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From:   Arnd Bergmann <arnd@kernel.org>
Date:   Wed, 9 Dec 2020 18:51:34 +0100
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Subject: Re: [PATCH v8 00/16] Add support for Clang LTO
To:     Sami Tolvanen <samitolvanen@google.com>
Cc:     Masahiro Yamada <masahiroy@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Will Deacon <will@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
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        linux-pci <linux-pci@vger.kernel.org>
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On Wed, Dec 9, 2020 at 5:25 PM 'Sami Tolvanen' via Clang Built Linux
<clang-built-linux@googlegroups.com> wrote:
>
> On Wed, Dec 9, 2020 at 4:36 AM Arnd Bergmann <arnd@kernel.org> wrote:
> >
> > On Tue, Dec 8, 2020 at 1:15 PM Arnd Bergmann <arnd@kernel.org> wrote:
> >
> >
> > It seems to happen because of CONFIG_TRIM_UNUSED_KSYMS,
> > which is a shame, since I think that is an option we'd always want to
> > have enabled with LTO, to allow more dead code to be eliminated.
>
> Ah yes, this is a known issue. We use TRIM_UNUSED_KSYMS with LTO in
> Android's Generic Kernel Image and the problem is that bitcode doesn't
> yet contain calls to these functions, so autoksyms won't see them. The
> solution is to use a symbol whitelist with LTO to prevent these from
> being trimmed. I suspect we would need a default whitelist for LTO
> builds.

A built-in allowlist sounds good to me. FWIW, in the randconfigs so far, I only
saw five symbols that would need to be on it:

memcpy(), memmove(), memset(), __stack_chk_fail() and __stack_chk_guard

       Arnd