > > > parameters, and had better be able to deal with it!
> > > >
> > > > AFAIU this won't work.
> > > >
> > > > C99 6.5.2.2 Function calls
> > > >
> > > > "If the function is defined with a type that is not compatible with the type (of the
> > > > expression) pointed to by the expression that denotes the called function, the behavior is
> > > > undefined."
> > >
> > > But is it really a problem in practice. I'm sure we could create an objtool
> > > function to check to make sure we don't break anything at build time.
> >
> > I think that as long as the function is completely empty (it never
> > touches any of the arguments) this should work in practise.
> >
> > That is:
> >
> > void tp_nop_func(void) { }
>
> or `void tp_nop_func()` if you plan to call it with different
> parameter types that are all unused in the body. If you do plan to
> use them, maybe a pointer to a tagged union would be safer?

This stub function will never use the parameters passed to it.

You can see the patch I have for the tracepoint issue here:

https://lore.kernel.org/r/20201118093405.7a6d2290@gandalf.local.home

I could change the stub from (void) to () if that would be better.

>
> >
> > can be used as an argument to any function pointer that has a void
> > return. In fact, I already do that, grep for __static_call_nop().
> >
> > I'm not sure what the LLVM-CFI crud makes of it, but that's their
> > problem.
>
> If you have instructions on how to exercise the code in question, we
> can help test it with CFI. Better to find any potential issues before
> they get committed.

If you apply the patch to the Linux kernel, and then apply:

https://lore.kernel.org/r/20201116181638.6b0de6f7@gandalf.local.home

Which will force the failed case (to use the stubs). And build and boot the
kernel with those patches applied, you can test it with:

# mount -t tracefs nodev /sys/kernel/tracing
# cd /sys/kernel/tracing
# echo 1 > events/sched/sched_switch/enable
# mkdir instances/foo
# echo 1 > instances/foo/events/sched/sched_switch/enable
# echo 0 > events/sched/sched_switch/enable

Which add two callbacks to the function array for the sched_switch
tracepoint. The remove the first one, which would add the stub instead.

-- Steve