Re: perf script, libperf: python binding bug (bytearrays vs. strings)

[Jiri Olsa]

On Sun, Sep 27, 2020 at 09:43:12AM +0200, Hagen Paul Pfeifer wrote:
> 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.
> 

patch below fixes it for me, but seems strange this was
working till now.. maybe you're the only one using this
with python3 ;-)

jirka


---
diff --git a/tools/perf/util/print_binary.c b/tools/perf/util/print_binary.c
index 599a1543871d..13fdc51c61d9 100644
--- a/tools/perf/util/print_binary.c
+++ b/tools/perf/util/print_binary.c
@@ -50,7 +50,7 @@ int is_printable_array(char *p, unsigned int len)
 
 	len--;
 
-	for (i = 0; i < len; i++) {
+	for (i = 0; i < len && p[i]; i++) {
 		if (!isprint(p[i]) && !isspace(p[i]))
 			return 0;
 	}