[PATCH 0/4] Enable deadcode elimination at link time

[PATCH 0/4] Enable deadcode elimination at link time

[Russell King - ARM Linux]

This series enables elimination of dead code from the kernel at link
time.  The changes are mostly ARM specific, except for one change in
EFI as the EFI stub must not be compiled with -fdata-sections.

 arch/arm/Kconfig                       |  1 +
 arch/arm/boot/compressed/vmlinux.lds.S |  2 +-
 arch/arm/kernel/vmlinux.lds.S          | 10 +++++-----
 arch/arm/kernel/vmlinux.lds.h          | 18 +++++++++---------
 drivers/firmware/efi/libstub/Makefile  |  2 +-
 5 files changed, 17 insertions(+), 16 deletions(-)

-- 
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[PATCH 1/4] EFI stub: remove -fdata-sections

[Russell King]

Remove -fdata-sections from the EFI stub build as this causes problems
for the ARM decompressor code.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
---
 drivers/firmware/efi/libstub/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index c51627660dbb..98133d0577ab 100644
--- a/drivers/firmware/efi/libstub/Makefile
+++ b/drivers/firmware/efi/libstub/Makefile
@@ -15,7 +15,7 @@ cflags-$(CONFIG_X86)		+= -m$(BITS) -D__KERNEL__ -O2 \
 # disable the stackleak plugin
 cflags-$(CONFIG_ARM64)		:= $(subst -pg,,$(KBUILD_CFLAGS)) -fpie \
 				   $(DISABLE_STACKLEAK_PLUGIN)
-cflags-$(CONFIG_ARM)		:= $(subst -pg,,$(KBUILD_CFLAGS)) \
+cflags-$(CONFIG_ARM)		:= $(filter-out -pg -fdata-sections,$(KBUILD_CFLAGS)) \
 				   -fno-builtin -fpic \
 				   $(call cc-option,-mno-single-pic-base)
 
-- 
2.7.4

Re: [PATCH 1/4] EFI stub: remove -fdata-sections

[Ard Biesheuvel]

On 6 November 2018 at 14:40, Russell King <rmk+kernel@armlinux.org.uk> wrote:
> Remove -fdata-sections from the EFI stub build as this causes problems
> for the ARM decompressor code.
>
Just out of curiosity: what kind of problems?

In any case: it doesn't make sense to obsess about .data size
optimizations here, so

Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

> Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
> ---
>  drivers/firmware/efi/libstub/Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
> index c51627660dbb..98133d0577ab 100644
> --- a/drivers/firmware/efi/libstub/Makefile
> +++ b/drivers/firmware/efi/libstub/Makefile
> @@ -15,7 +15,7 @@ cflags-$(CONFIG_X86)          += -m$(BITS) -D__KERNEL__ -O2 \
>  # disable the stackleak plugin
>  cflags-$(CONFIG_ARM64)         := $(subst -pg,,$(KBUILD_CFLAGS)) -fpie \
>                                    $(DISABLE_STACKLEAK_PLUGIN)
> -cflags-$(CONFIG_ARM)           := $(subst -pg,,$(KBUILD_CFLAGS)) \
> +cflags-$(CONFIG_ARM)           := $(filter-out -pg -fdata-sections,$(KBUILD_CFLAGS)) \
>                                    -fno-builtin -fpic \
>                                    $(call cc-option,-mno-single-pic-base)
>
> --
> 2.7.4
>

Re: [PATCH 1/4] EFI stub: remove -fdata-sections

[Russell King - ARM Linux]

On Tue, Nov 06, 2018 at 03:11:18PM +0100, Ard Biesheuvel wrote:
> On 6 November 2018 at 14:40, Russell King <rmk+kernel@armlinux.org.uk> wrote:
> > Remove -fdata-sections from the EFI stub build as this causes problems
> > for the ARM decompressor code.
> >
> Just out of curiosity: what kind of problems?

I'm afraid I don't remember - these patches are March 2018.  I suspect
it was a result of a randconfig build failing to link, as I don't have
any 32-bit ARM configs that have the EFI stub enabled.

In any case, this will break:

#
# ARM discards the .data section because it disallows r/w data in the
# decompressor. So move our .data to .data.efistub, which is preserved
# explicitly by the decompressor linker script.
#
STUBCOPY_FLAGS-$(CONFIG_ARM)    += --rename-section .data=.data.efistub

because with -fdata-sections enabled, there is no longer a single .data
section, but multiple .data.* sections.  Hence, the rename no longer
works, and all the EFI stub data ends up being discarded by the
decompressor link stage.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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Re: [PATCH 1/4] EFI stub: remove -fdata-sections

[Ard Biesheuvel]

On 6 November 2018 at 15:21, Russell King - ARM Linux
<linux@armlinux.org.uk> wrote:
> On Tue, Nov 06, 2018 at 03:11:18PM +0100, Ard Biesheuvel wrote:
>> On 6 November 2018 at 14:40, Russell King <rmk+kernel@armlinux.org.uk> wrote:
>> > Remove -fdata-sections from the EFI stub build as this causes problems
>> > for the ARM decompressor code.
>> >
>> Just out of curiosity: what kind of problems?
>
> I'm afraid I don't remember - these patches are March 2018.  I suspect
> it was a result of a randconfig build failing to link, as I don't have
> any 32-bit ARM configs that have the EFI stub enabled.
>
> In any case, this will break:
>
> #
> # ARM discards the .data section because it disallows r/w data in the
> # decompressor. So move our .data to .data.efistub, which is preserved
> # explicitly by the decompressor linker script.
> #
> STUBCOPY_FLAGS-$(CONFIG_ARM)    += --rename-section .data=.data.efistub
>
> because with -fdata-sections enabled, there is no longer a single .data
> section, but multiple .data.* sections.  Hence, the rename no longer
> works, and all the EFI stub data ends up being discarded by the
> decompressor link stage.
>

Ah, of course, that must be it.