Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20201118220731.925424-1-samitolvanen@google.com>
 <20201130120130.GF24563@willie-the-truck> <202012010929.3788AF5@keescook>
In-Reply-To: <202012010929.3788AF5@keescook>
From:   Masahiro Yamada <masahiroy@kernel.org>
Date:   Wed, 2 Dec 2020 11:42:21 +0900
Message-ID: <CAK7LNASQPOGohtUyzBM6n54pzpLN35kDXC7VbvWzX8QWUmqq9g@mail.gmail.com>
Subject: Re: [PATCH v7 00/17] Add support for Clang LTO
To:     Kees Cook <keescook@chromium.org>
Cc:     Will Deacon <will@kernel.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        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>,
        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 mailing list <linux-kbuild@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-pci@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Wed, Dec 2, 2020 at 2:31 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.


What I dislike about this implementation is
it cannot drop any unreachable function/data.
(and it is completely different from GCC LTO)

This is not real LTO.


> --
> Kees Cook
>
> --
> You received this message because you are subscribed to the Google Groups "Clang Built Linux" group.
> To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@googlegroups.com.
> To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/202012010929.3788AF5%40keescook.


-- 
Best Regards
Masahiro Yamada