netdev - Re: violating function pointer signature

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Message-ID: <1762005214.49230.1605715285133.JavaMail.zimbra@efficios.com>
Date:   Wed, 18 Nov 2020 11:01:25 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     rostedt <rostedt@...dmis.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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 <linux-toolchains@...r.kernel.org>
Subject: Re: violating function pointer signature

----- On Nov 18, 2020, at 9:02 AM, rostedt rostedt@...dmis.org wrote:

> 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.

It works for me. Bonus points if you can document in a comment that this
trick depends on the cdecl calling convention.

Thanks,

Mathieu

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

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com