[PATCH] ARM: cfi: Fix compilation corner case

[Linus Walleij <linus.walleij@linaro.org>]

When enabling expert mode CONFIG_EXPERT and using that power
user mode to disable the branch prediction hardening
!CONFIG_HARDEN_BRANCH_PREDICTOR, the picky assembly linker
in CLANG notices that some assembly in proc-v7.S does
not have corresponding C call sites, i.e. the prototypes
in proc-v7-bugs.c are enclosed in ifdef
CONFIG_HARDEN_BRANCH_PREDICTOR so this assembly:

SYM_TYPED_FUNC_START(cpu_v7_smc_switch_mm)
SYM_TYPED_FUNC_START(cpu_v7_hvc_switch_mm)

Results in:

ld.lld: error: undefined symbol: __kcfi_typeid_cpu_v7_smc_switch_mm
>>> referenced by proc-v7.S:94 (.../arch/arm/mm/proc-v7.S:94)
>>> arch/arm/mm/proc-v7.o:(.text+0x108) in archive vmlinux.a

ld.lld: error: undefined symbol: __kcfi_typeid_cpu_v7_hvc_switch_mm
>>> referenced by proc-v7.S:105 (.../arch/arm/mm/proc-v7.S:105)
>>> arch/arm/mm/proc-v7.o:(.text+0x124) in archive vmlinux.a

Fix this by adding an additional requirement that
CONFIG_HARDEN_BRANCH_PREDICTOR has to be enabled to compile
these assembly calls.

I suppose it wasn't a problem before because the linker is not
so picky that other assembly symbols are actually being
used.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202411041456.ZsoEiD7T-lkp@intel.com/
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
 arch/arm/mm/proc-v7.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm/mm/proc-v7.S b/arch/arm/mm/proc-v7.S
index 5fb9a6aecb00..2cd933342679 100644
--- a/arch/arm/mm/proc-v7.S
+++ b/arch/arm/mm/proc-v7.S
@@ -94,7 +94,7 @@ SYM_TYPED_FUNC_START(cpu_v7_dcache_clean_area)
 	ret	lr
 SYM_FUNC_END(cpu_v7_dcache_clean_area)
 
-#ifdef CONFIG_ARM_PSCI
+#if defined(CONFIG_ARM_PSCI) && defined(CONFIG_HARDEN_BRANCH_PREDICTOR)
 	.arch_extension sec
 SYM_TYPED_FUNC_START(cpu_v7_smc_switch_mm)
 	stmfd	sp!, {r0 - r3}

---
base-commit: 9852d85ec9d492ebef56dc5f229416c925758edc
change-id: 20241107-fix-kcfi-bug-ae3b08cbc167

Best regards,
-- 
Linus Walleij <linus.walleij@linaro.org>