linux-kernel - Re: violating function pointer signature

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Message-ID: <20201118135823.3f0d24b7@gandalf.local.home>
Date:   Wed, 18 Nov 2020 13:58:23 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Florian Weimer <fw@...eb.enyo.de>
Cc:     Segher Boessenkool <segher@...nel.crashing.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        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 19:31:50 +0100
Florian Weimer <fw@...eb.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