----- Original Message -----
> 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().
I see...
>
> 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
That's probably OK -- and at least "bt -F" would clarify it somewhat.
Thanks,
Dave
>
> 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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>
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