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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Fri, 20 Nov 2020 12:25:09 -0800
Message-ID: <CABCJKueTjhjSY=oOD3bWqn7E_1X9k-MK+6r_Z35AiCKDTNK18g@mail.gmail.com>
Subject: Re: [PATCH v7 00/17] Add support for Clang LTO
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     Masahiro Yamada <masahiroy@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Will Deacon <will@kernel.org>,
        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-kernel <linux-arm-kernel@lists.infradead.org>,
        linux-kbuild <linux-kbuild@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, linux-pci@vger.kernel.org
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On Thu, Nov 19, 2020 at 8:04 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> On Wed, Nov 18, 2020 at 02:07:14PM -0800, Sami Tolvanen wrote:
> > This patch series adds support for building the kernel 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 v7 brings back arm64 support as Will has now staged the
> > prerequisite memory ordering patches [1], and drops x86_64 while we work
> > on fixing the remaining objtool warnings [2].
>
> Sami,
>
> Here are some patches to fix the objtool issues (other than crypto which
> I'll work on next).
>
>   https://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git objtool-vmlinux

Thanks, Josh! I can confirm that these fix all the non-crypto objtool
warnings with LTO as well.

Sami