Re: [Crash-utility] [PATCH v5 3/4] 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&gt; --- 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 -- Crash-utility mailing list Crash-utility(a)redhat.com https://www.redhat.com/mailman/listinfo/crash-utility