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Date:   Wed, 18 Nov 2020 09:02:56 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Peter Zijlstra <peterz@infradead.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: Re: violating function pointer signature
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On Wed, 18 Nov 2020 14:21:36 +0100
Peter Zijlstra <peterz@infradead.org> wrote:

> 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) { }

My original version (the OP of this thread) had this:

+static void tp_stub_func(void)
+{
+	return;
+}

> 
> 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 it is already done elsewhere in the kernel, then I will call this
precedence, and keep the original version.

This way Alexei can't complain about adding a check in the fast path of
more than one callback attached.

-- Steve