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/* 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
#define loglvl r9
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
mov loglvl, r2
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
mov r3, loglvl
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
mov r2, loglvl
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, loglvl
mov r2, 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 "%sBacktrace 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
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