219 lines
6.8 KiB
ArmAsm
Executable file
219 lines
6.8 KiB
ArmAsm
Executable file
/* SPDX-License-Identifier: GPL-2.0-only */
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/*
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* linux/arch/arm/lib/backtrace-clang.S
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*
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* Copyright (C) 2019 Nathan Huckleberry
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*
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*/
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#include <linux/kern_levels.h>
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#include <linux/linkage.h>
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#include <asm/assembler.h>
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.text
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/* fp is 0 or stack frame */
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#define frame r4
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#define sv_fp r5
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#define sv_pc r6
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#define mask r7
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#define sv_lr r8
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ENTRY(c_backtrace)
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#if !defined(CONFIG_FRAME_POINTER) || !defined(CONFIG_PRINTK)
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ret lr
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ENDPROC(c_backtrace)
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#else
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/*
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* Clang does not store pc or sp in function prologues so we don't know exactly
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* where the function starts.
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*
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* We can treat the current frame's lr as the saved pc and the preceding
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* frame's lr as the current frame's lr, but we can't trace the most recent
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* call. Inserting a false stack frame allows us to reference the function
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* called last in the stacktrace.
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*
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* If the call instruction was a bl we can look at the callers branch
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* instruction to calculate the saved pc. We can recover the pc in most cases,
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* but in cases such as calling function pointers we cannot. In this case,
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* default to using the lr. This will be some address in the function, but will
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* not be the function start.
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*
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* Unfortunately due to the stack frame layout we can't dump r0 - r3, but these
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* are less frequently saved.
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*
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* Stack frame layout:
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* <larger addresses>
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* saved lr
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* frame=> saved fp
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* optionally saved caller registers (r4 - r10)
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* optionally saved arguments (r0 - r3)
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* <top of stack frame>
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* <smaller addresses>
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*
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* Functions start with the following code sequence:
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* corrected pc => stmfd sp!, {..., fp, lr}
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* add fp, sp, #x
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* stmfd sp!, {r0 - r3} (optional)
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*
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*
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*
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*
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*
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*
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* The diagram below shows an example stack setup for dump_stack.
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*
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* The frame for c_backtrace has pointers to the code of dump_stack. This is
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* why the frame of c_backtrace is used to for the pc calculation of
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* dump_stack. This is why we must move back a frame to print dump_stack.
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*
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* The stored locals for dump_stack are in dump_stack's frame. This means that
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* to fully print dump_stack's frame we need both the frame for dump_stack (for
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* locals) and the frame that was called by dump_stack (for pc).
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*
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* To print locals we must know where the function start is. If we read the
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* function prologue opcodes we can determine which variables are stored in the
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* stack frame.
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*
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* To find the function start of dump_stack we can look at the stored LR of
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* show_stack. It points at the instruction directly after the bl dump_stack.
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* We can then read the offset from the bl opcode to determine where the branch
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* takes us. The address calculated must be the start of dump_stack.
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*
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* c_backtrace frame dump_stack:
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* {[LR] } ============| ...
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* {[FP] } =======| | bl c_backtrace
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* | |=> ...
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* {[R4-R10]} |
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* {[R0-R3] } | show_stack:
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* dump_stack frame | ...
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* {[LR] } =============| bl dump_stack
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* {[FP] } <=======| |=> ...
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* {[R4-R10]}
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* {[R0-R3] }
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*/
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stmfd sp!, {r4 - r9, fp, lr} @ Save an extra register
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@ to ensure 8 byte alignment
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movs frame, r0 @ if frame pointer is zero
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beq no_frame @ we have no stack frames
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tst r1, #0x10 @ 26 or 32-bit mode?
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moveq mask, #0xfc000003
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movne mask, #0 @ mask for 32-bit
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/*
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* Switches the current frame to be the frame for dump_stack.
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*/
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add frame, sp, #24 @ switch to false frame
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for_each_frame: tst frame, mask @ Check for address exceptions
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bne no_frame
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/*
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* sv_fp is the stack frame with the locals for the current considered
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* function.
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*
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* sv_pc is the saved lr frame the frame above. This is a pointer to a code
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* address within the current considered function, but it is not the function
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* start. This value gets updated to be the function start later if it is
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* possible.
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*/
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1001: ldr sv_pc, [frame, #4] @ get saved 'pc'
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1002: ldr sv_fp, [frame, #0] @ get saved fp
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teq sv_fp, mask @ make sure next frame exists
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beq no_frame
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/*
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* sv_lr is the lr from the function that called the current function. This is
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* a pointer to a code address in the current function's caller. sv_lr-4 is
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* the instruction used to call the current function.
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*
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* This sv_lr can be used to calculate the function start if the function was
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* called using a bl instruction. If the function start can be recovered sv_pc
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* is overwritten with the function start.
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*
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* If the current function was called using a function pointer we cannot
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* recover the function start and instead continue with sv_pc as an arbitrary
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* value within the current function. If this is the case we cannot print
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* registers for the current function, but the stacktrace is still printed
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* properly.
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*/
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1003: ldr sv_lr, [sv_fp, #4] @ get saved lr from next frame
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1007: ldr r0, [sv_lr, #-4] @ get call instruction
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ldr r3, .Lopcode+4
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and r2, r3, r0 @ is this a bl call
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teq r2, r3
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bne finished_setup @ give up if it's not
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and r0, #0xffffff @ get call offset 24-bit int
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lsl r0, r0, #8 @ sign extend offset
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asr r0, r0, #8
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ldr sv_pc, [sv_fp, #4] @ get lr address
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add sv_pc, sv_pc, #-4 @ get call instruction address
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add sv_pc, sv_pc, #8 @ take care of prefetch
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add sv_pc, sv_pc, r0, lsl #2@ find function start
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finished_setup:
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bic sv_pc, sv_pc, mask @ mask PC/LR for the mode
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/*
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* Print the function (sv_pc) and where it was called from (sv_lr).
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*/
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1004: mov r0, sv_pc
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mov r1, sv_lr
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mov r2, frame
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bic r1, r1, mask @ mask PC/LR for the mode
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bl dump_backtrace_entry
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/*
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* Test if the function start is a stmfd instruction to determine which
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* registers were stored in the function prologue.
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*
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* If we could not recover the sv_pc because we were called through a function
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* pointer the comparison will fail and no registers will print. Unwinding will
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* continue as if there had been no registers stored in this frame.
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*/
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1005: ldr r1, [sv_pc, #0] @ if stmfd sp!, {..., fp, lr}
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ldr r3, .Lopcode @ instruction exists,
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teq r3, r1, lsr #11
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ldr r0, [frame] @ locals are stored in
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@ the preceding frame
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subeq r0, r0, #4
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bleq dump_backtrace_stm @ dump saved registers
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/*
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* If we are out of frames or if the next frame is invalid.
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*/
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teq sv_fp, #0 @ zero saved fp means
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beq no_frame @ no further frames
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cmp sv_fp, frame @ next frame must be
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mov frame, sv_fp @ above the current frame
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bhi for_each_frame
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1006: adr r0, .Lbad
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mov r1, frame
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bl printk
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no_frame: ldmfd sp!, {r4 - r9, fp, pc}
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ENDPROC(c_backtrace)
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.pushsection __ex_table,"a"
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.align 3
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.long 1001b, 1006b
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.long 1002b, 1006b
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.long 1003b, 1006b
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.long 1004b, 1006b
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.long 1005b, 1006b
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.long 1007b, 1006b
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.popsection
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.Lbad: .asciz "Backtrace aborted due to bad frame pointer <%p>\n"
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.align
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.Lopcode: .word 0xe92d4800 >> 11 @ stmfd sp!, {... fp, lr}
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.word 0x0b000000 @ bl if these bits are set
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#endif
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