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

[David Ahern <dsahern@gmail.com>]

(This time with actual backtrace; forgot to remove the leading '#' on the frames.)

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
2  0x0000000000444642 in perf_event__process_mmap (tool=0x7fffffffdd80, event=0x7ffff7fb4120, sample=0x7fffffffd210, machine=0x0)
    at util/event.c:668
3  0x0000000000470e0b in perf_session_deliver_event (session=0x915ca0, event=0x7ffff7fb4120, sample=0x7fffffffd210, tool=0x7fffffffdd80,
    file_offset=8480) at util/session.c:979
4  0x000000000047032e in flush_sample_queue (s=0x915ca0, tool=0x7fffffffdd80) at util/session.c:679
5  0x0000000000471c8d in __perf_session__process_events (session=0x915ca0, data_offset=400, data_size=150448, file_size=150848, tool=
    0x7fffffffdd80) at util/session.c:1363
6  0x0000000000471d42 in perf_session__process_events (self=0x915ca0, tool=0x7fffffffdd80) at util/session.c:1379
7  0x000000000042484a in __cmd_report (rep=0x7fffffffdd80) at builtin-report.c:368
8  0x0000000000425bf1 in cmd_report (argc=0, argv=0x915b00, prefix=0x0) at builtin-report.c:756
9  0x0000000000438505 in __cmd_report (argc=4, argv=0x7fffffffe260) at builtin-kvm.c:84
10 0x000000000043882a in cmd_kvm (argc=4, argv=0x7fffffffe260, prefix=0x0) at builtin-kvm.c:131
11 0x00000000004152cd in run_builtin (p=0x7a54e8, argc=9, argv=0x7fffffffe260) at perf.c:273
12 0x00000000004154c7 in handle_internal_command (argc=9, argv=0x7fffffffe260) at perf.c:345
13 0x0000000000415613 in run_argv (argcp=0x7fffffffe14c, argv=0x7fffffffe140) at perf.c:389
14 0x0000000000415899 in main (argc=9, argv=0x7fffffffe260) at perf.c:487

If no samples hit within a guest module then no build id events are written
to the header during the record. On the report side this cases no 'machine'
to be generated for the default guest (pid == 0) leading to a
machine == NULL in perf_session_deliver_event.

perf's handling of default kallsyms and guest-modules needs to be reworked.
Until then perf should at least not segfault.

Reported-by: Pradeep Kumar Surisetty <psuriset@linux.vnet.ibm.com>
CC: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: David Ahern <dsahern@gmail.com>

Signed-off-by: David Ahern <dsahern@gmail.com>
---
 tools/perf/util/session.c |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
index c3e399b..3999d5e 100644
--- a/tools/perf/util/session.c
+++ b/tools/perf/util/session.c
@@ -976,6 +976,8 @@ static int perf_session_deliver_event(struct perf_session *session,
 		}
 		return tool->sample(tool, event, sample, evsel, machine);
 	case PERF_RECORD_MMAP:
+		if (machine == NULL)
+			return 0;
 		return tool->mmap(tool, event, sample, machine);
 	case PERF_RECORD_COMM:
 		return tool->comm(tool, event, sample, machine);
-- 
1.7.10.1