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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Wed, 5 Jun 2019 13:48:31 -0700
Message-ID: <CAKwvOdnPcjESFrQRR_=cCVag3ZSnC0nBqF7+LFHrcDArT_segA@mail.gmail.com>
Subject: Re: Building arm64 EFI stub with -fpie breaks build of 4.9.x
 (undefined reference to `__efistub__GLOBAL_OFFSET_TABLE_')
To:     Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc:     Greg KH <gregkh@linuxfoundation.org>,
        Rolf Eike Beer <eb@emlix.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Matt Fleming <matt@codeblueprint.co.uk>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-efi <linux-efi@vger.kernel.org>,
        Linux Kernel Developers List <linux-kernel@vger.kernel.org>,
        stable <stable@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
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On Wed, Jun 5, 2019 at 11:42 AM Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:
> For the record, this is an example of why I think backporting those
> clang enablement patches is a bad idea.

There's always a risk involved with backports of any kind; more CI
coverage can help us mitigate some of these risks in an automated
fashion before we get user reports like this.  I meet with the
KernelCI folks weekly, so I'll double check on the coverage of the
stable tree's branches.  The 0day folks are also very responsive and
I've spoken with them a few times, so I'll try to get to the bottom of
why this wasn't reported by either of those.

Also, these patches help keep Android, CrOS, and Google internal
production kernels closer to their upstream sources.

> We can't actually build those
> kernels with clang, can we? So what is the point? </grumpy>

Here's last night's build:
https://travis-ci.com/ClangBuiltLinux/continuous-integration/builds/114388434

Also, Android and CrOS have shipped X million devices w/ 4.9 kernels
built with Clang.  I think this number will grow at least one order of
magnitude imminently.

> Alternatively, we can just revert this patch from 4.9

That would break at least the above devices next time Android and CrOS
pulled from stable.

> It would be helpful to get a relocation dump (objdump -r) of
> arm64-stub.o to figure out which symbol needs a 'hidden' annotation to
> prevent GCC from emitting it as a PIC reference requiring a GOT.

Sounds like the best way forward, as well as having more info on which
config/toolchain reliably reproduces the issue.
-- 
Thanks,
~Nick Desaulniers