Re: [PATCH/RFC 4/9] perf symbols: Load kernel module symbols ASAP

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Fri, Jun 23, 2017 at 02:48:22PM +0900, Namhyung Kim escreveu:
> When loading kernel symbols from /proc/kallsyms, it might have different
> addresses for modules.  We should honor the mmap event recorded in a
> perf.data so load the module symbols when it sees the event so that it
> cannot be overridden by symbols in /proc/kallsyms later.
 
> Cc: Adrian Hunter <adrian.hunter@intel.com>
> Cc: Wang Nan <wangnan0@huawei.com>
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> ---
>  tools/perf/util/machine.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
> index d838f893e639..799efe920f0c 100644
> --- a/tools/perf/util/machine.c
> +++ b/tools/perf/util/machine.c
> @@ -656,6 +656,9 @@ struct map *machine__findnew_module_map(struct machine *machine, u64 start,
>  
>  	map_groups__insert(&machine->kmaps, map);
>  
> +	if (dso->has_build_id)
> +		map__load(map);

How could this be set at this point? I just tried processing a
PERF_RECORD_MMAP event for a kernel module and at this point it is set
to false :-\

Humm, but I did it for video.ko, that has no hits in this case, so this
must be coming from reading the perf.data build-id table... /me tries
that...  yeah, when processing the buildid table in perf.data we
populate the machine->dsos list and set the dso->has_build_id flag:

[root@jouet ~]# perf buildid-list | grep .ko
57a47c2f0d85ef5726b80e8f55403c3f508a4eac /lib/modules/4.12.0-rc4+/kernel/drivers/net/wireless/intel/iwlwifi/iwlwifi.ko
[root@jouet ~]# 
(gdb) bt
#0  machine__findnew_module_map (machine=0x21c6c78, start=18446744072647282688, filename=0x7fd8fc057a00 "/lib/modules/4.12.0-rc4+/kernel/drivers/net/wireless/intel/iwlwifi/iwlwifi.ko")
    at util/machine.c:650
#1  0x00000000005094a3 in machine__process_kernel_mmap_event (machine=0x21c6c78, event=0x7fd8fc0579d8) at util/machine.c:1274
#2  0x00000000005099de in machine__process_mmap_event (machine=0x21c6c78, event=0x7fd8fc0579d8, sample=0x7fffffffbef0) at util/machine.c:1433
#3  0x00000000004cc021 in perf_event__process_mmap (tool=0x7fffffffc440, event=0x7fd8fc0579d8, sample=0x7fffffffbef0, machine=0x21c6c78) at util/event.c:1265
#4  0x0000000000512686 in machines__deliver_event (machines=0x21c6c78, evlist=0x21c6f30, event=0x7fd8fc0579d8, sample=0x7fffffffbef0, tool=0x7fffffffc440, file_offset=10712)
    at util/session.c:1250
#5  0x00000000005129ce in perf_session__deliver_event (session=0x21c6b90, event=0x7fd8fc0579d8, sample=0x7fffffffbef0, tool=0x7fffffffc440, file_offset=10712) at util/session.c:1310
#6  0x0000000000513255 in perf_session__process_event (session=0x21c6b90, event=0x7fd8fc0579d8, file_offset=10712) at util/session.c:1490
#7  0x0000000000513ff4 in __perf_session__process_events (session=0x21c6b90, data_offset=232, data_size=23120, file_size=23352) at util/session.c:1867
#8  0x00000000005141f2 in perf_session__process_events (session=0x21c6b90) at util/session.c:1921
#9  0x00000000004427c9 in __cmd_report (rep=0x7fffffffc440) at builtin-report.c:575
#10 0x000000000044427c in cmd_report (argc=0, argv=0x7fffffffe160) at builtin-report.c:1054
#11 0x00000000004bad91 in run_builtin (p=0xa28fd0 <commands+240>, argc=2, argv=0x7fffffffe160) at perf.c:296
#12 0x00000000004baffe in handle_internal_command (argc=2, argv=0x7fffffffe160) at perf.c:348
#13 0x00000000004bb150 in run_argv (argcp=0x7fffffffdfbc, argv=0x7fffffffdfb0) at perf.c:392
#14 0x00000000004bb52a in main (argc=2, argv=0x7fffffffe160) at perf.c:530
(gdb) p dso
$127 = (struct dso *) 0x21c7dc0
(gdb) p dso->has_build_id
$128 = 1 '\001'
(gdb) 

So what you're doing is to load it here... perhaps we should not do
that, and instead in the kallsyms loading code, when processing the
symbols for this specific module, discard them and leave its symbol
table unpopulated, then, later, when resolving a sample in this module,
it will see that it is not loaded, will see that it has a build-id and
will use that, and _refuse_ to get it from anywhere else than an ELF
file with that build-id.

I.e. in general, when we have a build-id for a given DSO, we should not
read it from kallsyms or any other file that don't have a matching
build-id.

The only possibility would be: hey, I have a build id, and reading the
currently loaded module (/sys/module/e1000e/notes/.note.gnu.build-id) I
see it _still_ has that same build-id, ok, I can read it from kallsyms.

What do you think?

- Arnaldo


> +
>  	/* Put the map here because map_groups__insert alread got it */
>  	map__put(map);
>  out:
> -- 
> 2.13.1