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Date:   Thu, 19 Nov 2020 09:36:48 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Segher Boessenkool <segher@kernel.crashing.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Florian Weimer <fw@deneb.enyo.de>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Sami Tolvanen <samitolvanen@google.com>,
        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, Nov 18, 2020 at 01:11:27PM -0600, Segher Boessenkool wrote:
> Calling this via a different declared function type is undefined
> behaviour, but that is independent of how the function is *defined*.
> Your program can make ducks appear from your nose even if that function
> is never called, if you do that.  Just don't do UB, not even once!

Ah, see, here I think we disagree. UB is a flaw of the spec, but the
real world often has very sane behaviour there (sometimes also very
much not).

In this particular instance the behaviour is UB because the C spec
doesn't want to pin down the calling convention, which is something I
can understand. But once you combine the C spec with the ABI(s) at hand,
there really isn't two ways about it. This has to work, under the
premise that the ABI defines a caller cleanup calling convention.

So in the view that the compiler is a glorified assembler, I'll take UB
every day if it means I can get the thing to do what I want it to.

Obviously in the interest of co-operation and longer term viability, it
would be nice if we can agree on the behaviour and get a language
extention covering it.

Note that we have a fairly extensive tradition of defining away UB with
language extentions, -fno-strict-overflow, -fno-strict-aliasing,
-fno-delete-null-pointer-checks etc..