linux-kernel - Re: [PATCH v5 00/29] Add support for Clang LTO

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Message-ID: <20201009210548.GB1448445@google.com>
Date:   Fri, 9 Oct 2020 14:05:48 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Will Deacon <will@...nel.org>,
        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@...glegroups.com,
        kernel-hardening@...ts.openwall.com, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        x86@...nel.org
Subject: Re: [PATCH v5 00/29] Add support for Clang LTO

On Fri, Oct 09, 2020 at 03:35:12PM -0400, Steven Rostedt wrote:
> On Fri,  9 Oct 2020 09:13:09 -0700
> Sami Tolvanen <samitolvanen@...gle.com> wrote:
> 
> > This patch series adds support for building x86_64 and arm64 kernels
> > with Clang's Link Time Optimization (LTO).
> > 
> > In addition to performance, the primary motivation for LTO is
> > to allow Clang's Control-Flow Integrity (CFI) to be used in the
> > kernel. Google has shipped millions of Pixel devices running three
> > major kernel versions with LTO+CFI since 2018.
> > 
> > Most of the patches are build system changes for handling LLVM
> > bitcode, which Clang produces with LTO instead of ELF object files,
> > postponing ELF processing until a later stage, and ensuring initcall
> > ordering.
> > 
> > Note that this version is based on tip/master to reduce the number
> > of prerequisite patches, and to make it easier to manage changes to
> > objtool. Patch 1 is from Masahiro's kbuild tree, and while it's not
> > directly related to LTO, it makes the module linker script changes
> > cleaner.
> > 
> 
> I went to test this, but it appears that the latest tip/master fails to
> build for me. This error is on tip/master, before I even applied a single
> patch.
> 
> (config attached)

Ah yes, X86_DECODER_SELFTEST seems to be broken in tip/master. If you
prefer, I have these patches on top of mainline here:

  https://github.com/samitolvanen/linux/tree/clang-lto

Testing your config with LTO on this tree, it does build and boot for
me, although I saw a couple of new objtool warnings, and with LLVM=1,
one warning from llvm-objdump.

Sami