Subject: Re: [PATCH] arm: fix Kbuild issue caused by per-task stack protector GCC plugin

From: Tony Lindgren <[email-masked]>
Date: Mon, 17 Feb 2020 13:20:27 -0800

Hi,

* Ard Biesheuvel <[email-masked]> [200217 20:31]:
> When using plugins, GCC requires that the -fplugin= options precedes
> any of its plugin arguments appearing on the command line as well.
> This is usually not a concern, but as it turns out, this requirement
> is causing some issues with ARM's per-task stack protector plugin
> and Kbuild's implementation of $(cc-option).
> 
> When the per-task stack protector plugin is enabled, and we tweak
> the implementation of cc-option not to pipe the stderr output of
> GCC to /dev/null, the following output is generated when GCC is
> executed in the context of cc-option:
> 
>   cc1: error: plugin arm_ssp_per_task_plugin should be specified before \
>          -fplugin-arg-arm_ssp_per_task_plugin-tso=1 in the command line
>   cc1: error: plugin arm_ssp_per_task_plugin should be specified before \
>          -fplugin-arg-arm_ssp_per_task_plugin-offset=24 in the command line
> 
> These errors will cause any option passed to cc-option to be treated
> as unsupported, which is obviously incorrect.
> 
> The cause of this issue is the fact that the -fplugin= argument is
> added to GCC_PLUGINS_CFLAGS, whereas the arguments above are added
> to KBUILD_CFLAGS, and the contents of the former get filtered out of
> the latter before being passed to the GCC running the cc-option test,
> and so the -fplugin= option does not appear at all on the GCC command
> line.
> 
> Adding the arguments to GCC_PLUGINS_CFLAGS instead of KBUILD_CFLAGS
> would be the correct approach here, if it weren't for the fact that we
> are using $(eval) to defer the moment that they are added until after
> asm-offsets.h is generated, which is after the point where the contents
> of GCC_PLUGINS_CFLAGS are added to KBUILD_CFLAGS. So instead, we have
> to add our plugin arguments to both.
> 
> For similar reasons, we cannot append DISABLE_ARM_SSP_PER_TASK_PLUGIN
> to KBUILD_CFLAGS, as it will be passed to GCC when executing in the
> context of cc-option, whereas the other plugin arguments will have
> been filtered out, resulting in a similar error and false negative
> result as above. So add it to ccflags-y instead.
> 
> Reported-by: Tony Lindgren <[email-masked]>
> Fixes: 189af4657186da08 ("ARM: smp: add support for per-task stack canaries")
> Cc: Kees Cook <[email-masked]>
> Cc: Russell King <[email-masked]>
> Cc: Arnd Bergmann <[email-masked]>
> Cc: Masahiro Yamada <[email-masked]>
> Signed-off-by: Ard Biesheuvel <[email-masked]>

This fixes the appended dtb boot test case for me:

Tested-by: Tony Lindgren <[email-masked]>

Also adding Merlijn also to Cc to test his, he originally
reported the issue. So I suggest you please also add:

Reported-by: Merlijn Wajer <[email-masked]>

Regards,

Tony

> ---
>  arch/arm/Makefile                 | 4 +++-
>  arch/arm/boot/compressed/Makefile | 4 ++--
>  2 files changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm/Makefile b/arch/arm/Makefile
> index db857d07114f..1fc32b611f8a 100644
> --- a/arch/arm/Makefile
> +++ b/arch/arm/Makefile
> @@ -307,13 +307,15 @@ endif
>  ifeq ($(CONFIG_STACKPROTECTOR_PER_TASK),y)
>  prepare: stack_protector_prepare
>  stack_protector_prepare: prepare0
> -	$(eval KBUILD_CFLAGS += \
> +	$(eval SSP_PLUGIN_CFLAGS := \
>  		-fplugin-arg-arm_ssp_per_task_plugin-tso=$(shell	\
>  			awk '{if ($$2 == "THREAD_SZ_ORDER") print $$3;}'\
>  				include/generated/asm-offsets.h)	\
>  		-fplugin-arg-arm_ssp_per_task_plugin-offset=$(shell	\
>  			awk '{if ($$2 == "TI_STACK_CANARY") print $$3;}'\
>  				include/generated/asm-offsets.h))
> +	$(eval KBUILD_CFLAGS += $(SSP_PLUGIN_CFLAGS))
> +	$(eval GCC_PLUGINS_CFLAGS += $(SSP_PLUGIN_CFLAGS))
>  endif
>  
>  all:	$(notdir $(KBUILD_IMAGE))
> diff --git a/arch/arm/boot/compressed/Makefile b/arch/arm/boot/compressed/Makefile
> index da599c3a1193..9c11e7490292 100644
> --- a/arch/arm/boot/compressed/Makefile
> +++ b/arch/arm/boot/compressed/Makefile
> @@ -101,7 +101,6 @@ clean-files += piggy_data lib1funcs.S ashldi3.S bswapsdi2.S \
>  		$(libfdt) $(libfdt_hdrs) hyp-stub.S
>  
>  KBUILD_CFLAGS += -DDISABLE_BRANCH_PROFILING
> -KBUILD_CFLAGS += $(DISABLE_ARM_SSP_PER_TASK_PLUGIN)
>  
>  ifeq ($(CONFIG_FUNCTION_TRACER),y)
>  ORIG_CFLAGS := $(KBUILD_CFLAGS)
> @@ -117,7 +116,8 @@ CFLAGS_fdt_ro.o := $(nossp-flags-y)
>  CFLAGS_fdt_rw.o := $(nossp-flags-y)
>  CFLAGS_fdt_wip.o := $(nossp-flags-y)
>  
> -ccflags-y := -fpic $(call cc-option,-mno-single-pic-base,) -fno-builtin -I$(obj)
> +ccflags-y := -fpic $(call cc-option,-mno-single-pic-base,) -fno-builtin \
> +	     -I$(obj) $(DISABLE_ARM_SSP_PER_TASK_PLUGIN)
>  asflags-y := -DZIMAGE
>  
>  # Supply kernel BSS size to the decompressor via a linker symbol.
> -- 
> 2.17.1
> 

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