Re: [PATCH v5 2/7] x86/boot/compressed: Force hidden visibility for all symbol references

Sedat Dilek
2020-07-15T08:54:34Z

On Wed, Jul 15, 2020 at 2:41 AM Arvind Sankar <nivedita@alum.mit.edu> 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: move hidden.h to include/linux instead of making a copy]
>

Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Reported-by: Sedat Dilek <sedat.dilek@gmail.com>

Reported hidden.h breakage and asked for a follow-up.

- Sedat -

[1] https://marc.info/?l=linux-kernel&m=159056070321982&w=2

> Reviewed-by: Kees Cook <keescook@chromium.org>
> 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/vmlinux.lds.S | 1 +
> drivers/firmware/efi/libstub/Makefile | 2 +-
> drivers/firmware/efi/libstub/hidden.h | 6 ------
> include/linux/hidden.h | 19 +++++++++++++++++++
> 5 files changed, 22 insertions(+), 7 deletions(-)
> delete mode 100644 drivers/firmware/efi/libstub/hidden.h
> create mode 100644 include/linux/hidden.h
>
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 7619742f91c9..c829d874dcac 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)/include/linux/hidden.h
>
> KBUILD_AFLAGS := $(KBUILD_CFLAGS) -D__ASSEMBLY__
> GCOV_PROFILE := n
> diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
> index b17d218ccdf9..4bcc943842ab 100644
> --- a/arch/x86/boot/compressed/vmlinux.lds.S
> +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> @@ -81,6 +81,7 @@ SECTIONS
> DISCARDS
> }
>
> +ASSERT(SIZEOF(.got) == 0, "Unexpected GOT entries detected!")
> #ifdef CONFIG_X86_64
> ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18, "Unexpected GOT/PLT entries detected!")
> #else
> diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
> index 4cce372edaf4..609157a40493 100644
> --- a/drivers/firmware/efi/libstub/Makefile
> +++ b/drivers/firmware/efi/libstub/Makefile
> @@ -27,7 +27,7 @@ cflags-$(CONFIG_ARM) := $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) \
> cflags-$(CONFIG_EFI_GENERIC_STUB) += -I$(srctree)/scripts/dtc/libfdt
>
> KBUILD_CFLAGS := $(cflags-y) -Os -DDISABLE_BRANCH_PROFILING \
> - -include $(srctree)/drivers/firmware/efi/libstub/hidden.h \
> + -include $(srctree)/include/linux/hidden.h \
> -D__NO_FORTIFY \
> $(call cc-option,-ffreestanding) \
> $(call cc-option,-fno-stack-protector) \
> diff --git a/drivers/firmware/efi/libstub/hidden.h b/drivers/firmware/efi/libstub/hidden.h
> deleted file mode 100644
> index 3493b041f419..000000000000
> --- a/drivers/firmware/efi/libstub/hidden.h
> +++ /dev/null
> @@ -1,6 +0,0 @@
> -/* SPDX-License-Identifier: GPL-2.0 */
> -/*
> - * To prevent the compiler from emitting GOT-indirected (and thus absolute)
> - * references to any global symbols, override their visibility as 'hidden'
> - */
> -#pragma GCC visibility push(hidden)
> diff --git a/include/linux/hidden.h b/include/linux/hidden.h
> new file mode 100644
> index 000000000000..49a17b6b5962
> --- /dev/null
> +++ b/include/linux/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)
> --
> 2.26.2
>