linux-kernel - Re: [PATCH 0/4] -ffreestanding/-fno-builtin-* patches

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Message-ID: <CAHk-=wjX6jaRH+yozTh3kV3ocF2Qv0v96o8JofvbV6=U_K2mXw@mail.gmail.com>
Date:   Fri, 21 Aug 2020 12:23:10 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Dávid Bolvanský <david.bolvansky@...il.com>,
        Eli Friedman <efriedma@...cinc.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Michal Marek <michal.lkml@...kovi.net>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Tony Luck <tony.luck@...el.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Joe Perches <joe@...ches.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Daniel Axtens <dja@...ens.net>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Alexandru Ardelean <alexandru.ardelean@...log.com>,
        Yury Norov <yury.norov@...il.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        Bruce Ashfield <bruce.ashfield@...il.com>,
        Marco Elver <elver@...gle.com>,
        Vamshi K Sthambamkadi <vamshi.k.sthambamkadi@...il.com>
Subject: Re: [PATCH 0/4] -ffreestanding/-fno-builtin-* patches

On Fri, Aug 21, 2020 at 12:15 PM Arvind Sankar <nivedita@...m.mit.edu> wrote:
>
> How are you testing it?
>
> https://godbolt.org/z/eahdGn

Ugh. I tested the reverse thing - that the builtin is still available
for manual use despite the -fno-builtin.

Because I - stupidly - assumed that fno-builtin would do *something*.

But if it's available for manual use _and_ it's used implicitly by the
compiler, then that was clearly very naive of me.

                     Linus