From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michael Petlan Subject: Re: bogus values of variables in userspace probes Date: Tue, 24 Nov 2015 19:30:39 +0100 Message-ID: <1448389839.24573.21.camel@redhat.com> References: <1448363902.24573.18.camel@redhat.com> <20151124150819.GD18140@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Return-path: Received: from mx1.redhat.com ([209.132.183.28]:57040 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754461AbbKXSam (ORCPT ); Tue, 24 Nov 2015 13:30:42 -0500 In-Reply-To: <20151124150819.GD18140@kernel.org> Sender: linux-perf-users-owner@vger.kernel.org List-ID: To: Arnaldo Carvalho de Melo Cc: linux-perf-users@vger.kernel.org, Masami Hiramatsu , Jiri Olsa , Ingo Molnar , David Ahern , Wang Nan On Tue, 2015-11-24 at 12:08 -0300, Arnaldo Carvalho de Melo wrote: > 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. :-) Hi Arnaldo, thanks for investigating the issue. I do not use fedora kernel, I use what I build from git, but of course I am some versions back. So I use 4.3.0 and 4.3.0-rc5 and both do the same thing. I have met this when writing new tests for perf-probe into the testsuite I had been speaking about some time ago [1]. But if needed, I may add it as a perf-test entry as you wish. Michael > > - Arnaldo