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

[Dave Anderson]

----- 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 at linaro.org>
> ---
>  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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