linux-kernel - Re: violating function pointer signature

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Message-ID: <20201118090256.55656208@gandalf.local.home>
Date:   Wed, 18 Nov 2020 09:02:56 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Matt Mullins <mmullins@...x.us>,
        Ingo Molnar <mingo@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        linux-toolchains@...r.kernel.org
Subject: Re: violating function pointer signature

On Wed, 18 Nov 2020 14:21:36 +0100
Peter Zijlstra <peterz@...radead.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