Re: [PATCH 1/4] Makefile: add -fno-builtin-stpcpy

From: Ard Biesheuvel <ardb@kernel.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: "Masahiro Yamada" <masahiroy@kernel.org>,
	"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@vger.kernel.org,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"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>,
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	"Daniel Axtens" <dja@axtens.net>,
	"Arvind Sankar" <nivedita@alum.mit.edu>,
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	"Yury Norov" <yury.norov@gmail.com>, "X86 ML" <x86@kernel.org>,
	"H . Peter Anvin" <hpa@zytor.com>,
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	"Marco Elver" <elver@google.com>,
	"Vamshi K Sthambamkadi" <vamshi.k.sthambamkadi@gmail.com>,
	"Andi Kleen" <ak@suse.de>,
	"Linus Torvalds" <torvalds@linux-foundation.org>,
	"Dávid Bolvanský" <david.bolvansky@gmail.com>,
	"Eli Friedman" <efriedma@quicinc.com>,
	"# 3.4.x" <stable@vger.kernel.org>,
	"Sami Tolvanen" <samitolvanen@google.com>
Subject: Re: [PATCH 1/4] Makefile: add -fno-builtin-stpcpy
Date: Tue, 18 Aug 2020 09:10:01 +0200	[thread overview]
Message-ID: <CAMj1kXH0gRCaoF0NziC870=eSEy0ghi8b0b6A+LMu8PMd58C0Q@mail.gmail.com> (raw)
In-Reply-To: <20200817220212.338670-2-ndesaulniers@google.com>

On Tue, 18 Aug 2020 at 00:02, Nick Desaulniers <ndesaulniers@google.com> wrote:
>
> LLVM implemented a recent "libcall optimization" that lowers calls to
> `sprintf(dest, "%s", str)` where the return value is used to
> `stpcpy(dest, str) - dest`. This generally avoids the machinery involved
> in parsing format strings. This optimization was introduced into
> clang-12. Because the kernel does not provide an implementation of
> stpcpy, we observe linkage failures for almost all targets when building
> with ToT clang.
>
> The interface is unsafe as it does not perform any bounds checking.
> Disable this "libcall optimization" via `-fno-builtin-stpcpy`.
>
> Unlike
> commit 5f074f3e192f ("lib/string.c: implement a basic bcmp")
> which cited failures with `-fno-builtin-*` flags being retained in LLVM
> LTO, that bug seems to have been fixed by
> https://reviews.llvm.org/D71193, so the above sha can now be reverted in
> favor of `-fno-builtin-bcmp`.
>
> Cc: stable@vger.kernel.org # 4.4

Why does a fix for Clang-12 have to be backported all the way to v4.4?
How does that meet the requirements for stable patches?

> Link: https://bugs.llvm.org/show_bug.cgi?id=47162
> Link: https://github.com/ClangBuiltLinux/linux/issues/1126
> Link: https://reviews.llvm.org/D85963
> Reported-by: Sami Tolvanen <samitolvanen@google.com>
> Suggested-by: Dávid Bolvanský <david.bolvansky@gmail.com>
> Suggested-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
>  Makefile | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/Makefile b/Makefile
> index 9cac6fde3479..211a1b6f6478 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -959,6 +959,12 @@ ifdef CONFIG_RETPOLINE
>  KBUILD_CFLAGS += $(call cc-option,-fcf-protection=none)
>  endif
>
> +# The compiler may "libcall optimize" certain function calls into the below
> +# functions, for architectures that don't use -ffreestanding. If we don't plan
> +# to provide implementations of these routines, then prevent the compiler from
> +# emitting calls to what will be undefined symbols.
> +KBUILD_CFLAGS  += -fno-builtin-stpcpy
> +
>  # include additional Makefiles when needed
>  include-y                      := scripts/Makefile.extrawarn
>  include-$(CONFIG_KASAN)                += scripts/Makefile.kasan
> --
> 2.28.0.220.ged08abb693-goog
>