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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Tue, 15 Sep 2020 15:34:39 -0700
Message-ID: <CAKwvOdmptEpi8fiOyWUo=AiZJiX+Z+VHJOM2buLPrWsMTwLnyw@mail.gmail.com>
Subject: Re: [tip:x86/seves] BUILD SUCCESS WITH WARNING e6eb15c9ba3165698488ae5c34920eea20eaa38e
To:     Josh Poimboeuf <jpoimboe@redhat.com>,
        Marco Elver <elver@google.com>
Cc:     Borislav Petkov <bp@alien8.de>, Rong Chen <rong.a.chen@intel.com>,
        kernel test robot <lkp@intel.com>,
        "Li, Philip" <philip.li@intel.com>, x86-ml <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        kasan-dev <kasan-dev@googlegroups.com>
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On Tue, Sep 15, 2020 at 2:02 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> panic() is noreturn, so the compiler is enforcing the fact that it
> doesn't return, by trapping if it does return.
>
> I seem to remember that's caused by CONFIG_UBSAN_TRAP.

Indeed, if I remove CONFIG_UBSAN_TRAP from the 0day report's
randconfig, these unreachable instruction warnings all go away.

So what's the right way to fix this?

CONFIG_UBSAN_TRAP enables -fsanitize-undefined-trap-on-error  (not
sure why that's wrapped in cc-option; it shouldn't be selectable via
Kconfig if unsupported by the toolchain).

Should clang not be emitting `ud2` trapping instructions for this flag
for no-return functions?

or

Should objtool be made aware of the config option and then not check
traps after no-returns?

I suspect the latter, but I'm not sure how feasible it is to
implement.  Josh, Marco, do you have thoughts on the above?
-- 
Thanks,
~Nick Desaulniers