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

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Date:   Fri, 23 Nov 2018 17:03:56 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Eric Saint-Etienne <eric.saint.etienne@...cle.com>
Cc:     Linux Kernel <linux-kernel@...r.kernel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Darren Kenny <darren.kenny@...cle.com>,
        Eric Saint-Etienne <eric.saintetienne@...il.com>
Subject: Re: [PATCH] perf symbols: Cannot disassemble some routines when
 debuginfo present

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@...cle.com>
> Reviewed-by: Darren Kenny <darren.kenny@...cle.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
>