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ARM: 8900/1: UNWINDER_FRAME_POINTER implementation for Clang

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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@googlegroups.com
Suggested-by: Tri Vo <trong@google.com>
Signed-off-by: Nathan Huckleberry <nhuck@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
This commit is contained in:
Nathan Huckleberry 2019-08-22 21:26:53 +01:00 committed by Russell King
parent 834020366d
commit 6dc5fd93b2
4 changed files with 229 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
arch/arm/Kconfig.debug
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@ -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

5
arch/arm/Makefile
View File

@ -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)

8
arch/arm/lib/Makefile
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@ -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

217
arch/arm/lib/backtrace-clang.S Normal file
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@ -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