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Date:   Mon, 29 Jun 2020 08:50:59 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        Nick Desaulniers <ndesaulniers@google.com>,
        Fangrui Song <maskray@google.com>,
        Dmitry Golovin <dima@golovin.in>,
        clang-built-linux@googlegroups.com,
        Ard Biesheuvel <ardb@kernel.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Daniel Kiper <daniel.kiper@oracle.com>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Arnd Bergmann <arnd@arndb.de>,
        "H . J . Lu" <hjl@sourceware.org>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 2/7] x86/boot/compressed: Force hidden visibility for
 all symbol references
Message-ID: <202006290849.940FAE8C9B@keescook>
References: <20200629140928.858507-1-nivedita@alum.mit.edu>
 <20200629140928.858507-3-nivedita@alum.mit.edu>
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On Mon, Jun 29, 2020 at 10:09:23AM -0400, Arvind Sankar wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
> 
> Eliminate all GOT entries in the decompressor binary, by forcing hidden
> visibility for all symbol references, which informs the compiler that
> such references will be resolved at link time without the need for
> allocating GOT entries.
> 
> To ensure that no GOT entries will creep back in, add an assertion to
> the decompressor linker script that will fire if the .got section has
> a non-zero size.
> 
> [Arvind: fixup -include hidden.h to -include $(srctree)/$(src)/hidden.h]
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> Acked-by: Arvind Sankar <nivedita@alum.mit.edu>
> Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
> From: Ard Biesheuvel <ardb@kernel.org>
> Link: https://lore.kernel.org/r/20200523120021.34996-3-ardb@kernel.org
> ---
>  arch/x86/boot/compressed/Makefile      |  1 +
>  arch/x86/boot/compressed/hidden.h      | 19 +++++++++++++++++++
>  arch/x86/boot/compressed/vmlinux.lds.S |  1 +
>  3 files changed, 21 insertions(+)
>  create mode 100644 arch/x86/boot/compressed/hidden.h
> 
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 7619742f91c9..b01c8aed0f23 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -42,6 +42,7 @@ KBUILD_CFLAGS += $(call cc-disable-warning, gnu)
>  KBUILD_CFLAGS += -Wno-pointer-sign
>  KBUILD_CFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
>  KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
> +KBUILD_CFLAGS += -include $(srctree)/$(src)/hidden.h
>  
>  KBUILD_AFLAGS  := $(KBUILD_CFLAGS) -D__ASSEMBLY__
>  GCOV_PROFILE := n
> diff --git a/arch/x86/boot/compressed/hidden.h b/arch/x86/boot/compressed/hidden.h
> new file mode 100644
> index 000000000000..49a17b6b5962
> --- /dev/null
> +++ b/arch/x86/boot/compressed/hidden.h
> @@ -0,0 +1,19 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * When building position independent code with GCC using the -fPIC option,
> + * (or even the -fPIE one on older versions), it will assume that we are
> + * building a dynamic object (either a shared library or an executable) that
> + * may have symbol references that can only be resolved at load time. For a
> + * variety of reasons (ELF symbol preemption, the CoW footprint of the section
> + * that is modified by the loader), this results in all references to symbols
> + * with external linkage to go via entries in the Global Offset Table (GOT),
> + * which carries absolute addresses which need to be fixed up when the
> + * executable image is loaded at an offset which is different from its link
> + * time offset.
> + *
> + * Fortunately, there is a way to inform the compiler that such symbol
> + * references will be satisfied at link time rather than at load time, by
> + * giving them 'hidden' visibility.
> + */
> +
> +#pragma GCC visibility push(hidden)

Is this recognized by Clang? I'm assuming so, since I see this already
being used in drivers/firmware/efi/libstub/hidden.h

> diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
> index b17d218ccdf9..4bcc943842ab 100644
> --- a/arch/x86/boot/compressed/vmlinux.lds.S
> +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> @@ -81,6 +81,7 @@ SECTIONS
>  	DISCARDS
>  }
>  
> +ASSERT(SIZEOF(.got) == 0, "Unexpected GOT entries detected!")
>  #ifdef CONFIG_X86_64
>  ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18, "Unexpected GOT/PLT entries detected!")
>  #else

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook