Em Tue, Nov 24, 2015 at 12:18:22PM +0100, Michael Petlan escreveu:
> 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?
I'm now trying to bisect that register error I found while trying to
reproduce the problem reported above, but please consider writing a
'perf test' entry that does exactly what you did, i.e. have a simple
program that you build with a known set of compile flags, then run it
using perf probe to catch a specific function, its argument, and then
check that the values it produces are the ones expected.
We would catch bugs in 'perf probe', the DWARF code, etc, much more
quickly with something like that in place. :-)
- Arnaldo
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