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From:   Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date:   Thu, 6 Jun 2019 11:34:54 +0200
Message-ID: <CAKv+Gu9qg0K44hWtKH9vycxhUF4e2zB87kLqw33Jt+Shc1+9HQ@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:     Greg KH <gregkh@linuxfoundation.org>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        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 Thu, 6 Jun 2019 at 10:58, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
>
> On Thu, 6 Jun 2019 at 09:08, Greg KH <gregkh@linuxfoundation.org> wrote:
> >
> > On Thu, Jun 06, 2019 at 08:55:29AM +0200, Ard Biesheuvel wrote:
> > > On Wed, 5 Jun 2019 at 22:48, Nick Desaulniers <ndesaulniers@google.com> wrote:
> > > >
> > > > 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
> > > >
> > >
> > > If you are saying that plain upstream 4.9-stable defconfig can be
> > > built with Clang, then I am pleasantly surprised.
> >
> > I know some specific configs can, there's no rule that I know of that
> > 'defconfig' support is required.  But then again, it might also work,
> > try it and see :)
> >
>
> Well, it is the rule that the arm64 maintainers use.
>
> > > > 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.
> > > >
> > >
> > > I know that (since you keep reminding me :-)), but obviously, Google
> > > does not care about changes that regress GCC support.
> >
> > What are you talking about?  Bugs happen all the time, what specifically
> > did "Google" do to break gcc support?  If you are referring to this
> > patch, and it is a regression, of course I will revert it.  But note
> > that gcc and 4.9 works just fine for all of the other users right now,
> > remember we do do a lot of testing of these releases.
> >
>
> Don't get me wrong: I am not blaming Google for this. But having
> strict Documented/ stable-rules, violating them by backporting patches
> that are clearly not bug fixes, and *then* saying 'bugs happen all the
> time' makes no sense to me at all.

BTW I hit the same issue immediately building 4.9.180 defconfig +
CONFIG_RANDOMIZE_BASE=y, using my distro GCC (6.3.0), so I'd say the
testing coverage is not sufficient.