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

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Message-ID: <20200819002052.GA3397377@rani.riverdale.lan>
Date:   Tue, 18 Aug 2020 20:20:52 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Eli Friedman <efriedma@...cinc.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>,
        "H . Peter Anvin" <hpa@...or.com>,
        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>,
        Andi Kleen <ak@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dávid Bolvanský <david.bolvansky@...il.com>
Subject: Re: [PATCH 0/4] -ffreestanding/-fno-builtin-* patches

On Tue, Aug 18, 2020 at 03:59:45PM -0700, Nick Desaulniers wrote:
> On Tue, Aug 18, 2020 at 3:25 PM Arvind Sankar <nivedita@...m.mit.edu> wrote:
> >
> > Another thing that needs to be fixed is that at least lib/string.c needs
> > to be compiled with -ffreestanding.
> >
> > gcc-10 optimizes the generic memset implementation in there into a call
> > to memset. Now that's on x86 which doesn't use the generic
> > implementation, but this is just waiting to bite us.
> >
> > https://godbolt.org/z/6EhG15
> 
> Admittedly, we've had the same shenanigans with memcpy implemented in
> terms of calls to __builtin_memcpy being lowered to infinitely
> recursive calls...which feels like the same kind of bug.  ("You wanted
> infinite recursion in the kexec purgatory image, right?" "No,
> compiler, I did not.")  example: https://godbolt.org/z/MzrTaM
> (probably should fix this in both implementations; at the least I feel
> like Clang's -Winfinite-recursion should try to help us out here).
> 

What's the other implementation we need to worry about? purgatory (at
least on x86) has freestanding already.