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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
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