Re: [PATCH] perf symbols: Cannot disassemble some routines when debuginfo present

Jiri Olsa
2018-11-23T16:04:02Z

On Fri, Nov 23, 2018 at 02:25:26AM -0800, Eric Saint-Etienne wrote:
> When the kernel is compiled with -ffunction-sections and perf uses the
> kernel debuginfo, perf fails the very first symbol lookup and ends up with
> an hex offset inside [kernel.vmlinux]. It's due to how perf loads the maps.
>
> Indeed only .text gets loaded by map_groups__find() into al->map.
> Consequently al->map address range encompass the whole code.
> But map__load() has just loaded many function maps by splitting al->map,
> which reduced al->map range drastically. Very likely the target address is
> now in one of those newly created function maps, so we need to lookup the
> map again to find that new map.
>
> This issue is not specific to the kernel but to how the image is linked.
> For the kernel, when we're not using the kernel debuginfo, perf will
> fallback to using kallsyms and then the first lookup will work.
>
> This patch makes sure that the event address we're looking-up is indeed
> within the map we've found, otherwise we lookup another map again.
> Only one extra lookup at most is required for the proper map to be found,
> if it exists.
>
> Signed-off-by: Eric Saint-Etienne <eric.saint.etienne@oracle.com>
> Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
> ---
> tools/perf/util/event.c | 23 ++++++++++++++++++++++-
> 1 file changed, 22 insertions(+), 1 deletion(-)
>
> diff --git a/tools/perf/util/event.c b/tools/perf/util/event.c
> index e9c108a..a69ef52 100644
> --- a/tools/perf/util/event.c
> +++ b/tools/perf/util/event.c
> @@ -1571,7 +1571,28 @@ struct map *thread__find_map(struct thread *thread, u8 cpumode, u64 addr,
> */
> if (load_map)
> map__load(al->map);
> - al->addr = al->map->map_ip(al->map, al->addr);
> +
> + /*
> + * When using -ffunction-sections, only .text gets loaded by
> + * map_groups__find() into al->map. Consequently al->map address
> + * range encompass the whole code.
> + *
> + * But map__load() has just loaded many function maps by
> + * splitting al->map, which reduced al->map range drastically.
> + * Very likely the target address is now in one of those newly
> + * created function maps, so we need to lookup the map again
> + * to find that new map.
> + */

hum, so map__load actualy can split the map to create new maps?

cold you please point me to that code? I haven't touch
this area for some time and I can't find it

thanks,
jirka

> + if (al->addr < al->map->start || al->addr >= al->map->end)
> + al->map = map_groups__find(mg, al->addr);
> +
> + /*
> + * The new map *ought* to exist because the initial al->map
> + * contained that address and subsequently has been split into
> + * many *contiguous* maps.
> + */
> + if (al->map != NULL)
> + al->addr = al->map->map_ip(al->map, al->addr);
> }
>
> return al->map;
> --
> 1.8.3.1
>