linux-kernel - Re: new objtool unreachable instruction warnings

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Date:   Wed, 26 Jul 2017 21:52:36 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: new objtool unreachable instruction warnings

On Wed, Jul 26, 2017 at 9:10 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> On Wed, Jul 26, 2017 at 04:46:40PM +0200, Arnd Bergmann wrote:
>> I only saw these warnings once, they are either very rare, or were introduced
>> recently:
>>
>> arch/x86/kvm/vmx.o: warning: objtool: .altinstr_replacement+0x0:
>> unreachable instruction
>> arch/x86/kvm/svm.o: warning: objtool: .altinstr_replacement+0x6e:
>> unreachable instruction
>>
>> I see this with gcc-4.6 though gcc-7, but not with gcc-4.3.
>>
>> The configuration file that triggered it is
>> https://pastebin.com/aMn45GYP
>>
>> I managed to trace the problem down to the CC_HAVE_ASM_GOTO
>> macro, without that, we don't run into the problem.
>
> Thanks for reporting all these!
>
> This should fix all the warnings you reported.  I'll split it up into
> real patches and submit them soon.  Let me know if you find anything
> else.

Looks good. I tried ten different randconfigs that found warnings in the
past few days, and all the warnings are gone now.

For the warnings that showed up only with gcc-7, I think we may want
a backport to stable kernels, the other ones seem to be recent additions
(after 4.12).

Thanks,

     Arnd