linux-kernel - Re: [PATCH v7 02/17] kbuild: add support for Clang LTO

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Date:   Fri, 20 Nov 2020 12:43:14 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Will Deacon <will@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-pci@...r.kernel.org
Subject: Re: [PATCH v7 02/17] kbuild: add support for Clang LTO

On Fri, Nov 20, 2020 at 01:29:35PM -0700, Nathan Chancellor wrote:
> On Fri, Nov 20, 2020 at 11:47:21AM -0800, Kees Cook wrote:
> > On Fri, Nov 20, 2020 at 08:23:11AM -0800, Sami Tolvanen wrote:
> > > Changing the ThinLTO config to a choice and moving it after the main
> > > LTO config sounds like a good idea to me. I'll see if I can change
> > > this in v8. Thanks!
> > 
> > Originally, I thought this might be a bit ugly once GCC LTO is added,
> > but this could be just a choice like we're done for the stack
> > initialization. Something like an "LTO" choice of NONE, CLANG_FULL,
> > CLANG_THIN, and in the future GCC, etc.
> 
> Having two separate choices might be a little bit cleaner though? One
> for the compiler (LTO_CLANG versus LTO_GCC) and one for the type
> (THINLTO versus FULLLTO). The type one could just have a "depends on
> CC_IS_CLANG" to ensure it only showed up when needed.

Right, that's how the stack init choice works. Kconfigs that aren't
supported by the compiler won't be shown. I.e. after Sami's future
patch, the only choice for GCC will be CONFIG_LTO_NONE. But building
under Clang, it would offer CONFIG_LTO_NONE, CONFIG_LTO_CLANG_FULL,
CONFIG_LTO_CLANG_THIN, or something.

(and I assume  CONFIG_LTO would be def_bool y, depends on !LTO_NONE)

-- 
Kees Cook