[Crash-utility] [PATCH] ARM: Fix idle task stack unwinding on Thumb-2 kernels

[Vincent Whitchurch]

ARM kernels built with the Thumb-2 instruction need R7 instead of FP for
unwinding stacks using the DWARF unwinder.

On a Thumb-2 kernel:

Before:
crash> bt 1
PID: 1      TASK: ee7e0000  CPU: 1   COMMAND: "systemd-shutdow"

After:
crash> bt 1
PID: 1      TASK: ee7e0000  CPU: 1   COMMAND: "systemd-shutdow"
 #0 [<805587a1>] (__schedule) from [<80558d2b>]
 #1 [<80558d2b>] (schedule) from [<8055c8bb>]
 #2 [<8055c8bb>] (schedule_hrtimeout_range_clock) from [<8055c937>]
 #3 [<8055c937>] (schedule_hrtimeout_range) from [<8012cbdf>]
 #4 [<8012cbdf>] (sys_rt_sigtimedwait) from [<80107259>]

Change-Id: Iec0c3a3ab089441d0ebf0002343790d65fe6ca78
---
 arm.c        | 15 +++++++++++----
 defs.h       |  1 +
 symbols.c    |  2 ++
 unwind_arm.c | 14 ++++++++++----
 4 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/arm.c b/arm.c
index 9652361..b05b0b3 100644
--- a/arm.c
+++ b/arm.c
@@ -301,6 +301,8 @@ arm_init(int when)
 		 * thread_info.cpu_context.
 		 */
 		STRUCT_SIZE_INIT(cpu_context_save, "cpu_context_save");
+		MEMBER_OFFSET_INIT(cpu_context_save_r7,
+			"cpu_context_save", "r7");
 		MEMBER_OFFSET_INIT(cpu_context_save_fp,
 			"cpu_context_save", "fp");
 		MEMBER_OFFSET_INIT(cpu_context_save_sp,
@@ -1313,13 +1315,18 @@ arm_get_frame(struct bt_info *bt, ulong *pcp, ulong *spp)
 	cpu_context = tt->thread_info + OFFSET(thread_info_cpu_context);
 
 #define GET_REG(ptr, cp, off) ((*ptr) = (*((ulong *)((cp) + OFFSET(off)))))
-	/*
-	 * Unwinding code needs FP value also so we pass it with bt.
-	 */
-	GET_REG(&bt->frameptr, cpu_context, cpu_context_save_fp);
 	GET_REG(spp, cpu_context, cpu_context_save_sp);
 	GET_REG(pcp, cpu_context, cpu_context_save_pc);
 
+	/*
+	 * Unwinding code needs FP (R7 for Thumb code) value also so we pass it
+	 * with bt.
+	 */
+	if (*pcp & 1)
+		GET_REG(&bt->frameptr, cpu_context, cpu_context_save_r7);
+	else
+		GET_REG(&bt->frameptr, cpu_context, cpu_context_save_fp);
+
 	return TRUE;
 }
 
diff --git a/defs.h b/defs.h
index 5841b1f..b4f6372 100644
--- a/defs.h
+++ b/defs.h
@@ -2066,6 +2066,7 @@ struct offset_table {                    /* stash of commonly-used offsets */
 	long xa_node_shift;
 	long hd_struct_dkstats;
 	long disk_stats_in_flight;
+	long cpu_context_save_r7;
 };
 
 struct size_table {         /* stash of commonly-used sizes */
diff --git a/symbols.c b/symbols.c
index e73e735..31a4d7b 100644
--- a/symbols.c
+++ b/symbols.c
@@ -10047,6 +10047,8 @@ dump_offset_table(char *spec, ulong makestruct)
 	fprintf(fp, "          s390_stack_frame_r14: %ld\n",
 		OFFSET(s390_stack_frame_r14));
 
+	fprintf(fp, "           cpu_context_save_r7: %ld\n",
+		OFFSET(cpu_context_save_r7));
 	fprintf(fp, "           cpu_context_save_fp: %ld\n",
 		OFFSET(cpu_context_save_fp));
 	fprintf(fp, "           cpu_context_save_sp: %ld\n",
diff --git a/unwind_arm.c b/unwind_arm.c
index 8667d3c..1a8f51e 100644
--- a/unwind_arm.c
+++ b/unwind_arm.c
@@ -87,6 +87,7 @@ struct stackframe {
 };
 
 enum regs {
+	R7 = 7,
 	FP = 11,
 	SP = 13,
 	LR = 14,
@@ -615,6 +616,7 @@ unwind_frame(struct stackframe *frame, ulong stacktop)
 	struct unwind_ctrl_block ctrl;
 	struct unwind_idx *idx;
 	ulong low, high;
+	int fpindex = FP;
 
 	low = frame->sp;
 	high = stacktop;
@@ -622,6 +624,10 @@ unwind_frame(struct stackframe *frame, ulong stacktop)
 	if (!is_kernel_text(frame->pc))
 		return FALSE;
 
+	/* Thumb needs R7 instead of FP */
+	if (frame->pc & 1)
+		fpindex = R7;
+
 	tbl = search_table(frame->pc);
 	if (!tbl) {
 		error(WARNING, "UNWIND: cannot find unwind table for %lx\n",
@@ -630,13 +636,13 @@ unwind_frame(struct stackframe *frame, ulong stacktop)
 	}
 	idx = search_index(tbl, frame->pc);
 
-	ctrl.vrs[FP] = frame->fp;
+	ctrl.vrs[fpindex] = frame->fp;
 	ctrl.vrs[SP] = frame->sp;
 	ctrl.vrs[LR] = frame->lr;
 	ctrl.vrs[PC] = 0;
 
 	if (CRASHDEBUG(5)) {
-		fprintf(fp, "UNWIND: >frame: FP=%lx\n", ctrl.vrs[FP]);
+		fprintf(fp, "UNWIND: >frame: FP=%lx\n", ctrl.vrs[fpindex]);
 		fprintf(fp, "UNWIND: >frame: SP=%lx\n", ctrl.vrs[SP]);
 		fprintf(fp, "UNWIND: >frame: LR=%lx\n", ctrl.vrs[LR]);
 		fprintf(fp, "UNWIND: >frame: PC=%lx\n", ctrl.vrs[PC]);
@@ -706,13 +712,13 @@ unwind_frame(struct stackframe *frame, ulong stacktop)
 	if (frame->pc == ctrl.vrs[PC])
 		return FALSE;
 
-	frame->fp = ctrl.vrs[FP];
+	frame->fp = ctrl.vrs[fpindex];
 	frame->sp = ctrl.vrs[SP];
 	frame->lr = ctrl.vrs[LR];
 	frame->pc = ctrl.vrs[PC];
 
 	if (CRASHDEBUG(5)) {
-		fprintf(fp, "UNWIND: <frame: FP=%lx\n", ctrl.vrs[FP]);
+		fprintf(fp, "UNWIND: <frame: FP=%lx\n", ctrl.vrs[fpindex]);
 		fprintf(fp, "UNWIND: <frame: SP=%lx\n", ctrl.vrs[SP]);
 		fprintf(fp, "UNWIND: <frame: LR=%lx\n", ctrl.vrs[LR]);
 		fprintf(fp, "UNWIND: <frame: PC=%lx\n", ctrl.vrs[PC]);
-- 
2.20.0