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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Mon, 29 Jun 2020 11:26:39 -0700
Message-ID: <CAKwvOdntZw2k8ZZ7txPC-FD+50cPpMwctGYTW0j2zw+FVj9SgA@mail.gmail.com>
Subject: Re: [PATCH 1/2] kbuild: remove cc-option test of -fno-stack-protector
To:     Masahiro Yamada <masahiroy@kernel.org>
Cc:     Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        linux-efi <linux-efi@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>,
        LKML <linux-kernel@vger.kernel.org>
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Sender: linux-kernel-owner@vger.kernel.org
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On Sat, Jun 27, 2020 at 4:59 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> On Sat, Jun 27, 2020 at 5:13 AM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
> >
> > On Fri, Jun 26, 2020 at 12:00 PM Masahiro Yamada <masahiroy@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 <masahiroy@kernel.org>
> >
> > Generally looks pretty good, just a few more questions before adding
> > my tag explicitly.
> >
> > > ---
> > >
> > >  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
> >
> > 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.
> >
> > >
> > > 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
> >
> > 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 `:=`.
>
>
> 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)

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

Reviewed-by: Nick Desaulniers <ndesaulniers@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