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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Tue, 21 Jun 2022 08:37:07 -0700
Message-ID: <CABCJKudwqKOGfE0Ha9b8DXaZ23bvKk+DhdVbJHoLneV5zJaZzQ@mail.gmail.com>
Subject: Re: [PATCH] kbuild: Ignore __this_module in gen_autoksyms.sh
To:     Masahiro Yamada <masahiroy@kernel.org>
Cc:     Nicolas Schier <nicolas@fjasle.eu>,
        Alexander Lobakin <alobakin@pm.me>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Steve Muckle <smuckle@google.com>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Sat, Jun 18, 2022 at 4:01 PM Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> On Fri, Jun 17, 2022 at 4:58 AM Sami Tolvanen <samitolvanen@google.com> wrote:
> >
> > Module object files can contain an undefined reference to __this_module,
> > which isn't resolved until we link the final .ko. The kernel doesn't
> > export this symbol, so ignore it in gen_autoksyms.sh.
>
> OK, I understand these two sentences.
>
> > This avoids an
> > unnecessary vmlinux rebuild with UNUSED_KSYMS_WHITELIST when we have a
> > symbol list that already contains all the module dependencies.
> >
>
> I do not understand how this can happen.
>
>
> Can you provide me steps to reproduce it?

This issue only happens when we have a whitelist that already contains
all the symbols we need to export. As autoksyms.h contains all the
necessary symbols in the initial vmlinux build, there should be no
need to link vmlinux again. However, as the code is looking at
undefined symbols in module .o files before __this_module is resolved,
adjust_autoksyms.sh thinks that __this_module is still missing and
triggers a rebuild, without actually changing anything.

I suspect this isn't a common situation, but it does happen when
building Android kernels, which specify a complete list of exported
symbols. Linking vmlinux.o takes a while with LTO and we would like to
avoid performing this step more than once.

Sami