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From: Ard Biesheuvel <ardb@kernel.org>
To: Kees Cook <keescook@chromium.org>
Cc: Will Deacon <will@kernel.org>, Dave Martin <Dave.Martin@arm.com>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Peter Collingbourne <pcc@google.com>,
	James Morse <james.morse@arm.com>, Borislav Petkov <bp@suse.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	Russell King <linux@armlinux.org.uk>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Arvind Sankar <nivedita@alum.mit.edu>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Nathan Chancellor <natechancellor@gmail.com>,
	Arnd Bergmann <arnd@arndb.de>, X86 ML <x86@kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	linux-efi <linux-efi@vger.kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 05/17] ctype: Work around Clang -mbranch-protection=none bug
Date: Mon, 29 Jun 2020 10:15:47 +0200
Message-ID: <CAMj1kXE+toCd=Bx-zw7D9bvDRNB2aPn5-_7CY7MOKcVGA-azVg@mail.gmail.com> (raw)
In-Reply-To: <20200629061840.4065483-6-keescook@chromium.org>

On Mon, 29 Jun 2020 at 08:18, Kees Cook <keescook@chromium.org> wrote:
>
> In preparation for building efi/libstub with -mbranch-protection=none
> (EFI does not support branch protection features[1]), add no-op code
> to work around a Clang bug that emits an unwanted .note.gnu.property
> section for object files without code[2].
>
> [1] https://lore.kernel.org/lkml/CAMj1kXHck12juGi=E=P4hWP_8vQhQ+-x3vBMc3TGeRWdQ-XkxQ@mail.gmail.com
> [2] https://bugs.llvm.org/show_bug.cgi?id=46480
>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: Will Deacon <will@kernel.org>
> Cc: Dave Martin <Dave.Martin@arm.com>
> Cc: clang-built-linux@googlegroups.com
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  lib/ctype.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/lib/ctype.c b/lib/ctype.c
> index c819fe269eb2..21245ed57d90 100644
> --- a/lib/ctype.c
> +++ b/lib/ctype.c
> @@ -36,3 +36,13 @@ _L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,     /* 224-239 */
>  _L,_L,_L,_L,_L,_L,_L,_P,_L,_L,_L,_L,_L,_L,_L,_L};      /* 240-255 */
>
>  EXPORT_SYMBOL(_ctype);
> +
> +/*
> + * Clang will generate .note.gnu.property sections for object files
> + * without code, even in the presence of -mbranch-protection=none.
> + * To work around this, define an unused static function.
> + * https://bugs.llvm.org/show_bug.cgi?id=46480
> + */
> +#ifdef CONFIG_CC_IS_CLANG
> +void __maybe_unused __clang_needs_code_here(void) { }
> +#endif
> --
> 2.25.1
>

I take it we don't need this horrible hack if we build the EFI stub
with branch protections and filter out the .note.gnu.property section
explicitly?

Sorry to backpedal, but that is probably a better approach after all,
given that the instructions don't hurt, and we will hopefully be able
to arm them once UEFI (as well as PE/COFF) gets around to describing
this in a way that both the firmware and the OS can consume.

  reply index

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-29  6:18 [PATCH v4 00/17] Warn on orphan section placement Kees Cook
2020-06-29  6:18 ` [PATCH v4 01/17] vmlinux.lds.h: Add .gnu.version* to DISCARDS Kees Cook
2020-06-29  6:18 ` [PATCH v4 02/17] vmlinux.lds.h: Avoid KASAN and KCSAN's unwanted sections Kees Cook
2020-06-29  6:18 ` [PATCH v4 03/17] vmlinux.lds.h: Split ELF_DETAILS from STABS_DEBUG Kees Cook
2020-06-29  6:18 ` [PATCH v4 04/17] vmlinux.lds.h: Add .symtab, .strtab, and .shstrtab to ELF_DETAILS Kees Cook
2020-06-29  6:18 ` [PATCH v4 05/17] ctype: Work around Clang -mbranch-protection=none bug Kees Cook
2020-06-29  8:15   ` Ard Biesheuvel [this message]
2020-06-29 14:43     ` Kees Cook
2020-06-29 15:06     ` Kees Cook
2020-06-29 15:08       ` Ard Biesheuvel
2020-06-29 15:16         ` Kees Cook
2020-06-29 18:02       ` Nick Desaulniers
2020-06-29 18:37         ` Kees Cook
2020-06-29  6:18 ` [PATCH v4 06/17] efi/libstub: Disable -mbranch-protection Kees Cook
2020-06-29  6:18 ` [PATCH v4 07/17] arm64/build: Use common DISCARDS in linker script Kees Cook
2020-06-29  6:18 ` [PATCH v4 08/17] arm64/mm: Remove needless section quotes Kees Cook
2020-06-29 19:53   ` Nick Desaulniers
2020-06-29 20:03     ` Joe Perches
2020-06-29 20:29       ` Nick Desaulniers
2020-06-29 20:04     ` Kees Cook
2020-06-29 20:54       ` Arvind Sankar
2020-06-29 21:06         ` Arvind Sankar
2020-06-29  6:18 ` [PATCH v4 09/17] arm64/build: Remove .eh_frame* sections due to unwind tables Kees Cook
2020-06-29  6:18 ` [PATCH v4 10/17] arm64/kernel: Remove needless Call Frame Information annotations Kees Cook
2020-06-29  6:18 ` [PATCH v4 11/17] arm64/build: Warn on orphan section placement Kees Cook
2020-06-29  6:18 ` [PATCH v4 12/17] arm/build: Refactor linker script headers Kees Cook
2020-06-29  6:18 ` [PATCH v4 13/17] arm/build: Explicitly keep .ARM.attributes sections Kees Cook
2020-06-29  6:18 ` [PATCH v4 14/17] arm/build: Warn on orphan section placement Kees Cook
2020-06-29 15:54   ` Arvind Sankar
2020-06-29 18:07     ` Russell King - ARM Linux admin
2020-06-29 18:15       ` Arvind Sankar
2020-06-29 19:03         ` Russell King - ARM Linux admin
2020-06-29 19:47   ` Nick Desaulniers
2020-06-29  6:18 ` [PATCH v4 15/17] arm/boot: " Kees Cook
2020-06-29  6:18 ` [PATCH v4 16/17] x86/build: " Kees Cook
2020-06-29  6:18 ` [PATCH v4 17/17] x86/boot: " Kees Cook

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