ARM: 8900/1: UNWINDER_FRAME_POINTER implementation for Clang

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

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)

# 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                        \

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

/* 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