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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Fri, 20 Nov 2020 12:58:41 -0800
Message-ID: <CABCJKucJ87wa73YJkN_dYUyE7foQT+12gdWJZw1PgZ_decFr4w@mail.gmail.com>
Subject: Re: [PATCH v7 02/17] kbuild: add support for Clang LTO
To:     Kees Cook <keescook@chromium.org>
Cc:     Nathan Chancellor <natechancellor@gmail.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Will Deacon <will@kernel.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>, linux-pci@vger.kernel.org
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On Fri, Nov 20, 2020 at 12:43 PM Kees Cook <keescook@chromium.org> wrote:
>
> 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)

I'm fine with adding ThinLTO as another option to the LTO choice, but
it would duplicate the dependencies and a lot of the help text. I
suppose we could add another config for the dependencies and have both
LTO options depend on that instead.

Sami