[PATCH 1/2] kbuild: remove cc-option test of -fno-stack-protector

[Masahiro Yamada]

Some Makefiles already pass -fno-stack-protector unconditionally.
For example, arch/arm64/kernel/vdso/Makefile, arch/x86/xen/Makefile.
No problem report so far about hard-coding this option. So, we can
assume all supported compilers know -fno-stack-protector.

GCC 4.8 and Clang support this option (https://godbolt.org/z/_HDGzN)

Get rid of cc-option from -fno-stack-protector.

Remove CONFIG_CC_HAS_STACKPROTECTOR_NONE, which should always be 'y'.

Note:
arch/mips/vdso/Makefile adds -fno-stack-protector twice, first
unconditionally, and second conditionally. I removed the second one.

Signed-off-by: Masahiro Yamada <masa...@kernel.org>
---

Documentation/kbuild/kconfig-language.rst | 4 ++--
Makefile | 2 +-
arch/Kconfig | 3 ---
arch/arm/boot/compressed/Makefile | 3 +--
arch/mips/vdso/Makefile | 3 +--
arch/powerpc/kernel/Makefile | 2 +-
arch/powerpc/platforms/powermac/Makefile | 2 +-
arch/sparc/vdso/Makefile | 4 ++--
arch/um/Makefile | 3 +--
arch/x86/Makefile | 2 +-
arch/x86/boot/compressed/Makefile | 2 +-
arch/x86/entry/vdso/Makefile | 4 ++--
arch/x86/kernel/cpu/Makefile | 3 +--
arch/x86/lib/Makefile | 2 +-
arch/x86/mm/Makefile | 7 +++----
arch/x86/power/Makefile | 3 +--
arch/x86/purgatory/Makefile | 2 +-
arch/x86/um/vdso/Makefile | 2 +-
arch/x86/xen/Makefile | 5 ++---
drivers/firmware/efi/libstub/Makefile | 2 +-
drivers/xen/Makefile | 3 +--
kernel/kcsan/Makefile | 3 +--
lib/Makefile | 4 ++--
mm/kasan/Makefile | 2 +-
24 files changed, 30 insertions(+), 42 deletions(-)

diff --git a/Documentation/kbuild/kconfig-language.rst b/Documentation/kbuild/kconfig-language.rst
index a1601ec3317b..2538e7cb08e6 100644
--- a/Documentation/kbuild/kconfig-language.rst
+++ b/Documentation/kbuild/kconfig-language.rst
@@ -540,8 +540,8 @@ followed by a test macro::
If you need to expose a compiler capability to makefiles and/or C source files,
`CC_HAS_` is the recommended prefix for the config option::

- config CC_HAS_STACKPROTECTOR_NONE
- def_bool $(cc-option,-fno-stack-protector)
+ config CC_HAS_ASM_GOTO
+ def_bool $(success,$(srctree)/scripts/gcc-goto.sh $(CC))

Build as module only
~~~~~~~~~~~~~~~~~~~~
diff --git a/Makefile b/Makefile
index 5496a32dffa6..73948798ce3f 100644
--- a/Makefile
+++ b/Makefile
@@ -762,7 +762,7 @@ ifneq ($(CONFIG_FRAME_WARN),0)
KBUILD_CFLAGS += -Wframe-larger-than=$(CONFIG_FRAME_WARN)
endif

-stackp-flags-$(CONFIG_CC_HAS_STACKPROTECTOR_NONE) := -fno-stack-protector
+stackp-flags-y := -fno-stack-protector
stackp-flags-$(CONFIG_STACKPROTECTOR) := -fstack-protector
stackp-flags-$(CONFIG_STACKPROTECTOR_STRONG) := -fstack-protector-strong

diff --git a/arch/Kconfig b/arch/Kconfig
index 8cc35dc556c7..1ea61290900a 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -478,9 +478,6 @@ config HAVE_STACKPROTECTOR
An arch should select this symbol if:
- it has implemented a stack canary (e.g. __stack_chk_guard)

-config CC_HAS_STACKPROTECTOR_NONE
- def_bool $(cc-option,-fno-stack-protector)
-
config STACKPROTECTOR
bool "Stack Protector buffer overflow detection"
depends on HAVE_STACKPROTECTOR
diff --git a/arch/arm/boot/compressed/Makefile b/arch/arm/boot/compressed/Makefile
index 00602a6fba04..cb7a56c6723c 100644
--- a/arch/arm/boot/compressed/Makefile
+++ b/arch/arm/boot/compressed/Makefile
@@ -84,9 +84,8 @@ endif

# -fstack-protector-strong triggers protection checks in this code,
# but it is being used too early to link to meaningful stack_chk logic.
-nossp-flags-$(CONFIG_CC_HAS_STACKPROTECTOR_NONE) := -fno-stack-protector
$(foreach o, $(libfdt_objs) atags_to_fdt.o, \
- $(eval CFLAGS_$(o) := -I $(srctree)/scripts/dtc/libfdt $(nossp-flags-y)))
+ $(eval CFLAGS_$(o) := -I $(srctree)/scripts/dtc/libfdt -fno-stack-protector))

# These were previously generated C files. When you are building the kernel
# with O=, make sure to remove the stale files in the output tree. Otherwise,
diff --git a/arch/mips/vdso/Makefile b/arch/mips/vdso/Makefile
index 2e64c7600eea..57fe83235281 100644
--- a/arch/mips/vdso/Makefile
+++ b/arch/mips/vdso/Makefile
@@ -35,8 +35,7 @@ cflags-vdso := $(ccflags-vdso) \
-O3 -g -fPIC -fno-strict-aliasing -fno-common -fno-builtin -G 0 \
-mrelax-pic-calls $(call cc-option, -mexplicit-relocs) \
-fno-stack-protector -fno-jump-tables -DDISABLE_BRANCH_PROFILING \
- $(call cc-option, -fno-asynchronous-unwind-tables) \
- $(call cc-option, -fno-stack-protector)
+ $(call cc-option, -fno-asynchronous-unwind-tables)
aflags-vdso := $(ccflags-vdso) \
-D__ASSEMBLY__ -Wa,-gdwarf-2

diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
index 244542ae2a91..3a83f2b876a5 100644
--- a/arch/powerpc/kernel/Makefile
+++ b/arch/powerpc/kernel/Makefile
@@ -16,7 +16,7 @@ CFLAGS_prom_init.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
CFLAGS_btext.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
CFLAGS_prom.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)

-CFLAGS_prom_init.o += $(call cc-option, -fno-stack-protector)
+CFLAGS_prom_init.o += -fno-stack-protector
CFLAGS_prom_init.o += -DDISABLE_BRANCH_PROFILING
CFLAGS_prom_init.o += -ffreestanding

diff --git a/arch/powerpc/platforms/powermac/Makefile b/arch/powerpc/platforms/powermac/Makefile
index f4247ade71ca..cf85f0662d0d 100644
--- a/arch/powerpc/platforms/powermac/Makefile
+++ b/arch/powerpc/platforms/powermac/Makefile
@@ -1,6 +1,6 @@
# SPDX-License-Identifier: GPL-2.0
CFLAGS_bootx_init.o += -fPIC
-CFLAGS_bootx_init.o += $(call cc-option, -fno-stack-protector)
+CFLAGS_bootx_init.o += -fno-stack-protector

KASAN_SANITIZE_bootx_init.o := n

diff --git a/arch/sparc/vdso/Makefile b/arch/sparc/vdso/Makefile
index 708cb6304c2d..f44355e46f31 100644
--- a/arch/sparc/vdso/Makefile
+++ b/arch/sparc/vdso/Makefile
@@ -54,7 +54,7 @@ $(obj)/vdso-image-%.c: $(obj)/vdso%.so.dbg $(obj)/vdso%.so $(obj)/vdso2c FORCE
# optimize sibling calls.
#
CFL := $(PROFILING) -mcmodel=medlow -fPIC -O2 -fasynchronous-unwind-tables -m64 \
- $(filter -g%,$(KBUILD_CFLAGS)) $(call cc-option, -fno-stack-protector) \
+ $(filter -g%,$(KBUILD_CFLAGS)) -fno-stack-protector \
-fno-omit-frame-pointer -foptimize-sibling-calls \
-DDISABLE_BRANCH_PROFILING -DBUILD_VDSO

@@ -93,7 +93,7 @@ KBUILD_CFLAGS_32 := $(filter-out -fno-pic,$(KBUILD_CFLAGS_32))
KBUILD_CFLAGS_32 := $(filter-out $(GCC_PLUGINS_CFLAGS),$(KBUILD_CFLAGS_32))
KBUILD_CFLAGS_32 := $(filter-out $(SPARC_REG_CFLAGS),$(KBUILD_CFLAGS_32))
KBUILD_CFLAGS_32 += -m32 -msoft-float -fpic
-KBUILD_CFLAGS_32 += $(call cc-option, -fno-stack-protector)
+KBUILD_CFLAGS_32 += -fno-stack-protector
KBUILD_CFLAGS_32 += $(call cc-option, -foptimize-sibling-calls)
KBUILD_CFLAGS_32 += -fno-omit-frame-pointer
KBUILD_CFLAGS_32 += -DDISABLE_BRANCH_PROFILING
diff --git a/arch/um/Makefile b/arch/um/Makefile
index 3f27aa3ec0a6..1cea46ff9bb7 100644
--- a/arch/um/Makefile
+++ b/arch/um/Makefile
@@ -121,8 +121,7 @@ LINK-$(CONFIG_LD_SCRIPT_STATIC) += -static
LINK-$(CONFIG_LD_SCRIPT_DYN) += -Wl,-rpath,/lib $(call cc-option, -no-pie)

CFLAGS_NO_HARDENING := $(call cc-option, -fno-PIC,) $(call cc-option, -fno-pic,) \
- $(call cc-option, -fno-stack-protector,) \
- $(call cc-option, -fno-stack-protector-all,)
+ -fno-stack-protector $(call cc-option, -fno-stack-protector-all)

# Options used by linker script
export LDS_START := $(START)
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 00e378de8bc0..89c3cdfba753 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -37,7 +37,7 @@ REALMODE_CFLAGS := $(M16_CFLAGS) -g -Os -DDISABLE_BRANCH_PROFILING \
-mno-mmx -mno-sse

REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), -ffreestanding)
-REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), -fno-stack-protector)
+REALMODE_CFLAGS += -fno-stack-protector
REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), -Wno-address-of-packed-member)
REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), $(cc_stack_align4))
export REALMODE_CFLAGS
diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index 7619742f91c9..c88a31569a5e 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -36,7 +36,7 @@ cflags-$(CONFIG_X86_64) := -mcmodel=small
KBUILD_CFLAGS += $(cflags-y)
KBUILD_CFLAGS += -mno-mmx -mno-sse
KBUILD_CFLAGS += $(call cc-option,-ffreestanding)
-KBUILD_CFLAGS += $(call cc-option,-fno-stack-protector)
+KBUILD_CFLAGS += -fno-stack-protector
KBUILD_CFLAGS += $(call cc-disable-warning, address-of-packed-member)
KBUILD_CFLAGS += $(call cc-disable-warning, gnu)
KBUILD_CFLAGS += -Wno-pointer-sign
diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile
index 04e65f0698f6..215376d975a2 100644
--- a/arch/x86/entry/vdso/Makefile
+++ b/arch/x86/entry/vdso/Makefile
@@ -82,7 +82,7 @@ $(obj)/vdso-image-%.c: $(obj)/vdso%.so.dbg $(obj)/vdso%.so $(obj)/vdso2c FORCE
# optimize sibling calls.
#
CFL := $(PROFILING) -mcmodel=small -fPIC -O2 -fasynchronous-unwind-tables -m64 \
- $(filter -g%,$(KBUILD_CFLAGS)) $(call cc-option, -fno-stack-protector) \
+ $(filter -g%,$(KBUILD_CFLAGS)) -fno-stack-protector \
-fno-omit-frame-pointer -foptimize-sibling-calls \
-DDISABLE_BRANCH_PROFILING -DBUILD_VDSO

@@ -151,7 +151,7 @@ KBUILD_CFLAGS_32 := $(filter-out -mfentry,$(KBUILD_CFLAGS_32))
KBUILD_CFLAGS_32 := $(filter-out $(GCC_PLUGINS_CFLAGS),$(KBUILD_CFLAGS_32))
KBUILD_CFLAGS_32 := $(filter-out $(RETPOLINE_CFLAGS),$(KBUILD_CFLAGS_32))
KBUILD_CFLAGS_32 += -m32 -msoft-float -mregparm=0 -fpic
-KBUILD_CFLAGS_32 += $(call cc-option, -fno-stack-protector)
+KBUILD_CFLAGS_32 += -fno-stack-protector
KBUILD_CFLAGS_32 += $(call cc-option, -foptimize-sibling-calls)
KBUILD_CFLAGS_32 += -fno-omit-frame-pointer
KBUILD_CFLAGS_32 += -DDISABLE_BRANCH_PROFILING
diff --git a/arch/x86/kernel/cpu/Makefile b/arch/x86/kernel/cpu/Makefile
index dba6a83bc349..93792b457b81 100644
--- a/arch/x86/kernel/cpu/Makefile
+++ b/arch/x86/kernel/cpu/Makefile
@@ -17,8 +17,7 @@ KCOV_INSTRUMENT_perf_event.o := n
KCSAN_SANITIZE_common.o := n

# Make sure load_percpu_segment has no stackprotector
-nostackp := $(call cc-option, -fno-stack-protector)
-CFLAGS_common.o := $(nostackp)
+CFLAGS_common.o := -fno-stack-protector

obj-y := cacheinfo.o scattered.o topology.o
obj-y += common.o
diff --git a/arch/x86/lib/Makefile b/arch/x86/lib/Makefile
index 6110bce7237b..d46fff11f06f 100644
--- a/arch/x86/lib/Makefile
+++ b/arch/x86/lib/Makefile
@@ -24,7 +24,7 @@ ifdef CONFIG_FUNCTION_TRACER
CFLAGS_REMOVE_cmdline.o = -pg
endif

-CFLAGS_cmdline.o := $(call cc-option, -fno-stack-protector)
+CFLAGS_cmdline.o := -fno-stack-protector
endif

inat_tables_script = $(srctree)/arch/x86/tools/gen-insn-attr-x86.awk
diff --git a/arch/x86/mm/Makefile b/arch/x86/mm/Makefile
index f7fd0e868c9c..5864219221ca 100644
--- a/arch/x86/mm/Makefile
+++ b/arch/x86/mm/Makefile
@@ -22,10 +22,9 @@ obj-y := init.o init_$(BITS).o fault.o ioremap.o extable.o mmap.o \
obj-y += pat/

# Make sure __phys_addr has no stackprotector
-nostackp := $(call cc-option, -fno-stack-protector)
-CFLAGS_physaddr.o := $(nostackp)
-CFLAGS_setup_nx.o := $(nostackp)
-CFLAGS_mem_encrypt_identity.o := $(nostackp)
+CFLAGS_physaddr.o := -fno-stack-protector
+CFLAGS_setup_nx.o := -fno-stack-protector
+CFLAGS_mem_encrypt_identity.o := -fno-stack-protector

CFLAGS_fault.o := -I $(srctree)/$(src)/../include/asm/trace

diff --git a/arch/x86/power/Makefile b/arch/x86/power/Makefile
index 37923d715741..6907b523e856 100644
--- a/arch/x86/power/Makefile
+++ b/arch/x86/power/Makefile
@@ -3,8 +3,7 @@ OBJECT_FILES_NON_STANDARD_hibernate_asm_$(BITS).o := y

# __restore_processor_state() restores %gs after S3 resume and so should not
# itself be stack-protected
-nostackp := $(call cc-option, -fno-stack-protector)
-CFLAGS_cpu.o := $(nostackp)
+CFLAGS_cpu.o := -fno-stack-protector

obj-$(CONFIG_PM_SLEEP) += cpu.o
obj-$(CONFIG_HIBERNATION) += hibernate_$(BITS).o hibernate_asm_$(BITS).o hibernate.o
diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
index 088bd764e0b7..183ac60e5990 100644
--- a/arch/x86/purgatory/Makefile
+++ b/arch/x86/purgatory/Makefile
@@ -34,7 +34,7 @@ KCOV_INSTRUMENT := n
PURGATORY_CFLAGS_REMOVE := -mcmodel=kernel
PURGATORY_CFLAGS := -mcmodel=large -ffreestanding -fno-zero-initialized-in-bss
PURGATORY_CFLAGS += $(DISABLE_STACKLEAK_PLUGIN) -DDISABLE_BRANCH_PROFILING
-PURGATORY_CFLAGS += $(call cc-option,-fno-stack-protector)
+PURGATORY_CFLAGS += -fno-stack-protector

# Default KBUILD_CFLAGS can have -pg option set when FTRACE is enabled. That
# in turn leaves some undefined symbols like __fentry__ in purgatory and not
diff --git a/arch/x86/um/vdso/Makefile b/arch/x86/um/vdso/Makefile
index 0caddd6acb22..5943387e3f35 100644
--- a/arch/x86/um/vdso/Makefile
+++ b/arch/x86/um/vdso/Makefile
@@ -42,7 +42,7 @@ $(obj)/%.so: $(obj)/%.so.dbg FORCE
# optimize sibling calls.
#
CFL := $(PROFILING) -mcmodel=small -fPIC -O2 -fasynchronous-unwind-tables -m64 \
- $(filter -g%,$(KBUILD_CFLAGS)) $(call cc-option, -fno-stack-protector) \
+ $(filter -g%,$(KBUILD_CFLAGS)) -fno-stack-protector \
-fno-omit-frame-pointer -foptimize-sibling-calls

$(vobjs): KBUILD_CFLAGS += $(CFL)
diff --git a/arch/x86/xen/Makefile b/arch/x86/xen/Makefile
index 084de77a109e..5f1db522d06b 100644
--- a/arch/x86/xen/Makefile
+++ b/arch/x86/xen/Makefile
@@ -9,9 +9,8 @@ CFLAGS_REMOVE_irq.o = -pg
endif

# Make sure early boot has no stackprotector
-nostackp := $(call cc-option, -fno-stack-protector)
-CFLAGS_enlighten_pv.o := $(nostackp)
-CFLAGS_mmu_pv.o := $(nostackp)
+CFLAGS_enlighten_pv.o := -fno-stack-protector
+CFLAGS_mmu_pv.o := -fno-stack-protector

obj-y += enlighten.o
obj-y += mmu.o
diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index 75daaf20374e..f8418763cd79 100644
--- a/drivers/firmware/efi/libstub/Makefile
+++ b/drivers/firmware/efi/libstub/Makefile
@@ -29,7 +29,7 @@ KBUILD_CFLAGS := $(cflags-y) -Os -DDISABLE_BRANCH_PROFILING \
-include $(srctree)/drivers/firmware/efi/libstub/hidden.h \
-D__NO_FORTIFY \
$(call cc-option,-ffreestanding) \
- $(call cc-option,-fno-stack-protector) \
+ -fno-stack-protector \
$(call cc-option,-fno-addrsig) \
-D__DISABLE_EXPORTS

diff --git a/drivers/xen/Makefile b/drivers/xen/Makefile
index 0d322f3d90cd..c25c9a699b48 100644
--- a/drivers/xen/Makefile
+++ b/drivers/xen/Makefile
@@ -5,8 +5,7 @@ obj-y += mem-reservation.o
obj-y += events/
obj-y += xenbus/

-nostackp := $(call cc-option, -fno-stack-protector)
-CFLAGS_features.o := $(nostackp)
+CFLAGS_features.o := -fno-stack-protector

dom0-$(CONFIG_ARM64) += arm-device.o
dom0-$(CONFIG_PCI) += pci.o
diff --git a/kernel/kcsan/Makefile b/kernel/kcsan/Makefile
index d4999b38d1be..d111f32bd5fd 100644
--- a/kernel/kcsan/Makefile
+++ b/kernel/kcsan/Makefile
@@ -7,8 +7,7 @@ CFLAGS_REMOVE_core.o = $(CC_FLAGS_FTRACE)
CFLAGS_REMOVE_debugfs.o = $(CC_FLAGS_FTRACE)
CFLAGS_REMOVE_report.o = $(CC_FLAGS_FTRACE)

-CFLAGS_core.o := $(call cc-option,-fno-conserve-stack,) \
- $(call cc-option,-fno-stack-protector,)
+CFLAGS_core.o := $(call cc-option,-fno-conserve-stack,) -fno-stack-protector

obj-y := core.o debugfs.o report.o
obj-$(CONFIG_KCSAN_SELFTEST) += test.o
diff --git a/lib/Makefile b/lib/Makefile
index b1c42c10073b..0cda70649f1c 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -22,7 +22,7 @@ KCOV_INSTRUMENT_fault-inject.o := n
ifdef CONFIG_AMD_MEM_ENCRYPT
KASAN_SANITIZE_string.o := n

-CFLAGS_string.o := $(call cc-option, -fno-stack-protector)
+CFLAGS_string.o := -fno-stack-protector
endif

# Used by KCSAN while enabled, avoid recursion.
@@ -300,7 +300,7 @@ endif
UBSAN_SANITIZE_ubsan.o := n
KASAN_SANITIZE_ubsan.o := n
KCSAN_SANITIZE_ubsan.o := n
-CFLAGS_ubsan.o := $(call cc-option, -fno-stack-protector) $(DISABLE_STACKLEAK_PLUGIN)
+CFLAGS_ubsan.o := -fno-stack-protector $(DISABLE_STACKLEAK_PLUGIN)

obj-$(CONFIG_SBITMAP) += sbitmap.o

diff --git a/mm/kasan/Makefile b/mm/kasan/Makefile
index d532c2587731..370d970e5ab5 100644
--- a/mm/kasan/Makefile
+++ b/mm/kasan/Makefile
@@ -16,7 +16,7 @@ CFLAGS_REMOVE_tags_report.o = $(CC_FLAGS_FTRACE)
# Function splitter causes unnecessary splits in __asan_load1/__asan_store1
# see: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63533
CC_FLAGS_KASAN_RUNTIME := $(call cc-option, -fno-conserve-stack)
-CC_FLAGS_KASAN_RUNTIME += $(call cc-option, -fno-stack-protector)
+CC_FLAGS_KASAN_RUNTIME += -fno-stack-protector
# Disable branch tracing to avoid recursion.
CC_FLAGS_KASAN_RUNTIME += -DDISABLE_BRANCH_PROFILING

--
2.25.1

[Nick Desaulniers]

Just curious, looks like we could do the same for
`-fno-stack-protector-all`, here or tree-wide, right? Wait, what
compiler recognizes -fno-stack-protector-all?
https://godbolt.org/z/QFQKE_

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--
Thanks,
~Nick Desaulniers

[Masahiro Yamada]

-fstack-protector
-fstack-protector-strong
-fstack-protector-all

are supported.

But,

-fno-stack-protector-strong
-fno-stack-protector-all

are unsupported.



Perheps, -fno-stack-protector is enough
to disable all variants of stack-protector.



--
Best Regards
Masahiro Yamada

[Kees Cook]

On Fri, Jun 26, 2020 at 12:09:37PM -0700, Nick Desaulniers wrote:
> On Fri, Jun 26, 2020 at 12:00 PM Masahiro Yamada <masa...@kernel.org> wrote:
> > CFLAGS_NO_HARDENING := $(call cc-option, -fno-PIC,) $(call cc-option, -fno-pic,) \
> > - $(call cc-option, -fno-stack-protector,) \
> > - $(call cc-option, -fno-stack-protector-all,)
> > + -fno-stack-protector $(call cc-option, -fno-stack-protector-all)
>
> Just curious, looks like we could do the same for
> `-fno-stack-protector-all`, here or tree-wide, right? Wait, what
> compiler recognizes -fno-stack-protector-all?
> https://godbolt.org/z/QFQKE_

That is from ancient times, IIRC. -fno-stack-protector should be
sufficient.

--
Kees Cook

[Nick Desaulniers]

On Fri, Jun 26, 2020 at 12:00 PM Masahiro Yamada <masa...@kernel.org> wrote:
>

> Some Makefiles already pass -fno-stack-protector unconditionally.
> For example, arch/arm64/kernel/vdso/Makefile, arch/x86/xen/Makefile.
> No problem report so far about hard-coding this option. So, we can
> assume all supported compilers know -fno-stack-protector.
>
> GCC 4.8 and Clang support this option (https://godbolt.org/z/_HDGzN)
>
> Get rid of cc-option from -fno-stack-protector.
>
> Remove CONFIG_CC_HAS_STACKPROTECTOR_NONE, which should always be 'y'.
>
> Note:
> arch/mips/vdso/Makefile adds -fno-stack-protector twice, first
> unconditionally, and second conditionally. I removed the second one.
>
> Signed-off-by: Masahiro Yamada <masa...@kernel.org>

Generally looks pretty good, just a few more questions before adding
my tag explicitly.

So it looks like the previous behavior always added
`-fno-stack-protector` (since CONFIG_CC_HAS_STACKPROTECTOR_NONE was
always true), but then we append either `-fstack-protector` or
`-fstack-protector-strong` based on configs. While that's ok, and you
patch doesn't change that behavior, and it's good to be explicit to
set the stack protector or not...it seems weird to have
`-fno-stack-protector -fstack-protector` in the command line flags. I
would prefer if we checked for not having CONFIG_STACKPROTECTOR or
CONFIG_STACKPROTECTOR_STRONG before adding `-fno-stack-protector`.
That doesn't have to be done in this patch, per se.

Any time I see `:=` assignment to a CFLAGS variable, it's a red flag
for overwriting existing CFLAGS, which is a common source of bugs. I
recognize the kernel is current a mix and match for:

CFLAGS_<file>.o

rules to either use `+=` or `:=`, but I wish we were consistent, and
consistent in our use of `+=`. For those rules, is there a difference
between the use of `+=` and `:=` like there is for the more general
case of appending to KBUILD_CFLAGS? If not, it's ok to match the
existing style, but it's curious to me in this patch to see a mixed
use of `+=` and `:=`.

--
Thanks,
~Nick Desaulniers

[Kees Cook]

On Sat, Jun 27, 2020 at 03:59:12AM +0900, Masahiro Yamada wrote:
> Some Makefiles already pass -fno-stack-protector unconditionally.
> For example, arch/arm64/kernel/vdso/Makefile, arch/x86/xen/Makefile.
> No problem report so far about hard-coding this option. So, we can
> assume all supported compilers know -fno-stack-protector.
>
> GCC 4.8 and Clang support this option (https://godbolt.org/z/_HDGzN)
>
> Get rid of cc-option from -fno-stack-protector.
>
> Remove CONFIG_CC_HAS_STACKPROTECTOR_NONE, which should always be 'y'.
>
> Note:
> arch/mips/vdso/Makefile adds -fno-stack-protector twice, first
> unconditionally, and second conditionally. I removed the second one.
>
> Signed-off-by: Masahiro Yamada <masa...@kernel.org>

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

--
Kees Cook

[Kees Cook]

On Fri, Jun 26, 2020 at 01:13:20PM -0700, Nick Desaulniers wrote:
> On Fri, Jun 26, 2020 at 12:00 PM Masahiro Yamada <masa...@kernel.org> wrote:
> >
> > +++ b/Makefile
> > @@ -762,7 +762,7 @@ ifneq ($(CONFIG_FRAME_WARN),0)
> > KBUILD_CFLAGS += -Wframe-larger-than=$(CONFIG_FRAME_WARN)
> > endif
> >
> > -stackp-flags-$(CONFIG_CC_HAS_STACKPROTECTOR_NONE) := -fno-stack-protector
> > +stackp-flags-y := -fno-stack-protector
> > stackp-flags-$(CONFIG_STACKPROTECTOR) := -fstack-protector
> > stackp-flags-$(CONFIG_STACKPROTECTOR_STRONG) := -fstack-protector-strong
>
> So it looks like the previous behavior always added
> `-fno-stack-protector` (since CONFIG_CC_HAS_STACKPROTECTOR_NONE was
> always true), but then we append either `-fstack-protector` or
> `-fstack-protector-strong` based on configs. While that's ok, and you
> patch doesn't change that behavior, and it's good to be explicit to
> set the stack protector or not...it seems weird to have
> `-fno-stack-protector -fstack-protector` in the command line flags. I
> would prefer if we checked for not having CONFIG_STACKPROTECTOR or
> CONFIG_STACKPROTECTOR_STRONG before adding `-fno-stack-protector`.
> That doesn't have to be done in this patch, per se.

No, it would add only what was latest and most selected. (They're all
":=" assignments.) If CONFIG_STACKPROTECTOR_STRONG, only
-fstack-protector-strong is set. If only CONFIG_STACKPROTECTOR, only
-fstack-protector is set. Otherwise -fno-stack-protector.

--
Kees Cook

[Nick Desaulniers]

Ah, right. Thanks for pointing that out. I'm still curious if the
CFLAGS_<file>.o rules get appended or overwrite all flags for that
translation unit?
--
Thanks,
~Nick Desaulniers

[Kees Cook]

On Fri, Jun 26, 2020 at 01:25:27PM -0700, Nick Desaulniers wrote:
> Ah, right. Thanks for pointing that out. I'm still curious if the
> CFLAGS_<file>.o rules get appended or overwrite all flags for that
> translation unit?

CFLAGS_file.o gets appended to the CFLAGS. For example, after my updated
patch[1], the command line for syscall_64.o looks like this:

gcc -Wp,-MMD,arch/x86/entry/.syscall_64.o.d -nostdinc -isystem \
...
-Wframe-larger-than=2048 -fstack-protector-strong \
...
-fno-stack-protector -Wno-override-init \
-DKBUILD_MODFILE='"arch/x86/entry/syscall_64"' \
-DKBUILD_BASENAME='"syscall_64"' \
-DKBUILD_MODNAME='"syscall_64"' \
-c -o arch/x86/entry/syscall_64.o \
arch/x86/entry/syscall_64.c

-Kees

[1] https://lore.kernel.org/lkml/202006261333.585319CA6B@keescook/

--
Kees Cook

[Ard Biesheuvel]

On Fri, 26 Jun 2020 at 21:00, Masahiro Yamada <masa...@kernel.org> wrote:
>
> Some Makefiles already pass -fno-stack-protector unconditionally.
> For example, arch/arm64/kernel/vdso/Makefile, arch/x86/xen/Makefile.
> No problem report so far about hard-coding this option. So, we can
> assume all supported compilers know -fno-stack-protector.
>
> GCC 4.8 and Clang support this option (https://godbolt.org/z/_HDGzN)
>
> Get rid of cc-option from -fno-stack-protector.
>
> Remove CONFIG_CC_HAS_STACKPROTECTOR_NONE, which should always be 'y'.
>
> Note:
> arch/mips/vdso/Makefile adds -fno-stack-protector twice, first
> unconditionally, and second conditionally. I removed the second one.
>
> Signed-off-by: Masahiro Yamada <masa...@kernel.org>

Acked-by: Ard Biesheuvel <ar...@kernel.org>

[Masahiro Yamada]

I think Kees mostly answered your question.

Let me add some comments.


'+=' is the most used in kernel Makefiles, but
':=' and '=' are also used.

So, you are right, we are inconsistent.
This applies to not only CFLAGS_<file>.o, but also obj-y, etc.

For example,
https://github.com/torvalds/linux/blob/v5.7/arch/arm64/kernel/Makefile#L15
'obj-y :=' works since it is the first assignment to obj-y in that file.
'obj-y +=' also works, of course.

We can consistently use '+=' everywhere, but I do not send
patches for churn.


You can use any assignment operator to CFLAGS_<file>.o
if it is the first assignment in the Makefile.
Using '+=' is robust for future code insertion/removal, though.


If the right-hand side contains variable references,
there is important difference in the behavior.

You may know two flavors in variables
(https://www.gnu.org/software/make/manual/make.html#Flavors)


CFLAGS_foo.o := $(call cc-option,-fno-stack-protector)
The cc-option is expanded immediately when this line is parsed.
(So, the compiler is invoked for 'make clean' too)


CFLAGS_foo.o += $(call cc-option,-fno-stack-protector)
If this is the first assignment in the file,
'+=' act as '=', so the evaluation of cc-option
is delayed until $(CFLAGS_foo.o) is expanded.
(So, the compiler is NOT invoked for 'make clean')

[Nick Desaulniers]

Cool, thanks for all the info. With that, I'm happy with this patch.

Reviewed-by: Nick Desaulniers <ndesau...@google.com>

> CFLAGS_foo.o := $(call cc-option,-fno-stack-protector)
> The cc-option is expanded immediately when this line is parsed.
> (So, the compiler is invoked for 'make clean' too)
>
>
> CFLAGS_foo.o += $(call cc-option,-fno-stack-protector)
> If this is the first assignment in the file,
> '+=' act as '=', so the evaluation of cc-option
> is delayed until $(CFLAGS_foo.o) is expanded.
> (So, the compiler is NOT invoked for 'make clean')

Ah, I think that may explain: I've been seeing the occasional warning
from $(NM) when running `make clean` for ARCH=arm, I'll bet that's
where this is coming from then. Next time I reproduce it, I'll try to
find maybe where we're using `:=` or `=` with `$(NM)`.

Maybe arch/arm/boot/compressed/Makefile, KBSS_SZ is evaluated for
`make clean`? (If you start an arm build, but kill it before vmlinux
is created, then `make clean` I suspect that KBSS_SZ is evaluated?)
--
Thanks,
~Nick Desaulniers

[Nick Desaulniers]

On Mon, Jun 29, 2020 at 11:26 AM Nick Desaulniers

$ ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- make LLVM=1 -j71
$ rm vmlinux
$ ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- make LLVM=1 -j71 clean
llvm-nm: error: arch/arm/boot/compressed/../../../../vmlinux: No such
file or directory.

I suspect I see this from doing an ARCH=arm build, then building a
different ARCH and killing the build before vmlinux is produced or
`make clean` for a different ARCH, then run a `make clean` for
ARCH=arm.

Is the above (regarding lazy evaluation) not true for LDFLAGS_vmlinux?
I would have thought this would work:

diff --git a/arch/arm/boot/compressed/Makefile
b/arch/arm/boot/compressed/Makefile
index 00602a6fba04..9e7343ccc279 100644
--- a/arch/arm/boot/compressed/Makefile
+++ b/arch/arm/boot/compressed/Makefile
@@ -113,10 +113,9 @@ ccflags-y := -fpic $(call
cc-option,-mno-single-pic-base,) -fno-builtin \
asflags-y := -DZIMAGE

# Supply kernel BSS size to the decompressor via a linker symbol.
-KBSS_SZ = $(shell echo $$(($$($(NM) $(obj)/../../../../vmlinux | \
+LDFLAGS_vmlinux = --defsym _kernel_bss_size=$(shell echo $$(($$($(NM)
$(obj)/../../../../vmlinux | \
sed -n -e 's/^\([^ ]*\) [AB] __bss_start$$/-0x\1/p' \
- -e 's/^\([^ ]*\) [AB] __bss_stop$$/+0x\1/p') )) )
-LDFLAGS_vmlinux = --defsym _kernel_bss_size=$(KBSS_SZ)
+ -e 's/^\([^ ]*\) [AB]
__bss_stop$$/+0x\1/p') )) )

--
Thanks,
~Nick Desaulniers

[Masahiro Yamada]

On Tue, Jun 30, 2020 at 7:39 AM Nick Desaulniers

yeah, I had noticed this noise, but
I did not get around to digging into it.

> I suspect I see this from doing an ARCH=arm build, then building a
> different ARCH and killing the build before vmlinux is produced or
> `make clean` for a different ARCH, then run a `make clean` for
> ARCH=arm.
>
> Is the above (regarding lazy evaluation) not true for LDFLAGS_vmlinux?
> I would have thought this would work:

No, this does not work.

LDFLAGS_vmlinux is assigned by '=',
so it already has the lazy expansion flavor.


Unexporting LDFLAGS_vmlinux will fix it.
(I may write a better patch, though)



diff --git a/arch/arm/boot/compressed/Makefile
b/arch/arm/boot/compressed/Makefile
index 00602a6fba04..fab54b1c39aa 100644
--- a/arch/arm/boot/compressed/Makefile
+++ b/arch/arm/boot/compressed/Makefile
@@ -131,6 +131,8 @@ LDFLAGS_vmlinux += -X
# Next argument is a linker script
LDFLAGS_vmlinux += -T

+unexport LDFLAGS_vmlinux
+
# For __aeabi_uidivmod
lib1funcs = $(obj)/lib1funcs.o

[Marco Elver]

Is it possible that this patch drops the KCSAN portion? We have a patch
that is part of a series intended for 5.9 that also removes the
cc-option from -fno-stack-protector, but can't easily be dropped from
the surrounding series.

Please see: https://lkml.kernel.org/r/20200624190236.GA6603@paulmck-ThinkPad-P72

Thanks,
-- Marco

[Masahiro Yamada]

On Tue, Jun 30, 2020 at 7:39 AM Nick Desaulniers

I described why this error message is displayed:

https://patchwork.kernel.org/patch/11637129/