Commit 15d5761a authored by Masahiro Yamada's avatar Masahiro Yamada
Browse files

kbuild: introduce ccflags-remove-y and asflags-remove-y 



CFLAGS_REMOVE_<file>.o filters out flags when compiling a particular
object, but there is no convenient way to do that for every object in
a directory.

Add ccflags-remove-y and asflags-remove-y to make it easily.

Use ccflags-remove-y to clean up some Makefiles.

The add/remove order works as follows:

 [1] KBUILD_CFLAGS specifies compiler flags used globally

 [2] ccflags-y adds compiler flags for all objects in the
     current Makefile

 [3] ccflags-remove-y removes compiler flags for all objects in the
     current Makefile (New feature)

 [4] CFLAGS_<file> adds compiler flags per file.

 [5] CFLAGS_REMOVE_<file> removes compiler flags per file.

Having [3] before [4] allows us to remove flags from most (but not all)
objects in the current Makefile.

For example, kernel/trace/Makefile removes $(CC_FLAGS_FTRACE)
from all objects in the directory, then adds it back to
trace_selftest_dynamic.o and CFLAGS_trace_kprobe_selftest.o

The same applies to lib/livepatch/Makefile.

Please note ccflags-remove-y has no effect to the sub-directories.
In contrast, the previous notation got rid of compiler flags also from
all the sub-directories.

The following are not affected because they have no sub-directories:

  arch/arm/boot/compressed/
  arch/powerpc/xmon/
  arch/sh/
  kernel/trace/

However, lib/ has several sub-directories.

To keep the behavior, I added ccflags-remove-y to all Makefiles
in subdirectories of lib/, except the following:

  lib/vdso/Makefile        - Kbuild does not descend into this Makefile
  lib/raid/test/Makefile   - This is not used for the kernel build

I think commit 2464a609 ("ftrace: do not trace library functions")
excluded too much. In the next commit, I will remove ccflags-remove-y
from the sub-directories of lib/.

Suggested-by: default avatarSami Tolvanen <samitolvanen@google.com>
Signed-off-by: default avatarMasahiro Yamada <masahiroy@kernel.org>
Acked-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Acked-by: Brendan Higgins <brendanhiggins@google.com> (KUnit)
Tested-by: default avatarAnders Roxell <anders.roxell@linaro.org>
parent 3ec8a5b3

Documentation/kbuild/makefiles.rst

+14 −0
Original line number Diff line number Diff line
@@ -368,6 +368,14 @@ more details, with real examples. 


		subdir-ccflags-y := -Werror



    ccflags-remove-y, asflags-remove-y

	These flags are used to remove particular flags for the compiler,

	assembler invocations.



	Example::



		ccflags-remove-$(CONFIG_MCOUNT) += -pg



    CFLAGS_$@, AFLAGS_$@

	CFLAGS_$@ and AFLAGS_$@ only apply to commands in current

	kbuild makefile.
@@ -375,6 +383,9 @@ more details, with real examples. 
	$(CFLAGS_$@) specifies per-file options for $(CC).  The $@

	part has a literal value which specifies the file that it is for.



	CFLAGS_$@ has the higher priority than ccflags-remove-y; CFLAGS_$@

	can re-add compiler flags that were removed by ccflags-remove-y.



	Example::



		# drivers/scsi/Makefile
@@ -387,6 +398,9 @@ more details, with real examples. 
	$(AFLAGS_$@) is a similar feature for source files in assembly

	languages.



	AFLAGS_$@ has the higher priority than asflags-remove-y; AFLAGS_$@

	can re-add assembler flags that were removed by asflags-remove-y.



	Example::



		# arch/arm/kernel/Makefile

arch/arm/boot/compressed/Makefile

+1 −5
Original line number Diff line number Diff line
@@ -102,13 +102,9 @@ clean-files += piggy_data lib1funcs.S ashldi3.S bswapsdi2.S hyp-stub.S 


KBUILD_CFLAGS += -DDISABLE_BRANCH_PROFILING



ifeq ($(CONFIG_FUNCTION_TRACER),y)

ORIG_CFLAGS := $(KBUILD_CFLAGS)

KBUILD_CFLAGS = $(subst -pg, , $(ORIG_CFLAGS))

endif



ccflags-y := -fpic $(call cc-option,-mno-single-pic-base,) -fno-builtin \

	     -I$(obj) $(DISABLE_ARM_SSP_PER_TASK_PLUGIN)

ccflags-remove-$(CONFIG_FUNCTION_TRACER) += -pg

asflags-y := -DZIMAGE



# Supply kernel BSS size to the decompressor via a linker symbol.

arch/powerpc/xmon/Makefile

+1 −2
Original line number Diff line number Diff line
@@ -7,8 +7,7 @@ UBSAN_SANITIZE := n 
KASAN_SANITIZE := n



# Disable ftrace for the entire directory

ORIG_CFLAGS := $(KBUILD_CFLAGS)

KBUILD_CFLAGS = $(subst $(CC_FLAGS_FTRACE),,$(ORIG_CFLAGS))

ccflags-remove-$(CONFIG_FUNCTION_TRACER) += $(CC_FLAGS_FTRACE)



ifdef CONFIG_CC_IS_CLANG

# clang stores addresses on the stack causing the frame size to blow

arch/sh/boot/compressed/Makefile

+1 −4
Original line number Diff line number Diff line
@@ -28,10 +28,7 @@ IMAGE_OFFSET := $(shell /bin/bash -c 'printf "0x%08x" \ 
			$(CONFIG_BOOT_LINK_OFFSET)]')

endif



ifeq ($(CONFIG_MCOUNT),y)

ORIG_CFLAGS := $(KBUILD_CFLAGS)

KBUILD_CFLAGS = $(subst -pg, , $(ORIG_CFLAGS))

endif

ccflags-remove-$(CONFIG_MCOUNT) += -pg



LDFLAGS_vmlinux := --oformat $(ld-bfd) -Ttext $(IMAGE_OFFSET) -e startup \

		   -T $(obj)/../../kernel/vmlinux.lds

kernel/trace/Makefile

+2 −2
Original line number Diff line number Diff line
@@ -2,9 +2,9 @@ 


# Do not instrument the tracer itself:



ccflags-remove-$(CONFIG_FUNCTION_TRACER) += $(CC_FLAGS_FTRACE)



ifdef CONFIG_FUNCTION_TRACER

ORIG_CFLAGS := $(KBUILD_CFLAGS)

KBUILD_CFLAGS = $(subst $(CC_FLAGS_FTRACE),,$(ORIG_CFLAGS))



# Avoid recursion due to instrumentation.

KCSAN_SANITIZE := n

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