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Date:   Wed, 18 Nov 2020 14:21:36 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Matt Mullins <mmullins@mmlx.us>,
        Ingo Molnar <mingo@redhat.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Dmitry Vyukov <dvyukov@google.com>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Andrii Nakryiko <andriin@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>,
        netdev <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        linux-toolchains@vger.kernel.org
Subject: violating function pointer signature
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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) { }

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.