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Message-ID: <d53477c1-9c74-4578-8d74-281b95dee11e@arm.com>
Date: Sat, 12 Oct 2024 15:21:12 +0100
From: Leo Yan <leo.yan@....com>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>,
Namhyung Kim <namhyung@...nel.org>, Mark Rutland <mark.rutland@....com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
Adrian Hunter <adrian.hunter@...el.com>,
"Liang, Kan" <kan.liang@...ux.intel.com>, Dima Kogan <dima@...retsauce.net>,
james.clark@...aro.org, linux-perf-users@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 3/3] perf probe: Generate hash event for long symbol
On 10/12/24 06:30, Masami Hiramatsu (Google) wrote:
[...]
>> ---8<---
>>
>> From 3b09a6f89c7e383c6b1d2b7e6bd80c6bfa658d5b Mon Sep 17 00:00:00 2001
>> From: Leo Yan <leo.yan@....com>
>> Date: Fri, 11 Oct 2024 07:58:08 +0000
>> Subject: [PATCH] perf probe: Correct demangled symbols in C++ program
>>
>> An issue can be observed when probe C++ demangled symbol with steps:
>>
>> # nm test_cpp_mangle | grep print_data
>> 0000000000000c94 t _GLOBAL__sub_I__Z10print_datai
>> 0000000000000afc T _Z10print_datai
>> 0000000000000b38 T _Z10print_dataR5Point
>>
>> # ./perf probe -x /home/niayan01/test_cpp_mangle -F --demangle
>> ...
>> print_data(Point&)
>> print_data(int)
>> ...
>>
>> # ./perf --debug verbose=3 probe -x test_cpp_mangle --add "test=print_data(int)"
>> probe-definition(0): test=print_data(int)
>> symbol:print_data(int) file:(null) line:0 offset:0 return:0 lazy:(null)
>> 0 arguments
>> Open Debuginfo file: /home/niayan01/test_cpp_mangle
>> Try to find probe point from debuginfo.
>> Symbol print_data(int) address found : afc
>> Matched function: print_data [2ccf]
>> Probe point found: print_data+0
>> Found 1 probe_trace_events.
>> Opening /sys/kernel/tracing//uprobe_events write=1
>> Opening /sys/kernel/tracing//README write=0
>> Writing event: p:probe_test_cpp_mangle/test /home/niayan01/test_cpp_mangle:0xb38
>> ...
>>
>> When tried to probe symbol "print_data(int)", the log show:
>>
>> Symbol print_data(int) address found : afc
>>
>> The found address is 0xafc - which is right if we connect with the
>> output result from nm. Afterwards when write event, the command uses
>> offset 0xb38 in the last log, which is a wrong address.
>>
>> The dwarf_diename() gets a common function name, in above case, it
>> returns string "print_data". As a result, the tool parses the offset
>> based on the common name. This leads to probe at the wrong symbol
>> "print_data(Point&)".
>>
>> To fix the issue, use the die_get_linkage_name() function to retrieve
>> the distinct linkage name - this is the mangled name for the C++ case.
>> Based on this unique name, the tool can get a correct offset for
>> probing. Based on DWARF doc, it is possible the linkage name is missed
>> in the DIE, it rolls back to use dwarf_diename().
>
> Can you add the result after applying this patch here?
Sure. The result with this patch is:
# perf --debug verbose=3 probe -x test_cpp_mangle --add "test=print_data(int)"
probe-definition(0): test=print_data(int)
symbol:print_data(int) file:(null) line:0 offset:0 return:0 lazy:(null)
0 arguments
Open Debuginfo file: /home/niayan01/test_cpp_mangle
Try to find probe point from debuginfo.
Symbol print_data(int) address found : afc
Matched function: print_data [2d06]
Probe point found: print_data+0
Found 1 probe_trace_events.
Opening /sys/kernel/tracing//uprobe_events write=1
Opening /sys/kernel/tracing//README write=0
Writing event: p:probe_test_cpp_mangle/test /home/niayan01/test_cpp_mangle:0xafc
Added new event:
probe_test_cpp_mangle:test (on print_data(int) in /home/niayan01/test_cpp_mangle)
You can now use it in all perf tools, such as:
perf record -e probe_test_cpp_mangle:test -aR sleep 1
# perf --debug verbose=3 probe -x test_cpp_mangle --add "test2=print_data(Point&)"
probe-definition(0): test2=print_data(Point&)
symbol:print_data(Point&) file:(null) line:0 offset:0 return:0 lazy:(null)
0 arguments
Open Debuginfo file: /home/niayan01/test_cpp_mangle
Try to find probe point from debuginfo.
Symbol print_data(Point&) address found : b38
Matched function: print_data [2ccf]
Probe point found: print_data+0
Found 1 probe_trace_events.
Opening /sys/kernel/tracing//uprobe_events write=1
Parsing probe_events: p:probe_test_cpp_mangle/test /home/niayan01/test_cpp_mangle:0x0000000000000afc
Group:probe_test_cpp_mangle Event:test probe:p
Opening /sys/kernel/tracing//README write=0
Writing event: p:probe_test_cpp_mangle/test2 /home/niayan01/test_cpp_mangle:0xb38
Added new event:
probe_test_cpp_mangle:test2 (on print_data(Point&) in /home/niayan01/test_cpp_mangle)
You can now use it in all perf tools, such as:
perf record -e probe_test_cpp_mangle:test2 -aR sleep 1
I have sent out a formal patch (with fixed tag):
https://lore.kernel.org/linux-perf-users/20241012141432.877894-1-leo.yan@arm.com/T/#u
Thanks for review!
Leo
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