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

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Message-ID: <CAK8P3a32HwzZYDK3i68fY0JLGCj18RH1iDMq70OZpTrsopyCcw@mail.gmail.com>
Date:   Tue, 8 Dec 2020 17:56:13 +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 5:53 PM 'Sami Tolvanen' via Clang Built Linux
<clang-built-linux@...glegroups.com> wrote:
>
> > A small update here: I see this behavior with every single module
> > build, including 'tinyconfig' with one module enabled, and 'defconfig'.
>
> The .o file here is a thin archive of the bitcode files for the
> module. We compile .lto.o from that before modpost, because we need an
> ELF binary to process, and then reuse the .lto.o file when linking the
> final module.
>
> At no point should we link the .o file again, especially not with
> .lto.o, because that would clearly cause every symbol to be
> duplicated, so I'm not sure what goes wrong here. Here's the relevant
> part of scripts/Makefile.modfinal:
>
> ifdef CONFIG_LTO_CLANG
> # With CONFIG_LTO_CLANG, reuse the object file we compiled for modpost to
> # avoid a second slow LTO link
> prelink-ext := .lto
> ...
> $(modules): %.ko: %$(prelink-ext).o %.mod.o scripts/module.lds FORCE
>         +$(call if_changed,ld_ko_o)

Ah, it's probably a local problem now, as I had a merge conflict against
linux-next in this Makefile and I must have resolved the conflict incorrectly.

        Arnd