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

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Date:   Tue, 8 Jun 2021 11:18:15 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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 Tue, Jun 8, 2021 at 10:30 AM Nathan Chancellor <nathan@...nel.org> wrote:
>
> 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:
> >>> 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.

Assuming this is an optimization and not required to boot/run; you
could test that quickly by putting a return statement as the first
statement in the list_for_each_entry loop in arch_rewrite_retpolines.
If that works, you could instead use a counter to try to see which
symbol is bad; once you bisect a counter value where things start/stop
booting, you could try to print the corresponding symbol (ie `name`).
(Optimization Fuel)  (Sorry if any of that is unclear, let's follow up
off thread if so).  Maybe that symbol will give us further clues?  I
think that would tell us whether it's a problematic jump vs call, and
via which register.
-- 
Thanks,
~Nick Desaulniers