linux-kernel - Re: objtool warnings in prerelease clang-9

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Date:   Tue, 2 Jul 2019 23:58:27 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
cc:     Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kees Cook <keescook@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Craig Topper <craig.topper@...el.com>,
        Alexander Potapenko <glider@...gle.com>,
        Bill Wendling <morbo@...gle.com>,
        Stephen Hines <srhines@...gle.com>
Subject: Re: objtool warnings in prerelease clang-9

Nick,

On Tue, 2 Jul 2019, Nick Desaulniers wrote:

> TL;DR
> LLVM currently has a bug when unrolling loops containing asm goto and
> we have a fix in hand.

<snip>

> 
> Conservatively, we can block loop unrolling when we see asm goto in a loop:

Makes sense in order to make progress.

> This causes objtool to not find any issues in
> arch/x86/kernel/cpu/mtrr/generic.o.  I don't observe any duplication
> in the __jump_table section of the resulting .o file.  It also cuts
> down the objtool warnings I observe in a defconfig (listed at the
> beginning of the email) from 4 to 2. (platform-quirks.o predates asm
> goto,

It does not have asm goto inside :)

> i915_gem_execbuffer.o is likely a separate bug).

platform-quirks.o:

        if (x86_platform.set_legacy_features)
  74:   4c 8b 1d 00 00 00 00    mov    0x0(%rip),%r11        # 7b <x86_early_init_platform_quirks+0x7b>
  7b:   4d 85 db                test   %r11,%r11
  7e:   0f 85 00 00 00 00       jne    84 <x86_early_init_platform_quirks+0x84>
                x86_platform.set_legacy_features();
}
  84:   c3                      retq   

That jne jumps to __x86_indirect_thunk_r11, aka. ratpoutine.

No idea why objtool thinks that the instruction at 0x84 is not
reachable. Josh?

Thanks,

	tglx