bogus values of variables in userspace probes

[Michael Petlan]

Hi *,

I have been playing with perf-probe tool and I found out that some bogus
values of a function argument are obtained by perf-record.

How to reproduce:

gcc -O0 -g -o dummy dummy.c
perf probe -x ./dummy --add 'isprime a'
perf record -e probe_dummy:isprime ./dummy
perf script

The actual output looks like the following:

dummy 32476 [000] 3534401.838454: probe_dummy:isprime: (400530) a=32767
dummy 32476 [000] 3534401.838504: probe_dummy:isprime: (400530) a=32714
dummy 32476 [000] 3534401.838513: probe_dummy:isprime: (400530) a=3
dummy 32476 [000] 3534401.838519: probe_dummy:isprime: (400530) a=4
dummy 32476 [000] 3534401.838525: probe_dummy:isprime: (400530) a=5
dummy 32476 [000] 3534401.838531: probe_dummy:isprime: (400530) a=6
dummy 32476 [000] 3534401.838537: probe_dummy:isprime: (400530) a=7
dummy 32476 [000] 3534401.838543: probe_dummy:isprime: (400530) a=13
dummy 32476 [000] 3534401.838561: probe_dummy:isprime: (400530) a=17

But if you look into the source, you can see that the function isprime()
is called with the following arguments:

int numbers[] = { 2, 3, 4, 5, 6, 7, 13, 17, 19 };

So the first and last ones are omitted, there are some bogus numbers instead
of them and all that is shifted somehow.

Note that when I probe for %ax register it looks correct.

The version of kernel/perf is 4.3.0. The architecture is x86_64.
Am I missing something or is it a bug?

Thank you!
Michael


#include <stdlib.h>
#include <stdio.h>

int isprime(int a)
{
	int i;
	if(a <= 1)
		return 0;
	for(i = 2; i <= a / 2; i++)
		if(!(a % i))
			return 0;
	return 1;
}

int main(int argc, char **argv)
{
	int numbers[] = { 2, 3, 4, 5, 6, 7, 13, 17, 19 };
	int i;

	for(i = 0; i < 9; i++)
	{
		printf("%i %s prime\n", numbers[i], (isprime(numbers[i]))? "is" : "is not");
	}

	return 0;
}