arm64: unwind: add asynchronous unwind tables to kernel and modules

Enable asynchronous unwind table generation for both the core kernel as
well as modules, and emit the resulting .eh_frame sections as init code
so we can use the unwind directives for code patching at boot or module
load time.

This will be used by dynamic shadow call stack support, which will rely
on code patching rather than compiler codegen to emit the shadow call
stack push and pop instructions.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Tested-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lore.kernel.org/r/20221027155908.1940624-2-ardb@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/Kconfig

@@ -370,6 +370,9 @@ config KASAN_SHADOW_OFFSET
 	default 0xeffffff800000000 if ARM64_VA_BITS_36 && KASAN_SW_TAGS
 	default 0xffffffffffffffff
 
+config UNWIND_TABLES
+	bool
+
 source "arch/arm64/Kconfig.platforms"
 
 menu "Kernel Features"

arch/arm64/Makefile

@@ -45,8 +45,13 @@ KBUILD_CFLAGS	+= $(call cc-option,-mabi=lp64)
 KBUILD_AFLAGS	+= $(call cc-option,-mabi=lp64)
 
 # Avoid generating .eh_frame* sections.
+ifneq ($(CONFIG_UNWIND_TABLES),y)
 KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables -fno-unwind-tables
 KBUILD_AFLAGS	+= -fno-asynchronous-unwind-tables -fno-unwind-tables
+else
+KBUILD_CFLAGS	+= -fasynchronous-unwind-tables
+KBUILD_AFLAGS	+= -fasynchronous-unwind-tables
+endif
 
 ifeq ($(CONFIG_STACKPROTECTOR_PER_TASK),y)
 prepare: stack_protector_prepare

arch/arm64/include/asm/module.lds.h

@@ -17,4 +17,12 @@ SECTIONS {
 	 */
 	.text.hot : { *(.text.hot) }
 #endif
+
+#ifdef CONFIG_UNWIND_TABLES
+	/*
+	 * Currently, we only use unwind info at module load time, so we can
+	 * put it into the .init allocation.
+	 */
+	.init.eh_frame : { *(.eh_frame) }
+#endif
 }

arch/arm64/kernel/pi/Makefile

@@ -7,6 +7,7 @@ KBUILD_CFLAGS	:= $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) -fpie \
 		   -I$(srctree)/scripts/dtc/libfdt -fno-stack-protector \
 		   -include $(srctree)/include/linux/hidden.h \
 		   -D__DISABLE_EXPORTS -ffreestanding -D__NO_FORTIFY \
+		   -fno-asynchronous-unwind-tables -fno-unwind-tables \
 		   $(call cc-option,-fno-addrsig)
 
 # remove SCS flags from all objects in this directory

arch/arm64/kernel/vmlinux.lds.S

@@ -121,6 +121,17 @@ jiffies = jiffies_64;
 #define TRAMP_TEXT
 #endif
 
+#ifdef CONFIG_UNWIND_TABLES
+#define UNWIND_DATA_SECTIONS				\
+	.eh_frame : {					\
+		__eh_frame_start = .;			\
+		*(.eh_frame)				\
+		__eh_frame_end = .;			\
+	}
+#else
+#define UNWIND_DATA_SECTIONS
+#endif
+
 /*
  * The size of the PE/COFF section that covers the kernel image, which
  * runs from _stext to _edata, must be a round multiple of the PE/COFF
@@ -231,6 +242,8 @@ SECTIONS
 		__alt_instructions_end = .;
 	}
 
+	UNWIND_DATA_SECTIONS
+
 	. = ALIGN(SEGMENT_ALIGN);
 	__inittext_end = .;
 	__initdata_begin = .;

arch/arm64/kvm/hyp/nvhe/Makefile

@@ -96,6 +96,7 @@ KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) $(CC_FLAGS_CFI)
 # when profile optimization is applied. gen-hyprel does not support SHT_REL and
 # causes a build failure. Remove profile optimization flags.
 KBUILD_CFLAGS := $(filter-out -fprofile-sample-use=% -fprofile-use=%, $(KBUILD_CFLAGS))
+KBUILD_CFLAGS += -fno-asynchronous-unwind-tables -fno-unwind-tables
 
 # KVM nVHE code is run at a different exception code with a different map, so
 # compiler instrumentation that inserts callbacks or checks into the code may

drivers/firmware/efi/libstub/Makefile

@@ -20,6 +20,7 @@ cflags-$(CONFIG_X86)		+= -m$(BITS) -D__KERNEL__ \
 # disable the stackleak plugin
 cflags-$(CONFIG_ARM64)		:= $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) \
 				   -fpie $(DISABLE_STACKLEAK_PLUGIN) \
+				   -fno-unwind-tables -fno-asynchronous-unwind-tables \
 				   $(call cc-option,-mbranch-protection=none)
 cflags-$(CONFIG_ARM)		:= $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) \
 				   -fno-builtin -fpic \

include/asm-generic/vmlinux.lds.h

@@ -1027,14 +1027,19 @@
  * keep any .init_array.* sections.
  * https://bugs.llvm.org/show_bug.cgi?id=46478
  */
+#ifdef CONFIG_UNWIND_TABLES
+#define DISCARD_EH_FRAME
+#else
+#define DISCARD_EH_FRAME	*(.eh_frame)
+#endif
 #if defined(CONFIG_GCOV_KERNEL) || defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KCSAN)
 # ifdef CONFIG_CONSTRUCTORS
 #  define SANITIZER_DISCARDS						\
-	*(.eh_frame)
+	DISCARD_EH_FRAME
 # else
 #  define SANITIZER_DISCARDS						\
 	*(.init_array) *(.init_array.*)					\
-	*(.eh_frame)
+	DISCARD_EH_FRAME
 # endif
 #else
 # define SANITIZER_DISCARDS

scripts/module.lds.S

@@ -3,6 +3,12 @@
  * Archs are free to supply their own linker scripts.  ld will
  * combine them automatically.
  */
+#ifdef CONFIG_UNWIND_TABLES
+#define DISCARD_EH_FRAME
+#else
+#define DISCARD_EH_FRAME	*(.eh_frame)
+#endif
+
 SECTIONS {
 	/DISCARD/ : {
 		*(.discard)