[PATCH v2 0/2] powerpc: Prepare for clang's per-task stack protector support

[PATCH v2 0/2] powerpc: Prepare for clang's per-task stack protector support

[Nathan Chancellor]

This series prepares the powerpc Kconfig and Kbuild files for clang's
per-task stack protector support. clang requires
'-mstack-protector-guard-offset' to always be passed with the other
'-mstack-protector-guard' flags, which does not always happen with the
powerpc implementation, unlike arm, arm64, and riscv implementations.
This series brings powerpc in line with those other architectures, which
allows clang's support to work right away when it is merged.
Additionally, there is one other fix needed for the Kconfig test to work
correctly when targeting 32-bit.

I have tested this series in QEMU against LKDTM's REPORT_STACK_CANARY
with ppc64le_guest_defconfig and pmac32_defconfig built with a toolchain
that contains Keith's in-progress pull request, which should land for
LLVM 20:

https://github.com/llvm/llvm-project/pull/110928

---
Changes in v2:
- Combined patch 1 and 3, as they are fixing the same test for similar
  reasons; adjust commit message accordingly (Christophe)
- Moved stack protector guard flags on one line in Makefile (Christophe)
- Add 'Cc: stable' targeting 6.1 and newer for the sake of simplicity,
  as it is the oldest stable release where this series applies cleanly
  (folks who want it on earlier releases can request or perform a
  backport separately).
- Pick up Keith's Reviewed-by and Tested-by on both patches.
- Add a blurb to commit message of patch 1 explaining why clang's
  register selection behavior differs from GCC.
- Link to v1: https://lore.kernel.org/r/20241007-powerpc-fix-stackprotector-test-clang-v1-0-08c15b2694e4@kernel.org

---
Nathan Chancellor (2):
      powerpc: Fix stack protector Kconfig test for clang
      powerpc: Adjust adding stack protector flags to KBUILD_CLAGS for clang

 arch/powerpc/Kconfig  |  4 ++--
 arch/powerpc/Makefile | 13 ++++---------
 2 files changed, 6 insertions(+), 11 deletions(-)
---
base-commit: 8cf0b93919e13d1e8d4466eb4080a4c4d9d66d7b
change-id: 20241004-powerpc-fix-stackprotector-test-clang-84e67ed82f62

Best regards,
-- 
Nathan Chancellor <nathan@kernel.org>

[PATCH v2 1/2] powerpc: Fix stack protector Kconfig test for clang

[Nathan Chancellor]

Clang's in-progress per-task stack protector support [1] does not work
with the current Kconfig checks because '-mstack-protector-guard-offset'
is not provided, unlike all other architecture Kconfig checks.

  $ fd Kconfig -x rg -l mstack-protector-guard-offset
  ./arch/arm/Kconfig
  ./arch/riscv/Kconfig
  ./arch/arm64/Kconfig

This produces an error from clang, which is interpreted as the flags not
being supported at all when they really are.

  $ clang --target=powerpc64-linux-gnu \
          -mstack-protector-guard=tls \
          -mstack-protector-guard-reg=r13 \
          -c -o /dev/null -x c /dev/null
  clang: error: '-mstack-protector-guard=tls' is used without '-mstack-protector-guard-offset', and there is no default

This argument will always be provided by the build system, so mirror
other architectures and use '-mstack-protector-guard-offset=0' for
testing support, which fixes the issue for clang and does not regress
support with GCC.

Even with the first problem addressed, the 32-bit test continues to fail
because Kbuild uses the powerpc64le-linux-gnu target for clang and
nothing flips the target to 32-bit, resulting in an error about an
invalid register valid:

  $ clang --target=powerpc64le-linux-gnu \
          -mstack-protector-guard=tls
          -mstack-protector-guard-reg=r2 \
          -mstack-protector-guard-offset=0 \
          -x c -c -o /dev/null /dev/null
  clang: error: invalid value 'r2' in 'mstack-protector-guard-reg=', expected one of: r13

While GCC allows arbitrary registers, the implementation of
'-mstack-protector-guard=tls' in LLVM shares the same code path as the
user space thread local storage implementation, which uses a fixed
register (2 for 32-bit and 13 for 62-bit), so the command line parsing
enforces this limitation.

Use the Kconfig macro '$(m32-flag)', which expands to '-m32' when
supported, in the stack protector support cc-option call to properly
switch the target to a 32-bit one, which matches what happens in Kbuild.
While the 64-bit macro does not strictly need it, add the equivalent
64-bit option for symmetry.

Cc: stable@vger.kernel.org # 6.1+
Link: https://github.com/llvm/llvm-project/pull/110928 [1]
Reviewed-by: Keith Packard <keithp@keithp.com>
Tested-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 arch/powerpc/Kconfig | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 8094a01974cca1d27002720e706f66bec2a2d035..6aaca48955a34b2a38af1415bfa36f74f35c3f3e 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -275,8 +275,8 @@ config PPC
 	select HAVE_RSEQ
 	select HAVE_SETUP_PER_CPU_AREA		if PPC64
 	select HAVE_SOFTIRQ_ON_OWN_STACK
-	select HAVE_STACKPROTECTOR		if PPC32 && $(cc-option,-mstack-protector-guard=tls -mstack-protector-guard-reg=r2)
-	select HAVE_STACKPROTECTOR		if PPC64 && $(cc-option,-mstack-protector-guard=tls -mstack-protector-guard-reg=r13)
+	select HAVE_STACKPROTECTOR		if PPC32 && $(cc-option,$(m32-flag) -mstack-protector-guard=tls -mstack-protector-guard-reg=r2 -mstack-protector-guard-offset=0)
+	select HAVE_STACKPROTECTOR		if PPC64 && $(cc-option,$(m64-flag) -mstack-protector-guard=tls -mstack-protector-guard-reg=r13 -mstack-protector-guard-offset=0)
 	select HAVE_STATIC_CALL			if PPC32
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_VIRT_CPU_ACCOUNTING

-- 
2.47.0

[PATCH v2 2/2] powerpc: Adjust adding stack protector flags to KBUILD_CLAGS for clang

[Nathan Chancellor]

After fixing the HAVE_STACKPROTECTER checks for clang's in-progress
per-task stack protector support [1], the build fails during prepare0
because '-mstack-protector-guard-offset' has not been added to
KBUILD_CFLAGS yet but the other '-mstack-protector-guard' flags have.

  clang: error: '-mstack-protector-guard=tls' is used without '-mstack-protector-guard-offset', and there is no default
  clang: error: '-mstack-protector-guard=tls' is used without '-mstack-protector-guard-offset', and there is no default
  make[4]: *** [scripts/Makefile.build:229: scripts/mod/empty.o] Error 1
  make[4]: *** [scripts/Makefile.build:102: scripts/mod/devicetable-offsets.s] Error 1

Mirror other architectures and add all '-mstack-protector-guard' flags
to KBUILD_CFLAGS atomically during stack_protector_prepare, which
resolves the issue and allows clang's implementation to fully work with
the kernel.

Cc: stable@vger.kernel.org # 6.1+
Link: https://github.com/llvm/llvm-project/pull/110928 [1]
Reviewed-by: Keith Packard <keithp@keithp.com>
Tested-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 arch/powerpc/Makefile | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index bbfe4a1f06ef9db9b2f2e48e02096b1e0500a14b..cbb353ddacb7adc5de28cd1fde893de3efdd8272 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -100,13 +100,6 @@ KBUILD_AFLAGS	+= -m$(BITS)
 KBUILD_LDFLAGS	+= -m elf$(BITS)$(LDEMULATION)
 endif
 
-cflags-$(CONFIG_STACKPROTECTOR)	+= -mstack-protector-guard=tls
-ifdef CONFIG_PPC64
-cflags-$(CONFIG_STACKPROTECTOR)	+= -mstack-protector-guard-reg=r13
-else
-cflags-$(CONFIG_STACKPROTECTOR)	+= -mstack-protector-guard-reg=r2
-endif
-
 LDFLAGS_vmlinux-y := -Bstatic
 LDFLAGS_vmlinux-$(CONFIG_RELOCATABLE) := -pie
 LDFLAGS_vmlinux-$(CONFIG_RELOCATABLE) += -z notext
@@ -402,9 +395,11 @@ prepare: stack_protector_prepare
 PHONY += stack_protector_prepare
 stack_protector_prepare: prepare0
 ifdef CONFIG_PPC64
-	$(eval KBUILD_CFLAGS += -mstack-protector-guard-offset=$(shell awk '{if ($$2 == "PACA_CANARY") print $$3;}' include/generated/asm-offsets.h))
+	$(eval KBUILD_CFLAGS += -mstack-protector-guard=tls -mstack-protector-guard-reg=r13 \
+				-mstack-protector-guard-offset=$(shell awk '{if ($$2 == "PACA_CANARY") print $$3;}' include/generated/asm-offsets.h))
 else
-	$(eval KBUILD_CFLAGS += -mstack-protector-guard-offset=$(shell awk '{if ($$2 == "TASK_CANARY") print $$3;}' include/generated/asm-offsets.h))
+	$(eval KBUILD_CFLAGS += -mstack-protector-guard=tls -mstack-protector-guard-reg=r2 \
+				-mstack-protector-guard-offset=$(shell awk '{if ($$2 == "TASK_CANARY") print $$3;}' include/generated/asm-offsets.h))
 endif
 endif
 

-- 
2.47.0

Re: [PATCH v2 0/2] powerpc: Prepare for clang's per-task stack protector support

[Michael Ellerman]

On Wed, 09 Oct 2024 12:26:07 -0700, Nathan Chancellor wrote:
> This series prepares the powerpc Kconfig and Kbuild files for clang's
> per-task stack protector support. clang requires
> '-mstack-protector-guard-offset' to always be passed with the other
> '-mstack-protector-guard' flags, which does not always happen with the
> powerpc implementation, unlike arm, arm64, and riscv implementations.
> This series brings powerpc in line with those other architectures, which
> allows clang's support to work right away when it is merged.
> Additionally, there is one other fix needed for the Kconfig test to work
> correctly when targeting 32-bit.
> 
> [...]

Applied to powerpc/next.

[1/2] powerpc: Fix stack protector Kconfig test for clang
      https://git.kernel.org/powerpc/c/46e1879deea22eed31e9425d58635895fc0e8040
[2/2] powerpc: Adjust adding stack protector flags to KBUILD_CLAGS for clang
      https://git.kernel.org/powerpc/c/bee08a9e6ab03caf14481d97b35a258400ffab8f

cheers