Re: [PATCH 1/3] perf probe: Set retprobe flag when probe in address-based alternative mode

He Kuang
2015-04-13T15:22:44Z

On 04/13/2015 10:39 PM, Arnaldo Carvalho de Melo wrote:
> Em Mon, Apr 13, 2015 at 07:41:28PM +0800, He Kuang escreveu:
>> Perf probe misses to set retprobe flag back when falling back to
>> address-based alternative mode.
>
> And when is that happens, can you explain? Because I tried to follow your
> instructions, but in my case it worked in the same way before and after your
> patch.
>
> Can you please provide more context for reviewing your patch?
>

More details:

[root]# perf probe -v -k vmlinux --add='sys_write%return'
probe-definition(0): sys_write%return
symbol:sys_write file:(null) line:0 offset:0 return:1 lazy:(null)
0 arguments
Use vmlinux: vmlinux
map_groups__set_modules_path_dir: cannot open /lib/modules/4.0.0-rc6+ dir
Problems setting modules path maps, continuing anyway...
Using vmlinux for symbols

Open Debuginfo file: vmlinux
>>>==========================

Try to find probe point from debuginfo.
Symbol sys_write address found : ffffffff811a8cf0
Probe point found: SyS_write+0
Found 1 probe_trace_events.
Opening /sys/kernel/debug/tracing/kprobe_events write=1
Added new event:
Writing event: p:probe/sys_write _stext+1739560
probe:sys_write (on sys_write%return)

You can now use it in all perf tools, such as:

perf record -e probe:sys_write -aR sleep 1

[root@buildroot tmp]# cat /sys/kernel/debug/tracing/kprobe_events
p:probe/sys_write _stext+1739560

The line I marked is different from your result, which is:
"""Could not open debuginfo. Try to use symbols."""

When perf probe searched in debuginfo and failed, it tried with
alternative, in function get_alternative_probe_event():

memcpy(tmp, &pev->point, sizeof(*tmp));
memset(&pev->point, 0, sizeof(pev->point));

In this case, it drops the retprobe flag and forgets to set it back in
find_alternative_probe_point(), so the problem occurred.

> Here are my results:
>
> Before:
>
> [root@ssdandy ~]# perf probe -v --add='sys_write%return'
> probe-definition(0): sys_write%return
> symbol:sys_write file:(null) line:0 offset:0 return:1 lazy:(null)
> 0 arguments
> Using /root/.debug/.build-id/dd/32e51921ede0fd46f034091b7f6a0f2e01ebda for symbols
> Could not open debuginfo. Try to use symbols.
> Opening /sys/kernel/debug/tracing/kprobe_events write=1
> Added new event:
> Writing event: r:probe/sys_write sys_write+0
> probe:sys_write (on sys_write%return)
>
> You can now use it in all perf tools, such as:
>
> perf record -e probe:sys_write -aR sleep 1
> [root@ssdandy ~]# cat /sys/kernel/debug/tracing/kprobe_events
> r:probe/sys_write sys_write
>
> Remove it:
>
> [root@ssdandy ~]# perf probe --del *:*
> Removed event: probe:sys_write
>
> After:
>
> [root@ssdandy ~]# perf probe -v --add='sys_write%return'
> probe-definition(0): sys_write%return
> symbol:sys_write file:(null) line:0 offset:0 return:1 lazy:(null)
> 0 arguments
> Using /root/.debug/.build-id/dd/32e51921ede0fd46f034091b7f6a0f2e01ebda for symbols
> Could not open debuginfo. Try to use symbols.
> Opening /sys/kernel/debug/tracing/kprobe_events write=1
> Added new event:
> Writing event: r:probe/sys_write sys_write+0
> probe:sys_write (on sys_write%return)
>
> You can now use it in all perf tools, such as:
>
> perf record -e probe:sys_write -aR sleep 1
>
> [root@ssdandy ~]#
> [root@ssdandy ~]# cat /sys/kernel/debug/tracing/kprobe_events
> r:probe/sys_write sys_write
>
> Humm, noticed one other problem, but not with your patch, about this message:
>
> "Could not open debuginfo. Try to use symbols."
>
> That is really not clear, specially that "try to use symbols" :-)
>
> [root@ssdandy ~]# ls -la /root/.debug/.build-id/dd/32e51921ede0fd46f034091b7f6a0f2e01ebda
> lrwxrwxrwx. 1 root root 86 Apr 10 18:02 /root/.debug/.build-id/dd/32e51921ede0fd46f034091b7f6a0f2e01ebda -> ../../home/acme/git/build/v4.0.0-rc6+/vmlinux/dd32e51921ede0fd46f034091b7f6a0f2e01ebda
> [root@ssdandy ~]# ls -la /root/.debug/.build-id/dd/../../home/acme/git/build/v4.0.0-rc6+/vmlinux/dd32e51921ede0fd46f034091b7f6a0f2e01ebda
> -rwxr-xr-x. 1 root root 22698661 Apr 10 18:02 /root/.debug/.build-id/dd/../../home/acme/git/build/v4.0.0-rc6+/vmlinux/dd32e51921ede0fd46f034091b7f6a0f2e01ebda
> [root@ssdandy ~]# ls -la /root/.debug/.build-id/dd/../../home/acme/git/build/v4.0.0-rc6+/vmlinux/dd32e51921ede0fd46f034091b7f6a0f2e01ebda
>
> I.e. it managed to read the debuginfo, its just that it has no symbols in it :-)
>
> Anyway, digression ended.
>
> - Arnaldo
>
>> Can be reproduced as following:
>>
>> $ perf probe -v -k vmlinux --add='sys_write%return'
>> ...
>> Added new event:
>> Writing event: p:probe/sys_write _stext+1584952
>> probe:sys_write (on sys_write%return)
>>
>> $ cat /sys/kernel/debug/tracing/kprobe_events
>> p:probe/sys_write _stext+1584952
>>
>> After this patch:
>>
>> $ perf probe -v -k vmlinux --add='sys_write%return'
>> Added new event:
>> Writing event: r:probe/sys_write SyS_write+0
>> probe:sys_write (on sys_write%return)
>>
>> $ cat /sys/kernel/debug/tracing/kprobe_events
>> r:probe/sys_write SyS_write
>>
>> Signed-off-by: He Kuang <hekuang@huawei.com>
>> ---
>> tools/perf/util/probe-event.c | 1 +
>> 1 file changed, 1 insertion(+)
>>
>> diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
>> index 30545ce..5483d98 100644
>> --- a/tools/perf/util/probe-event.c
>> +++ b/tools/perf/util/probe-event.c
>> @@ -332,6 +332,7 @@ static int find_alternative_probe_point(struct debuginfo *dinfo,
>> else {
>> result->offset += pp->offset;
>> result->line += pp->line;
>> + result->retprobe = pp->retprobe;
>> ret = 0;
>> }
>>
>> --
>> 2.3.3.220.g9ab698f
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