[PATCH v5 3/4] arm64: correct a PC shown in bt

[AKASHI Takahiro <takahiro.akashi@xxxxxxxxxx>]

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@xxxxxxxxxx>
---
 arm64.c | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/arm64.c b/arm64.c
index b74d2c4..bad36aa 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

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