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Date:   Wed, 18 Nov 2020 13:11:27 -0600
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Florian Weimer <fw@deneb.enyo.de>,
        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
Message-ID: <20201118191127.GM2672@gate.crashing.org>
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On Wed, Nov 18, 2020 at 01:58:23PM -0500, Steven Rostedt wrote:
> 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.

I don't see how you imagine a compiler to mess this up?

void void_stub(void) { }

will do fine?

        .globl  void_stub
        .type   void_stub, @function
void_stub:
.LFB0:
        .cfi_startproc
        ret
        .cfi_endproc
.LFE0:
        .size   void_stub, .-void_stub


Calling this via a different declared function type is undefined
behaviour, but that is independent of how the function is *defined*.
Your program can make ducks appear from your nose even if that function
is never called, if you do that.  Just don't do UB, not even once!


Segher