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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Tue, 8 Jun 2021 11:18:15 -0700
Message-ID: <CAKwvOdmgx8HG5i=MMsOtvJcpMnF-R3FxLrMhd5bHYNSGxZWp=w@mail.gmail.com>
Subject: Re: [PATCH v3 16/16] objtool,x86: Rewrite retpoline thunk calls
To:     Nathan Chancellor <nathan@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        =?UTF-8?B?RsSBbmctcnXDrCBTw7JuZw==?= <maskray@google.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>, lma@semihalf.com,
        Guenter Roeck <groeck@google.com>,
        Juergen Gross <jgross@suse.com>, lb@semihalf.com,
        LKML <linux-kernel@vger.kernel.org>, mbenes@suse.com,
        =?UTF-8?Q?Rados=C5=82aw_Biernacki?= <rad@semihalf.com>,
        upstream@semihalf.com,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Sami Tolvanen <samitolvanen@google.com>
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On Tue, Jun 8, 2021 at 10:30 AM Nathan Chancellor <nathan@kernel.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@archlinux-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