[PATCH v3 39/59] x86/Kconfig: Introduce function padding

Peter Zijlstra
2022-09-15T11:44:00Z

From: Thomas Gleixner <tglx@linutronix.de>

Now that all functions are 16 byte aligned, add 16 bytes of NOP
padding in front of each function. This prepares things for software
call stack tracking and kCFI/FineIBT.

This significantly increases kernel .text size, around 5.1% on a
x86_64-defconfig-ish build.

However, per the random access argument used for alignment, these 16
extra bytes are code that wouldn't be used. Performance measurements
back this up by showing no significant performance regressions.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
arch/x86/Kconfig | 20 +++++++++++++++++++-
arch/x86/Makefile | 6 ++++++
arch/x86/entry/vdso/Makefile | 3 ++-
arch/x86/include/asm/linkage.h | 15 +++++++++++++--
4 files changed, 40 insertions(+), 4 deletions(-)

--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2429,9 +2429,27 @@ config CC_HAS_SLS
config CC_HAS_RETURN_THUNK
def_bool $(cc-option,-mfunction-return=thunk-extern)

+config CC_HAS_ENTRY_PADDING
+ def_bool $(cc-option,-fpatchable-function-entry=16,16)
+
+config FUNCTION_PADDING_CFI
+ int
+ default 59 if FUNCTION_ALIGNMENT_64B
+ default 27 if FUNCTION_ALIGNMENT_32B
+ default 11 if FUNCTION_ALIGNMENT_16B
+ default 3 if FUNCTION_ALIGNMENT_8B
+ default 0
+
+# Basically: FUNCTION_ALIGNMENT - 5*CFI_CLANG
+# except Kconfig can't do arithmetic :/
+config FUNCTION_PADDING_BYTES
+ int
+ default FUNCTION_PADDING_CFI if CFI_CLANG
+ default FUNCTION_ALIGNMENT
+
config HAVE_CALL_THUNKS
def_bool y
- depends on RETHUNK && OBJTOOL
+ depends on CC_HAS_ENTRY_PADDING && RETHUNK && OBJTOOL

config CALL_THUNKS
def_bool n
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -198,6 +198,12 @@ ifdef CONFIG_SLS
KBUILD_CFLAGS += -mharden-sls=all
endif

+ifdef CONFIG_CALL_THUNKS
+PADDING_CFLAGS := -fpatchable-function-entry=$(CONFIG_FUNCTION_PADDING_BYTES),$(CONFIG_FUNCTION_PADDING_BYTES)
+KBUILD_CFLAGS += $(PADDING_CFLAGS)
+export PADDING_CFLAGS
+endif
+
KBUILD_LDFLAGS += -m elf_$(UTS_MACHINE)

ifdef CONFIG_LTO_CLANG
--- a/arch/x86/entry/vdso/Makefile
+++ b/arch/x86/entry/vdso/Makefile
@@ -92,7 +92,7 @@ ifneq ($(RETPOLINE_VDSO_CFLAGS),)
endif
endif

-$(vobjs): KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_LTO) $(RANDSTRUCT_CFLAGS) $(GCC_PLUGINS_CFLAGS) $(RETPOLINE_CFLAGS),$(KBUILD_CFLAGS)) $(CFL)
+$(vobjs): KBUILD_CFLAGS := $(filter-out $(PADDING_CFLAGS) $(CC_FLAGS_LTO) $(RANDSTRUCT_CFLAGS) $(GCC_PLUGINS_CFLAGS) $(RETPOLINE_CFLAGS),$(KBUILD_CFLAGS)) $(CFL)
$(vobjs): KBUILD_AFLAGS += -DBUILD_VDSO

#
@@ -154,6 +154,7 @@ KBUILD_CFLAGS_32 := $(filter-out $(RANDS
KBUILD_CFLAGS_32 := $(filter-out $(GCC_PLUGINS_CFLAGS),$(KBUILD_CFLAGS_32))
KBUILD_CFLAGS_32 := $(filter-out $(RETPOLINE_CFLAGS),$(KBUILD_CFLAGS_32))
KBUILD_CFLAGS_32 := $(filter-out $(CC_FLAGS_LTO),$(KBUILD_CFLAGS_32))
+KBUILD_CFLAGS_32 := $(filter-out $(PADDING_CFLAGS),$(KBUILD_CFLAGS_32))
KBUILD_CFLAGS_32 += -m32 -msoft-float -mregparm=0 -fpic
KBUILD_CFLAGS_32 += -fno-stack-protector
KBUILD_CFLAGS_32 += $(call cc-option, -foptimize-sibling-calls)
--- a/arch/x86/include/asm/linkage.h
+++ b/arch/x86/include/asm/linkage.h
@@ -15,8 +15,19 @@
#define __ALIGN .balign CONFIG_FUNCTION_ALIGNMENT, 0x90;
#define __ALIGN_STR __stringify(__ALIGN)

-#define ASM_FUNC_ALIGN __ALIGN_STR
-#define __FUNC_ALIGN __ALIGN
+#if defined(CONFIG_CALL_THUNKS) && !defined(__DISABLE_EXPORTS) && !defined(BUILD_VDSO)
+#define FUNCTION_PADDING .skip CONFIG_FUNCTION_ALIGNMENT, 0x90;
+#else
+#define FUNCTION_PADDING
+#endif
+
+#if (CONFIG_FUNCTION_ALIGNMENT > 8) && !defined(__DISABLE_EXPORTS) && !defined(BULID_VDSO)
+# define __FUNC_ALIGN __ALIGN; FUNCTION_PADDING
+#else
+# define __FUNC_ALIGN __ALIGN
+#endif
+
+#define ASM_FUNC_ALIGN __stringify(__FUNC_ALIGN)
#define SYM_F_ALIGN __FUNC_ALIGN

#ifdef __ASSEMBLY__