linux-kernel - Re: [PATCH] kallsyms: fix nonconverging kallsyms table with lld

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Date:   Wed, 9 Jun 2021 17:23:28 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        David Brazdil <dbrazdil@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Mikhail Petrov <Mikhail.Petrov@....dev>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        mfaltesek@...gle.com
Subject: Re: [PATCH] kallsyms: fix nonconverging kallsyms table with lld

On Wed, Jun 9, 2021 at 5:16 PM Guenter Roeck <linux@...ck-us.net> wrote:
>
> On Wed, Jun 09, 2021 at 01:24:18PM +0200, Arnd Bergmann wrote:
> > On Wed, Jun 9, 2021 at 1:05 PM Guenter Roeck <linux@...ck-us.net> wrote:
> > > On Thu, Feb 04, 2021 at 04:29:47PM +0100, Arnd Bergmann wrote:
> > > > From: Arnd Bergmann <arnd@...db.de>
> > > >
> > > > ARM randconfig builds with lld sometimes show a build failure
> > > > from kallsyms:
> > > >
> > > >   Inconsistent kallsyms data
> > > >   Try make KALLSYMS_EXTRA_PASS=1 as a workaround
> > > >
> > > > The problem is the veneers/thunks getting added by the linker extend
> > > > the symbol table, which in turn leads to more veneers being needed,
> > > > so it may take a few extra iterations to converge.
> > > >
> > > > This bug has been fixed multiple times before, but comes back every time
> > > > a new symbol name is used. lld uses a different set of idenitifiers from
> > > > ld.bfd, so the additional ones need to be added as well.
> > > >
> > > > I looked through the sources and found that arm64 and mips define similar
> > > > prefixes, so I'm adding those as well, aside from the ones I observed. I'm
> > > > not sure about powerpc64, which seems to already be handled through a
> > > > section match, but if it comes back, the "__long_branch_" and "__plt_"
> > > > prefixes would have to get added as well.
> > > >
> > >
> > > This is such a whack-a-mole. The problem is hitting us yet again. I suspect
> > > it may be due to a new version of lld using new symbols, but I didn't really
> > > try to track it down. Is there an easy way to search for missed symbols ?
> >
> > The way I did it previously was to hack Kbuild to not remove the temporary
> > files after a failure, and then compare the "objdump --syms" output of the
> > last two stages.
>
> Problem with that is that we have a non-deterministic problem: The build
> fails for us on some build servers, but we are unable to reproduce the
> problem when building the same image manually on a development server.
> That is similar to what I had observed before, where powerpc builds would
> pass on one server, but the same kernel with the same configuration would
> fail to build on a second almost identical server. It would really be great
> if we can find a better solution.

Right, that sucks. I suppose removing the ignore-lists from scripts/kallsyms.c
would make it more easily reproducible after a few local randconfig builds,
at least enough to add some form of scripting that is able to print the names
of the generated symbols.

       Arnd