linux-kernel - perf script, libperf: python binding bug (bytearrays vs. strings)

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Date:   Sun, 27 Sep 2020 09:43:12 +0200
From:   Hagen Paul Pfeifer <hagen@...u.net>
To:     linux-kernel@...r.kernel.org
Cc:     Jiri Olsa <jolsa@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: perf script, libperf: python binding bug (bytearrays vs. strings)

Hallo Jiri, Arnaldo,

after updating Debian (probably with the advent of Python 3.8.5, guessing)
I get a wired behavior with python scripting. The error is that the python type
for prev_comm and next_comm are not strings anymore, rather bytearrays. Which
are incompatible types and scripts will not work anymore. NOTE: common_comm is
still fine (see swapper & mutex-thread-co), so they must be treated internal
differently compared to prev_comm and next_comm and possibly show a way to solve
this problem!

After bisecting the kernel (perf) even back to v5.6 the problem still exist.
Compiling perf with PYTHON=python2 do not show any problems - no problems in
the Python2 world. So I assume with Python 3.8.5 (or other helper library)
something changed internally. I assume the cause exists in perf forever but
is now triggered with the new Python3 version.

How to reproduce:

make PYTHON=python3
./perf record -e sched:sched_switch -a -- sleep 5
./perf script --gen-script py
./perf script -s ./perf-script.py

[..]
sched__sched_switch      7 563231.759525792        0 swapper              prev_comm=bytearray(b'swapper/7\x00\x00\x00\x00\x00\x00\x00'), prev_pid=0, prev_prio=120, prev_state=, next_comm=bytearray(b'mutex-thread-co\x00'), next_pid=3447985, next_prio=120
Sample: {addr=0, cpu=7, datasrc=84410401, datasrc_decode=N/A|SNP N/A|TLB N/A|LCK N/A, ip=18446744072189289569, period=1, phys_addr=0, pid=0, tid=0, time=563231759525792, transaction=0, values=[(0, 0)], weight=0}

sched__sched_switch      7 563231.759582596  3447985 mutex-thread-co      prev_comm=bytearray(b'mutex-thread-co\x00'), prev_pid=3447985, prev_prio=120, prev_state=, next_comm=bytearray(b'swapper/7\x00\x00\x00\x00\x00\x00\x00'), next_pid=0, next_prio=120
Sample: {addr=0, cpu=7, datasrc=84410401, datasrc_decode=N/A|SNP N/A|TLB N/A|LCK N/A, ip=18446744072189289569, period=1, phys_addr=0, pid=3447983, tid=3447985, time=563231759582596, transaction=0, values=[(0, 0)], weight=0}

See =bytearray(b'swapper/7\x00\x00\x00\x00\x00\x00\x00') - should be swapper/7

Note: the byte array has the length of 16 - exactly like the kernel
(TASK_COMM_LEN). I assume this is somehow copied directly into the variables
and not stringified anymore.

Even worse: I discovered bytearrays which are not correctly "memseted":
	bytearray(b'chrome\x00sandbox\x00\x00')

chrome should be the comm name, but is 'chromesandbox' somehow. See the null
bytes in between.

Jiri, Arnaldo - I tried to fix this. But the Python binding magic for the
automatically generated events are hard to get comfy.

Hagen

PS: assume this fix is also kernel stable relevant.