[Crash-utility] [PATCH v4 3/3] arm64: correct a PC shown in bt
On arm64, the link register (LR) holds a return address, which is the one
just after a branch instruction. So using a saved lr as PC for backtracing
might cause some confusion.
For example, in kernel/entry.S,
work_resched:
...
bl schedule
ret_to_user:
...
The current code shows "ret_o_user", instead of "work_resched",
as a caller of schedule().
This patch corrects a PC by decrementing it by 4.
But please note that this change may also make people a bit confused
because a value of LR in the stack dump of "bt -f" doesn't match with
an address in one-line summary.
#2 [ffffcc7511407eb0] schedule at ffff0000d628aee0
ffffcc7511407eb0: ffffcc6d22f23080 ffff0000d5b44d6c <= LR
ffffcc7511407ec0: ffffcc7511407ed0 0000000000000000
#3 [ffffcc7511407ed0] work_resched at ffff0000d5b44d68 <= correcrted PC
Signed-off-by: AKASHI Takahiro <takahiro.akashi(a)linaro.org>
---
arm64.c | 23 ++++++++++++++---------
1 file changed, 14 insertions(+), 9 deletions(-)
diff --git a/arm64.c b/arm64.c
index 5327370..554f639 100644
--- a/arm64.c
+++ b/arm64.c
@@ -1504,35 +1504,40 @@ static int
arm64_print_stackframe_entry(struct bt_info *bt, int level, struct arm64_stackframe
*frame, FILE *ofp)
{
char *name, *name_plus_offset;
- ulong symbol_offset;
+ ulong pc, symbol_offset;
struct syment *sp;
struct load_module *lm;
char buf[BUFSIZE];
- name = closest_symbol(frame->pc);
+ /*
+ * if pc comes from a saved lr, it actually points to an instruction
+ * after branch. To avoid any confusion, decrement pc by 4.
+ * See, for example, "bl schedule" before ret_to_user().
+ */
+ pc = frame->pc - 0x4;
+ name = closest_symbol(pc);
name_plus_offset = NULL;
if (bt->flags & BT_SYMBOL_OFFSET) {
- sp = value_search(frame->pc, &symbol_offset);
+ sp = value_search(pc, &symbol_offset);
if (sp && symbol_offset)
- name_plus_offset =
- value_to_symstr(frame->pc, buf, bt->radix);
+ name_plus_offset = value_to_symstr(pc, buf, bt->radix);
}
fprintf(ofp, "%s#%d [%8lx] %s at %lx", level < 10 ? " " :
"", level,
frame->fp ? frame->fp : bt->stacktop - USER_EFRAME_OFFSET,
- name_plus_offset ? name_plus_offset : name, frame->pc);
+ name_plus_offset ? name_plus_offset : name, pc);
if (BT_REFERENCE_CHECK(bt))
- arm64_do_bt_reference_check(bt, frame->pc, name);
+ arm64_do_bt_reference_check(bt, pc, name);
- if (module_symbol(frame->pc, NULL, &lm, NULL, 0))
+ if (module_symbol(pc, NULL, &lm, NULL, 0))
fprintf(ofp, " [%s]", lm->mod_name);
fprintf(ofp, "\n");
if (bt->flags & BT_LINE_NUMBERS) {
- get_line_number(frame->pc, buf, FALSE);
+ get_line_number(pc, buf, FALSE);
if (strlen(buf))
fprintf(ofp, " %s\n", buf);
}
--
2.9.0