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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Tue, 1 Dec 2020 13:38:17 -0800
Message-ID: <CABCJKudBfoE-Ya6_ZuS4d+DqOWObhFFU1bNe_NpfHroLvfOiyQ@mail.gmail.com>
Subject: Re: [PATCH v7 00/17] Add support for Clang LTO
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Kees Cook <keescook@chromium.org>, Will Deacon <will@kernel.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        PCI <linux-pci@vger.kernel.org>
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On Tue, Dec 1, 2020 at 11:51 AM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@googlegroups.com> wrote:
>
> On Tue, Dec 1, 2020 at 9:31 AM Kees Cook <keescook@chromium.org> wrote:
> >
> > On Mon, Nov 30, 2020 at 12:01:31PM +0000, Will Deacon wrote:
> > > Hi Sami,
> > >
> > > 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].
> > >
> > > Sounds like you're going to post a v8, but that's the plan for merging
> > > that? The arm64 parts look pretty good to me now.
> >
> > I haven't seen Masahiro comment on this in a while, so given the review
> > history and its use (for years now) in Android, I will carry v8 (assuming
> > all is fine with it) it in -next unless there are objections.
>
> I had some minor stylistic feedback on the Kconfig changes; I'm happy
> for you to land the bulk of the changes and then I follow up with
> patches to the Kconfig after.

These are included in v8, which I just sent out.

Sami