linux-kernel - Re: [PATCH v4] Makefile.compiler: replace cc-ifversion with compiler-specific macros

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Message-ID: <CAKwvOd=2zAV_mizvzLFdyHE_4OzBY5OVu6KLWuQPOMZK37vsmQ@mail.gmail.com>
Date:   Mon, 22 May 2023 12:52:13 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Greg KH <gregkh@...uxfoundation.org>,
        Maksim Panchenko <maks@...a.com>,
        Ricardo Cañuelo <ricardo.canuelo@...labora.com>
Cc:     Shreeya Patel <shreeya.patel@...labora.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <llvm@...ts.linux.dev>,
        Bill Wendling <morbo@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        regressions@...ts.linux.dev,
        "gustavo.padovan@...labora.com" <gustavo.padovan@...labora.com>,
        Guillaume Charles Tucker <guillaume.tucker@...labora.com>,
        denys.f@...labora.com, kernelci@...ts.linux.dev
Subject: Re: [PATCH v4] Makefile.compiler: replace cc-ifversion with
 compiler-specific macros

On Mon, May 22, 2023 at 9:52 AM Greg KH <gregkh@...uxfoundation.org> wrote:
>
> On Mon, May 22, 2023 at 12:09:34PM +0200, Ricardo Cañuelo wrote:
> > On vie, may 19 2023 at 08:57:24, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
> > > It could be; if the link order was changed, it's possible that this
> > > target may be hitting something along the lines of:
> > > https://isocpp.org/wiki/faq/ctors#static-init-order i.e. the "static
> > > initialization order fiasco"
> > >
> > > I'm struggling to think of how this appears in C codebases, but I
> > > swear years ago I had a discussion with GKH (maybe?) about this. I
> > > think I was playing with converting Kbuild to use Ninja rather than
> > > Make; the resulting kernel image wouldn't boot because I had modified
> > > the order the object files were linked in.  If you were to randomly
> > > shuffle the object files in the kernel, I recall some hazard that may
> > > prevent boot.
> >
> > I thought that was specifically a C++ problem? But then again, the
> > kernel docs explicitly say that the ordering of obj-y goals in kbuild is
> > significant in some instances [1]:
>
> Yes, it matters, you can not change it.  If you do, systems will break.
> It is the only way we have of properly ordering our init calls within
> the same "level".

Ah, right it was the initcall ordering. Thanks for the reminder.

(There's a joke in there similar to the use of regexes to solve a
problem resulting in two new problems; initcalls have levels for
ordering, but we still have (unexpressed) dependencies between calls
of the same level; brittle!).

+Maksim, since that might be relevant info for the BOLT+Kernel work.

Ricardo,
https://elinux.org/images/e/e8/2020_ELCE_initcalls_myjosserand.pdf
mentions that there's a kernel command line param `initcall_debug`.
Perhaps that can be used to see if
5750121ae7382ebac8d47ce6d68012d6cd1d7926 somehow changed initcall
ordering, resulting in a config that cannot boot?
-- 
Thanks,
~Nick Desaulniers