linux-kernel - Re: [PATCH v8 00/16] Add support for Clang LTO

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Date:   Wed, 9 Dec 2020 13:35:54 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Will Deacon <will@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-pci <linux-pci@...r.kernel.org>
Subject: Re: [PATCH v8 00/16] Add support for Clang LTO

On Tue, Dec 8, 2020 at 1:15 PM Arnd Bergmann <arnd@...nel.org> wrote:
>
> - one build seems to have dropped all symbols the string operations
> from vmlinux,
>   so while the link goes through, modules cannot be loaded:
>  ERROR: modpost: "memmove" [drivers/media/rc/rc-core.ko] undefined!
>  ERROR: modpost: "memcpy" [net/wireless/cfg80211.ko] undefined!
>  ERROR: modpost: "memcpy" [net/8021q/8021q.ko] undefined!
>  ERROR: modpost: "memset" [net/8021q/8021q.ko] undefined!
>  ERROR: modpost: "memcpy" [net/unix/unix.ko] undefined!
>  ERROR: modpost: "memset" [net/sched/cls_u32.ko] undefined!
>  ERROR: modpost: "memcpy" [net/sched/cls_u32.ko] undefined!
>  ERROR: modpost: "memset" [net/sched/sch_skbprio.ko] undefined!
>  ERROR: modpost: "memcpy" [net/802/garp.ko] undefined!
>  I first thought this was related to a clang-12 bug I saw the other day, but
>  this also happens with clang-11

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.

       Arnd