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Date:   Wed, 18 Nov 2020 13:58:23 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Florian Weimer <fw@deneb.enyo.de>
Cc:     Segher Boessenkool <segher@kernel.crashing.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        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: Re: violating function pointer signature
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On Wed, 18 Nov 2020 19:31:50 +0100
Florian Weimer <fw@deneb.enyo.de> wrote:

> * Segher Boessenkool:
> 
> > On Wed, Nov 18, 2020 at 12:17:30PM -0500, Steven Rostedt wrote:  
> >> I could change the stub from (void) to () if that would be better.  
> >
> > Don't?  In a function definition they mean exactly the same thing (and
> > the kernel uses (void) everywhere else, which many people find clearer).  
> 
> And I think () functions expected a caller-provided parameter save
> area on powerpc64le, while (void) functions do not.  It does not
> matter for an empty function, but GCC prefers to use the parameter
> save area instead of setting up a stack frame if it is present.  So
> you get stack corruption if you call a () function as a (void)
> function.  (The other way round is fine.)

I wonder if we should define on all architectures a void void_stub(void),
in assembly, that just does a return, an not worry about gcc messing up the
creation of the stub function.

On x86_64:

GLOBAL(void_stub)
	retq


And so on.

-- Steve