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Subject[RFC PATCH] ARM: 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.

Cc: clang-built-linux@googlegroups.com
Suggested-by: Tri Vo <trong@google.com>
Signed-off-by: Nathan Huckleberry <nhuck@google.com>
---
arch/arm/Kconfig.debug | 4 +-
arch/arm/Makefile | 2 +-
arch/arm/lib/backtrace.S | 134 ++++++++++++++++++++++++++++++++++++---
3 files changed, 128 insertions(+), 12 deletions(-)

diff --git a/arch/arm/Kconfig.debug b/arch/arm/Kconfig.debug
index 85710e078afb..92fca7463e21 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
@@ -1872,7 +1872,7 @@ config DEBUG_UNCOMPRESS
When this option is set, the selected DEBUG_LL output method
will be re-used for normal decompressor output on multiplatform
kernels.
-
+

config UNCOMPRESS_INCLUDE
string
diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index c3624ca6c0bc..a593d9c4e18a 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -36,7 +36,7 @@ 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 $(call cc-option,-mapcs,) $(call cc-option,-mno-sched-prolog,)
endif

ifeq ($(CONFIG_CPU_BIG_ENDIAN),y)
diff --git a/arch/arm/lib/backtrace.S b/arch/arm/lib/backtrace.S
index 1d5210eb4776..fd64eec9f6ae 100644
--- a/arch/arm/lib/backtrace.S
+++ b/arch/arm/lib/backtrace.S
@@ -14,10 +14,7 @@
@ fp is 0 or stack frame

#define frame r4
-#define sv_fp r5
-#define sv_pc r6
#define mask r7
-#define offset r8

ENTRY(c_backtrace)

@@ -25,7 +22,8 @@ ENTRY(c_backtrace)
ret lr
ENDPROC(c_backtrace)
#else
- stmfd sp!, {r4 - r8, lr} @ Save an extra register so we have a location...
+ stmfd sp!, {r4 - r8, fp, lr} @ Save an extra register
+ @ so we have a location..
movs frame, r0 @ if frame pointer is zero
beq no_frame @ we have no stack frames

@@ -35,11 +33,119 @@ ENDPROC(c_backtrace)
THUMB( orreq mask, #0x03 )
movne mask, #0 @ mask for 32-bit

-1: stmfd sp!, {pc} @ calculate offset of PC stored
- ldr r0, [sp], #4 @ by stmfd for this CPU
- adr r1, 1b
- sub offset, r0, r1

+#if defined(CONFIG_CC_IS_CLANG)
+/*
+ * 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 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 addressses>
+ *
+ * Functions start with the following code sequence:
+ * corrected pc => stmfd sp!, {..., fp, lr}
+ * add fp, sp, #x
+ * stmfd sp!, {r0 - r3} (optional)
+ */
+#define sv_fp r5
+#define sv_pc r6
+#define sv_lr r8
+ add frame, sp, #20 @ switch to false frame
+for_each_frame: tst frame, mask @ Check for address exceptions
+ bne no_frame
+
+1001: ldr sv_pc, [frame, #4] @ get saved 'pc'
+1002: ldr sv_fp, [frame, #0] @ get saved fp
+
+ teq sv_fp, #0 @ make sure next frame exists
+ beq no_frame
+
+1003: ldr sv_lr, [sv_fp, #4] @ get saved lr from next frame
+
+ //try to find function start
+ ldr r0, [sv_lr, #-4] @ get call instruction
+ ldr r3, .Ldsi+8
+ 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
+ b finished_setup
+
+finished_setup:
+
+ bic sv_pc, sv_pc, mask @ mask PC/LR for the mode
+
+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
+
+1005: ldr r1, [sv_pc, #0] @ if stmfd sp!, {..., fp, lr}
+ ldr r3, .Ldsi @ 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
+
+ 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 - r8, fp, pc}
+ENDPROC(c_backtrace)
+ .pushsection __ex_table,"a"
+ .align 3
+ .long 1001b, 1006b
+ .long 1002b, 1006b
+ .long 1003b, 1006b
+ .long 1004b, 1006b
+ .popsection
+
+.Lbad: .asciz "Backtrace aborted due to bad frame pointer <%p>\n"
+ .align
+.Ldsi: .word 0xe92d4800 >> 11 @ stmfd sp!, {... fp, lr}
+ .word 0xe92d0000 >> 11 @ stmfd sp!, {}
+ .word 0x0b000000 @ bl if these bits are set
+
+ENDPROC(c_backtrace)
+
+#else
/*
* Stack frame layout:
* optionally saved caller registers (r4 - r10)
@@ -55,6 +161,15 @@ ENDPROC(c_backtrace)
* stmfd sp!, {r0 - r3} (optional)
* corrected pc => stmfd sp!, {..., fp, ip, lr, pc}
*/
+#define sv_fp r5
+#define sv_pc r6
+#define offset r8
+
+1: stmfd sp!, {pc} @ calculate offset of PC stored
+ ldr r0, [sp], #4 @ by stmfd for this CPU
+ adr r1, 1b
+ sub offset, r0, r1
+
for_each_frame: tst frame, mask @ Check for address exceptions
bne no_frame

@@ -98,7 +213,7 @@ for_each_frame: tst frame, mask @ Check for address exceptions
1006: adr r0, .Lbad
mov r1, frame
bl printk
-no_frame: ldmfd sp!, {r4 - r8, pc}
+no_frame: ldmfd sp!, {r4 - r8, fp, pc}
ENDPROC(c_backtrace)

.pushsection __ex_table,"a"
@@ -115,3 +230,4 @@ ENDPROC(c_backtrace)
.word 0xe92d0000 >> 11 @ stmfd sp!, {}

#endif
+#endif
--
2.22.0.770.g0f2c4a37fd-goog
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