perf tools: Fix MMAP event synthesis broken by MAP_HUGETLB change

Patch "perf record: Mark MAP_HUGETLB when synthesizing mmap events") breaks
MMAP event synthesis.  The executable name comparison will match any name
if the length is zero, resulting in all the user space maps becoming
anonymous.  This is particularly noticeable with system-wide traces.
Example:

	perf record -a sleep 1
	perf script --show-mmap-events

Committer note:

That is not the case when, say, one has a qemu instance and libvirt actually
mounts hugetlbfs. To test this I had to first umount it:

[root@jouet ~]# mount | grep hugetlbfs
hugetlbfs on /dev/hugepages type hugetlbfs (rw,relatime,seclabel)
[root@jouet ~]#

After unmount it the error fixed by this patch manifests itself:

  # perf record -a sleep 1
  # perf script --show-mmap-events | grep PERF_RECORD_MMAP2 | head -5
    systemd 0 [000] 0.000000: PERF_RECORD_MMAP2 1/1: [0x557d47ed8000(0x167000) @ 0 fd:00 3146896 7362875424355726126]: r-xp //anon
    systemd 0 [000] 0.000000: PERF_RECORD_MMAP2 1/1: [0x7f96c488d000(0x4000) @ 0 fd:00 3153214 7362875424355726126]: r-xp //anon
    systemd 0 [000] 0.000000: PERF_RECORD_MMAP2 1/1: [0x7f96c4a92000(0x3d000) @ 0 fd:00 3159276 7362875424355726126]: r-xp //anon
    systemd 0 [000] 0.000000: PERF_RECORD_MMAP2 1/1: [0x7f96c4cd5000(0x15000) @ 0 fd:00 3153725 7362875424355726126]: r-xp //anon
    systemd 0 [000] 0.000000: PERF_RECORD_MMAP2 1/1: [0x7f96c4eeb000(0x25000) @ 0 fd:00 3153260 7362875424355726126]: r-xp //anon
  #

Fixed version:

  # perf record -a sleep 1
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 1.419 MB perf.data (182 samples) ]
  # perf script --show-mmap-events | grep PERF_RECORD_MMAP2 | head -5
    systemd 0 [000] 0.000000: PERF_RECORD_MMAP2 1/1: [0x557d47ed8000(0x167000) @ 0 fd:00 3146896 7362875424355726126]: r-xp /usr/lib/systemd/systemd
    systemd 0 [000] 0.000000: PERF_RECORD_MMAP2 1/1: [0x7f96c488d000(0x4000) @ 0 fd:00 3153214 7362875424355726126]: r-xp /usr/lib64/libuuid.so.1.3.0
    systemd 0 [000] 0.000000: PERF_RECORD_MMAP2 1/1: [0x7f96c4a92000(0x3d000) @ 0 fd:00 3159276 7362875424355726126]: r-xp /usr/lib64/libblkid.so.1.1.0
    systemd 0 [000] 0.000000: PERF_RECORD_MMAP2 1/1: [0x7f96c4cd5000(0x15000) @ 0 fd:00 3153725 7362875424355726126]: r-xp /usr/lib64/libz.so.1.2.8
    systemd 0 [000] 0.000000: PERF_RECORD_MMAP2 1/1: [0x7f96c4eeb000(0x25000) @ 0 fd:00 3153260 7362875424355726126]: r-xp /usr/lib64/liblzma.so.5.2.2
[root@jouet ~]#
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: http://lkml.kernel.org/r/1474641528-18776-3-git-send-email-adrian.hunter@intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/event.c

@@ -346,7 +346,8 @@ int perf_event__synthesize_mmap_events(struct perf_tool *tool,
 		if (!strcmp(execname, ""))
 			strcpy(execname, anonstr);
 
-		if (!strncmp(execname, hugetlbfs_mnt, hugetlbfs_mnt_len)) {
+		if (hugetlbfs_mnt_len &&
+		    !strncmp(execname, hugetlbfs_mnt, hugetlbfs_mnt_len)) {
 			strcpy(execname, anonstr);
 			event->mmap2.flags |= MAP_HUGETLB;
 		}