linux-kernel - Re: violating function pointer signature

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Date:   Thu, 19 Nov 2020 09:21:57 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Florian Weimer <fw@...eb.enyo.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        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, Nov 18, 2020 at 01:48:37PM -0600, Segher Boessenkool wrote:

> If you have at most four or so args, what you wnat to do will work on
> all systems the kernel currently supports, as far as I can tell.  It
> is not valid C, and none of the compilers have an extension for this
> either.  But it will likely work.

So this is where we rely on the calling convention being caller-cleanup
(cdecl has that).

I looked at the GCC preprocessor symbols but couldn't find anything that
seems relevant to the calling convention in use, so barring that, the
best option might to be have a boot-time self-test that triggers this.
Then we'll quickly know if all architectures handle this correctly.