Re: [PATCH] perf kvm: fix segfault with default guest arguments

[David Ahern <dsahern@gmail.com>]

On 6/28/12 8:44 AM, Arnaldo Carvalho de Melo wrote:
> Em Sun, Jun 24, 2012 at 10:41:26PM -0600, David Ahern escreveu:
>> File generated with:
>> $ perf kvm --host --guest --guestkallsyms=/tmp/guest-kallsyms
>>      --guestmodules=/tmp/guest-modules record -a
>>
>> The report command:
>> $ perf kvm --host --guest --guestkallsyms=/tmp/guest-kallsyms
>>      --guestmodules=/tmp/guest-modules report
>>
>> dies with a SEGFAULT:
>> Program received signal SIGSEGV, Segmentation fault.
>> 0x000000000046dd7b in machine__mmap_name (self=0x0, bf=0x7fffffffbd20 "q\021", size=4096) at util/map.c:715
>> 715			snprintf(bf, size, "[%s.%d]", "guest.kernel.kallsyms", self->pid);
>>
>> (gdb) bt
>> 0  0x000000000046dd7b in machine__mmap_name (self=0x0, bf=0x7fffffffbd20 "q\021", size=4096) at util/map.c:715
>> 1  0x0000000000444161 in perf_event__process_kernel_mmap (tool=0x7fffffffdd80, event=0x7ffff7fb4120, machine=0x0) at util/event.c:562
>

So this is the root cause of the SEGFAULT:

743eb868657bdb1b26c7b24077ca21c67c82c777 is the first bad commit
commit 743eb868657bdb1b26c7b24077ca21c67c82c777
Author: Arnaldo Carvalho de Melo <acme@redhat.com>
Date:   Mon Nov 28 07:56:39 2011 -0200

     perf tools: Resolve machine earlier and pass it to perf_event_ops

     Reducing the exposure of perf_session further, so that we can use the
     classes in cases where no perf.data file is created.

     Cc: David Ahern <dsahern@gmail.com>
     Cc: Frederic Weisbecker <fweisbec@gmail.com>
     Cc: Mike Galbraith <efault@gmx.de>
     Cc: Paul Mackerras <paulus@samba.org>
     Cc: Peter Zijlstra <peterz@infradead.org>
     Cc: Stephane Eranian <eranian@google.com>
     Link: 
http://lkml.kernel.org/n/tip-stua66dcscsezzrcdugvbmvd@git.kernel.org
     Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

:040000 040000 414f7e982a230f633440b3da2900da4d2d9c30ef
3231e148910bd596c61814e18bba77ecf7ad5dfb M  tools


specifically, this part:

-   machine = perf_session__findnew_machine(session, event->mmap.pid);
-   if (!machine) {
-       pr_err("Can't find id %d's machine\n", event->mmap.pid);
-       goto out_problem;
-   }
-

Suggestions on how to fix that part?

More to the perf-kvm story; details in future emails.

David