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

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Date:   Tue, 8 Jun 2021 10:29:56 -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 10:22 AM, Peter Zijlstra wrote:
> On Tue, Jun 08, 2021 at 09:58:03AM -0700, Nathan Chancellor wrote:
>> On 6/7/2021 1:54 PM, 'Nick Desaulniers' via Clang Built Linux wrote:
>>> On Mon, Jun 7, 2021 at 2:46 AM Peter Zijlstra <peterz@...radead.org> wrote:
>>>>
>>>
>>> Thanks, the below diff resolves the linker error reported in
>>> https://github.com/ClangBuiltLinux/linux/issues/1388
>>>
>>> Both readelf implementations seem happy with the results, too.
>>>
>>> Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
>>>
>>> Nathan,
>>> Can you please test the below diff and see if that resolves your boot
>>> issue reported in:
>>> https://github.com/ClangBuiltLinux/linux/issues/1384
>>
>> Unfortunately, it does not appear to resolve that issue.
>>
>> $ git log -2 --decorate=no --oneline
>> eea6a9d6d277 Peter's fix
>> 614124bea77e Linux 5.13-rc5
>>
>> $ strings /mnt/c/Users/natec/Linux/kernel-investigation | grep microsoft
>> 5.13.0-rc5-microsoft-standard-WSL2-00001-geea6a9d6d277
>> (nathan@...hlinux-ax161) #3 SMP Tue Jun 8 09:46:19 MST 2021
>>
>> My VM still never makes it to userspace.
> 
> Since it's a VM, can you use the gdb-stub to ask it where it's stuck?
> 

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.

Cheers,
Nathan