linux-kernel - Re: [PATCH v3 16/16] objtool,x86: Rewrite retpoline thunk calls

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Date:   Tue, 8 Jun 2021 11:49:47 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Fāng-ruì Sòng <maskray@...gle.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>, lma@...ihalf.com,
        Guenter Roeck <groeck@...gle.com>,
        Juergen Gross <jgross@...e.com>, lb@...ihalf.com,
        LKML <linux-kernel@...r.kernel.org>, mbenes@...e.com,
        Radosław Biernacki <rad@...ihalf.com>,
        upstream@...ihalf.com,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: Re: [PATCH v3 16/16] objtool,x86: Rewrite retpoline thunk calls

On 6/8/2021 11:17 AM, Peter Zijlstra wrote:
> On Tue, Jun 08, 2021 at 10:29:56AM -0700, Nathan Chancellor wrote:
>> Unfortunately, this is the VM provided by the Windows Subsystem for Linux so
>> examining it is nigh-impossible :/ I am considering bisecting the transforms
>> that objtool does to try and figure out the one that causes the machine to
>> fail to boot or try to reproduce in a different hypervisor, unless you have
>> any other ideas.
> 
> Does breaking Windows earn points similar to breaking the binary
> drivers? :-) :-)

:)

> The below should kill this latest transform and would quickly confirm if
> the that is causing your problem. If that's not it, what was your last
> known working version?

Yes, that diff gets me back to booting. I will see if I can figure out 
the exact rewrite that blows everything up.

> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index e5947fbb9e7a..d0f231b9c5a1 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -1857,10 +1857,10 @@ static int decode_sections(struct objtool_file *file)
>   	 * Must be after add_special_section_alts(), since this will emit
>   	 * alternatives. Must be after add_{jump,call}_destination(), since
>   	 * those create the call insn lists.
> -	 */
>   	ret = arch_rewrite_retpolines(file);
>   	if (ret)
>   		return ret;
> +	 */
>   
>   	return 0;
>   }
> 

Cheers,
Nathan