linux-kernel - Re: [PATCH v5 25/29] arm64: allow LTO

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Date:   Mon, 12 Oct 2020 14:02:35 -0700
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>,
        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,
        x86@...nel.org
Subject: Re: [PATCH v5 25/29] arm64: allow LTO_CLANG and THINLTO to be
 selected

On Mon, Oct 12, 2020 at 09:51:09PM +0100, Will Deacon wrote:
> On Mon, Oct 12, 2020 at 01:44:56PM -0700, Kees Cook wrote:
> > On Mon, Oct 12, 2020 at 09:31:16AM +0100, Will Deacon wrote:
> > > On Fri, Oct 09, 2020 at 09:13:34AM -0700, Sami Tolvanen wrote:
> > > > Allow CONFIG_LTO_CLANG and CONFIG_THINLTO to be enabled.
> > > > 
> > > > Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> > > > Reviewed-by: Kees Cook <keescook@...omium.org>
> > > > ---
> > > >  arch/arm64/Kconfig | 2 ++
> > > >  1 file changed, 2 insertions(+)
> > > > 
> > > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > > > index ad522b021f35..7016d193864f 100644
> > > > --- a/arch/arm64/Kconfig
> > > > +++ b/arch/arm64/Kconfig
> > > > @@ -72,6 +72,8 @@ config ARM64
> > > >  	select ARCH_USE_SYM_ANNOTATIONS
> > > >  	select ARCH_SUPPORTS_MEMORY_FAILURE
> > > >  	select ARCH_SUPPORTS_SHADOW_CALL_STACK if CC_HAVE_SHADOW_CALL_STACK
> > > > +	select ARCH_SUPPORTS_LTO_CLANG
> > > > +	select ARCH_SUPPORTS_THINLTO
> > > 
> > > Please don't enable this for arm64 until we have the dependency stuff sorted
> > > out. I posted patches [1] for this before, but I think they should be part
> > > of this series as they don't make sense on their own.
> > 
> > Oh, hm. We've been trying to trim down this series, since it's already
> > quite large. Why can't [1] land first? It would make this easier to deal
> > with, IMO.
> 
> I'm happy to handle [1] along with the LTO Kconfig change when the time
> comes, if that helps. I just don't want to merge dead code!

Okay, understood. Thanks!

> 
> Will
> 
> > > [1] https://git.kernel.org/pub/scm/linux/kernel/git/will/linux.git/log/?h=rwonce/read-barrier-depends

-- 
Kees Cook