linux-kernel - Re: [PATCH v7 00/17] Add support for Clang LTO

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Date:   Tue, 1 Dec 2020 09:31:00 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Will Deacon <will@...nel.org>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...nel.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
Subject: Re: [PATCH v7 00/17] Add support for Clang LTO

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.

-- 
Kees Cook