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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Tue, 25 Aug 2020 00:31:18 -0700
Message-ID: <CAKwvOdmRz6d9touuZnotpF3vs0m5M9MbbKX8M_XmTCZ+8Fe6-A@mail.gmail.com>
Subject: Re: [PATCH 0/4] -ffreestanding/-fno-builtin-* patches
To:     Masahiro Yamada <masahiroy@kernel.org>
Cc:     =?UTF-8?B?RMOhdmlkIEJvbHZhbnNrw70=?= <david.bolvansky@gmail.com>,
        Eli Friedman <efriedma@quicinc.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Michal Marek <michal.lkml@markovi.net>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
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        Kees Cook <keescook@chromium.org>,
        Tony Luck <tony.luck@intel.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Joe Perches <joe@perches.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Daniel Axtens <dja@axtens.net>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Alexandru Ardelean <alexandru.ardelean@analog.com>,
        Yury Norov <yury.norov@gmail.com>,
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        "Paul E . McKenney" <paulmck@kernel.org>,
        Daniel Kiper <daniel.kiper@oracle.com>,
        Bruce Ashfield <bruce.ashfield@gmail.com>,
        Marco Elver <elver@google.com>,
        Vamshi K Sthambamkadi <vamshi.k.sthambamkadi@gmail.com>,
        Arvind Sankar <nivedita@alum.mit.edu>
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On Tue, Aug 25, 2020 at 12:10 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Mon, Aug 24, 2020 at 10:34 AM Arvind Sankar <nivedita@alum.mit.edu> wrote:
> >
> > On Tue, Aug 25, 2020 at 12:57:22AM +0900, Masahiro Yamada wrote:
> > >
> > >
> > > To prevent transformation from foo() into bar(),
> > > there are two ways in Clang to do that;
> > > -fno-builtin-foo, and -fno-builtin-bar.
> > > There is only one in GCC; -fno-buitin-foo.
> > >
> > > Is this correct?
> > >
> >
> > It looked that way from previous experimentation, but...
> >
> > >
> > >
> > > I just played the optimization
> > > from printf("helloworld\n") to puts("helloworld").
> > >
> > > https://godbolt.org/z/5s4ded
> > >
> > >
> > > -fno-builtin-puts cannot prevent clang
> > > from emitting puts.
> > > Is it because clang does not support
> > > -fno-builtin-puts?
> >
> > Ugh. clang doesn't have __builtin_puts() but it optimizes printf() into
> > puts(). It doesn't have __builtin_putchar() but will optimize
> > printf("c") into putchar('c').
>
> Bah, merely a <strikethrough>flesh
> wound</strikethrough><strikethrough>compiler bug</strikethrough>rather
> long TODO in the compiler.
> https://github.com/llvm/llvm-project/blob/be2bc7d4cef2edd66c7fb74b70adf62fc68754db/clang/include/clang/Basic/Builtins.def#L943
>
> Anyways, give me a week and I'll hack through the rest of them
> https://reviews.llvm.org/D86508.  Certainly made HPA's point hit home,
> that's a lot of functionality to implement or disable in an
> environment.
>
> Masahiro, are you implying that we shouldn't take the
> -fno-builtin-stpcpy patch, because Clang is inconsistent? (That can be
> fixed.) Even though -fno-builtin-stpcpy works here as intended?
> https://lore.kernel.org/lkml/20200817220212.338670-2-ndesaulniers@google.com/

Sorry, the above link ^ should be this hunk (beyond tired, getting up
in 4.5hrs for plumbers):

diff --git a/Makefile b/Makefile
index c4470a4e131f..6a08cdfa58ae 100644
--- a/Makefile
+++ b/Makefile
@@ -577,6 +577,7 @@ ifneq ($(LLVM_IAS),1)
 CLANG_FLAGS    += -no-integrated-as
 endif
 CLANG_FLAGS    += -Werror=unknown-warning-option
+CLANG_FLAGS    += -fno-builtin-stpcpy
 KBUILD_CFLAGS  += $(CLANG_FLAGS)
 KBUILD_AFLAGS  += $(CLANG_FLAGS)
 export CLANG_FLAGS

>
> Otherwise we need to provide an implementation of this symbol in the kernel.
> https://lore.kernel.org/lkml/20200815020946.1538085-1-ndesaulniers@google.com/
>
> Please, pick your poison.
> --
> Thanks,
> ~Nick Desaulniers


-- 
Thanks,
~Nick Desaulniers