[PATCH v3] ARM: UNWINDER_FRAME_POINTER implementation for Clang

[Nathan Huckleberry]

The stackframe setup when compiled with clang is different.
Since the stack unwinder expects the gcc stackframe setup it
fails to print backtraces. This patch adds support for the
clang stackframe setup.

Link: https://github.com/ClangBuiltLinux/linux/issues/35
Cc: clang-built-linux@xxxxxxxxxxxxxxxx
Suggested-by: Tri Vo <trong@xxxxxxxxxx>
Signed-off-by: Nathan Huckleberry <nhuck@xxxxxxxxxx>
Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
---
Changes from v2->v3
* Fix indentation on code
* Fix comment formatting

 arch/arm/Kconfig.debug         |   2 +-
 arch/arm/Makefile              |   5 +-
 arch/arm/lib/Makefile          |   8 +-
 arch/arm/lib/backtrace-clang.S | 217 +++++++++++++++++++++++++++++++++
 4 files changed, 229 insertions(+), 3 deletions(-)
 create mode 100644 arch/arm/lib/backtrace-clang.S

diff --git a/arch/arm/Kconfig.debug b/arch/arm/Kconfig.debug
index 85710e078afb..b9c674ec19e0 100644
--- a/arch/arm/Kconfig.debug
+++ b/arch/arm/Kconfig.debug
@@ -56,7 +56,7 @@ choice
 
 config UNWINDER_FRAME_POINTER
 	bool "Frame pointer unwinder"
-	depends on !THUMB2_KERNEL && !CC_IS_CLANG
+	depends on !THUMB2_KERNEL
 	select ARCH_WANT_FRAME_POINTERS
 	select FRAME_POINTER
 	help
diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index c3624ca6c0bc..6f251c201db0 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -36,7 +36,10 @@ KBUILD_CFLAGS	+= $(call cc-option,-mno-unaligned-access)
 endif
 
 ifeq ($(CONFIG_FRAME_POINTER),y)
-KBUILD_CFLAGS	+=-fno-omit-frame-pointer -mapcs -mno-sched-prolog
+KBUILD_CFLAGS	+=-fno-omit-frame-pointer
+ifeq ($(CONFIG_CC_IS_GCC),y)
+KBUILD_CFLAGS += -mapcs -mno-sched-prolog
+endif
 endif
 
 ifeq ($(CONFIG_CPU_BIG_ENDIAN),y)
diff --git a/arch/arm/lib/Makefile b/arch/arm/lib/Makefile
index b25c54585048..6d2ba454f25b 100644
--- a/arch/arm/lib/Makefile
+++ b/arch/arm/lib/Makefile
@@ -5,7 +5,7 @@
 # Copyright (C) 1995-2000 Russell King
 #
 
-lib-y		:= backtrace.o changebit.o csumipv6.o csumpartial.o   \
+lib-y		:= changebit.o csumipv6.o csumpartial.o               \
 		   csumpartialcopy.o csumpartialcopyuser.o clearbit.o \
 		   delay.o delay-loop.o findbit.o memchr.o memcpy.o   \
 		   memmove.o memset.o setbit.o                        \
@@ -19,6 +19,12 @@ lib-y		:= backtrace.o changebit.o csumipv6.o csumpartial.o   \
 mmu-y		:= clear_user.o copy_page.o getuser.o putuser.o       \
 		   copy_from_user.o copy_to_user.o
 
+ifdef CONFIG_CC_IS_CLANG
+  lib-y	+= backtrace-clang.o
+else
+  lib-y	+= backtrace.o
+endif
+
 # using lib_ here won't override already available weak symbols
 obj-$(CONFIG_UACCESS_WITH_MEMCPY) += uaccess_with_memcpy.o
 
diff --git a/arch/arm/lib/backtrace-clang.S b/arch/arm/lib/backtrace-clang.S
new file mode 100644
index 000000000000..2ff375144b55
--- /dev/null
+++ b/arch/arm/lib/backtrace-clang.S
@@ -0,0 +1,217 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ *  linux/arch/arm/lib/backtrace-clang.S
+ *
+ *  Copyright (C) 2019 Nathan Huckleberry
+ *
+ */
+#include <linux/kern_levels.h>
+#include <linux/linkage.h>
+#include <asm/assembler.h>
+		.text
+
+/* fp is 0 or stack frame */
+
+#define frame	r4
+#define sv_fp	r5
+#define sv_pc	r6
+#define mask	r7
+#define sv_lr	r8
+
+ENTRY(c_backtrace)
+
+#if !defined(CONFIG_FRAME_POINTER) || !defined(CONFIG_PRINTK)
+		ret	lr
+ENDPROC(c_backtrace)
+#else
+
+
+/*
+ * Clang does not store pc or sp in function prologues so we don't know exactly
+ * where the function starts.
+ *
+ * We can treat the current frame's lr as the saved pc and the preceding
+ * frame's lr as the current frame's lr, but we can't trace the most recent
+ * call.  Inserting a false stack frame allows us to reference the function
+ * called last in the stacktrace.
+ *
+ * If the call instruction was a bl we can look at the callers branch
+ * instruction to calculate the saved pc.  We can recover the pc in most cases,
+ * but in cases such as calling function pointers we cannot. In this case,
+ * default to using the lr. This will be some address in the function, but will
+ * not be the function start.
+ *
+ * Unfortunately due to the stack frame layout we can't dump r0 - r3, but these
+ * are less frequently saved.
+ *
+ * Stack frame layout:
+ * 		<larger addresses>
+ * 		saved lr
+ * 	frame=> saved fp
+ * 		optionally saved caller registers (r4 - r10)
+ * 		optionally saved arguments (r0 - r3)
+ * 		<top of stack frame>
+ * 		<smaller addresses>
+ *
+ * Functions start with the following code sequence:
+ * corrected pc =>  stmfd sp!, {..., fp, lr}
+ *		add fp, sp, #x
+ *		stmfd sp!, {r0 - r3} (optional)
+ *
+ *
+ *
+ *
+ *
+ *
+ * The diagram below shows an example stack setup for dump_stack.
+ *
+ * The frame for c_backtrace has pointers to the code of dump_stack. This is
+ * why the frame of c_backtrace is used to for the pc calculation of
+ * dump_stack. This is why we must move back a frame to print dump_stack.
+ *
+ * The stored locals for dump_stack are in dump_stack's frame. This means that
+ * to fully print dump_stack's frame we need both the frame for dump_stack (for
+ * locals) and the frame that was called by dump_stack (for pc).
+ *
+ * To print locals we must know where the function start is. If we read the
+ * function prologue opcodes we can determine which variables are stored in the
+ * stack frame.
+ *
+ * To find the function start of dump_stack we can look at the stored LR of
+ * show_stack. It points at the instruction directly after the bl dump_stack.
+ * We can then read the offset from the bl opcode to determine where the branch
+ * takes us.  The address calculated must be the start of dump_stack.
+ *
+ * c_backtrace frame           dump_stack:
+ * {[LR]    }  ============|   ...
+ * {[FP]    }  =======|    |   bl c_backtrace
+ *                    |    |=> ...
+ * {[R4-R10]}         |
+ * {[R0-R3] }         |        show_stack:
+ * dump_stack frame   |        ...
+ * {[LR]    } =============|   bl dump_stack
+ * {[FP]    } <=======|    |=> ...
+ * {[R4-R10]}
+ * {[R0-R3] }
+ */
+
+		stmfd	sp!, {r4 - r9, fp, lr}	@ Save an extra register
+						@ to ensure 8 byte alignment
+		movs	frame, r0		@ if frame pointer is zero
+		beq	no_frame		@ we have no stack frames
+		tst	r1, #0x10		@ 26 or 32-bit mode?
+		moveq	mask, #0xfc000003
+		movne	mask, #0		@ mask for 32-bit
+
+/*
+ * Switches the current frame to be the frame for dump_stack.
+ */
+		add	frame, sp, #24		@ switch to false frame
+for_each_frame:	tst	frame, mask		@ Check for address exceptions
+		bne	no_frame
+
+/*
+ * sv_fp is the stack frame with the locals for the current considered
+ * function.
+ *
+ * sv_pc is the saved lr frame the frame above. This is a pointer to a code
+ * address within the current considered function, but it is not the function
+ * start. This value gets updated to be the function start later if it is
+ * possible.
+ */
+1001:		ldr	sv_pc, [frame, #4]	@ get saved 'pc'
+1002:		ldr	sv_fp, [frame, #0]	@ get saved fp
+
+		teq	sv_fp, mask		@ make sure next frame exists
+		beq	no_frame
+
+/*
+ * sv_lr is the lr from the function that called the current function. This is
+ * a pointer to a code address in the current function's caller.  sv_lr-4 is
+ * the instruction used to call the current function.
+ *
+ * This sv_lr can be used to calculate the function start if the function was
+ * called using a bl instruction. If the function start can be recovered sv_pc
+ * is overwritten with the function start.
+ *
+ * If the current function was called using a function pointer we cannot
+ * recover the function start and instead continue with sv_pc as an arbitrary
+ * value within the current function. If this is the case we cannot print
+ * registers for the current function, but the stacktrace is still printed
+ * properly.
+ */
+1003:		ldr	sv_lr, [sv_fp, #4]	@ get saved lr from next frame
+
+		ldr	r0, [sv_lr, #-4]	@ get call instruction
+		ldr	r3, .Lopcode+4
+		and	r2, r3, r0		@ is this a bl call
+		teq	r2, r3
+		bne	finished_setup		@ give up if it's not
+		and	r0, #0xffffff		@ get call offset 24-bit int
+		lsl	r0, r0, #8		@ sign extend offset
+		asr	r0, r0, #8
+		ldr	sv_pc, [sv_fp, #4]	@ get lr address
+		add	sv_pc, sv_pc, #-4	@ get call instruction address
+		add	sv_pc, sv_pc, #8	@ take care of prefetch
+		add	sv_pc, sv_pc, r0, lsl #2@ find function start
+
+finished_setup:
+
+		bic	sv_pc, sv_pc, mask	@ mask PC/LR for the mode
+
+/*
+ * Print the function (sv_pc) and where it was called from (sv_lr).
+ */
+1004:		mov	r0, sv_pc
+
+		mov	r1, sv_lr
+		mov	r2, frame
+		bic	r1, r1, mask		@ mask PC/LR for the mode
+		bl	dump_backtrace_entry
+
+/*
+ * Test if the function start is a stmfd instruction to determine which
+ * registers were stored in the function prologue.
+ *
+ * If we could not recover the sv_pc because we were called through a function
+ * pointer the comparison will fail and no registers will print. Unwinding will
+ * continue as if there had been no registers stored in this frame.
+ */
+1005:		ldr	r1, [sv_pc, #0]		@ if stmfd sp!, {..., fp, lr}
+		ldr	r3, .Lopcode		@ instruction exists,
+		teq	r3, r1, lsr #11
+		ldr	r0, [frame]		@ locals are stored in
+						@ the preceding frame
+		subeq	r0, r0, #4
+		bleq	dump_backtrace_stm	@ dump saved registers
+
+/*
+ * If we are out of frames or if the next frame is invalid.
+ */
+		teq	sv_fp, #0		@ zero saved fp means
+		beq	no_frame		@ no further frames
+
+		cmp	sv_fp, frame		@ next frame must be
+		mov	frame, sv_fp		@ above the current frame
+		bhi	for_each_frame
+
+1006:		adr	r0, .Lbad
+		mov	r1, frame
+		bl	printk
+no_frame:	ldmfd	sp!, {r4 - r9, fp, pc}
+ENDPROC(c_backtrace)
+		.pushsection __ex_table,"a"
+		.align	3
+		.long	1001b, 1006b
+		.long	1002b, 1006b
+		.long	1003b, 1006b
+		.long	1004b, 1006b
+		.long   1005b, 1006b
+		.popsection
+
+.Lbad:		.asciz	"Backtrace aborted due to bad frame pointer <%p>\n"
+		.align
+.Lopcode:	.word	0xe92d4800 >> 11	@ stmfd sp!, {... fp, lr}
+		.word	0x0b000000		@ bl if these bits are set
+
+#endif
-- 
2.23.0.187.g17f5b7556c-goog