[PATCH v3 2/3] perf: make build_cpu_topology skip offline/absent CPUs

[Jan Stancek]

When build_cpu_topo() encounters offline/absent CPUs,
it fails to find any sysfs entries and returns failure.
This leads to build_cpu_topology() and write_cpu_topology()
failing as well.

Because HEADER_CPU_TOPOLOGY has not been written, read leaves
cpu_topology_map NULL and we get NULL ptr deref at:

 ...
  cmd_test
   __cmd_test
    test_and_print
     run_test
      test_session_topology
       check_cpu_topology

36: Session topology                           :
--- start ---
test child forked, pid 14902
templ file: /tmp/perf-test-4CKocW
failed to write feature HEADER_CPU_TOPOLOGY
perf: Segmentation fault
Obtained 9 stack frames.
./perf(sighandler_dump_stack+0x41) [0x5095f1]
/lib64/libc.so.6(+0x35250) [0x7f4b7c3c9250]
./perf(test_session_topology+0x1db) [0x490ceb]
./perf() [0x475b68]
./perf(cmd_test+0x5b9) [0x4763c9]
./perf() [0x4945a3]
./perf(main+0x69f) [0x427e8f]
/lib64/libc.so.6(__libc_start_main+0xf5) [0x7f4b7c3b5b35]
./perf() [0x427fb9]
test child interrupted
---- end ----
Session topology: FAILED!

This patch makes build_cpu_topology() skip offline/absent CPUs,
by checking their presence against cpu_map built from online CPUs.

Signed-off-by: Jan Stancek <jstancek@xxxxxxxxxx>
---
 tools/perf/util/header.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

Changes in v3:
- make ret = -1 by default, so we fail in case build_cpu_topo
  doesn't run at all

diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
index d89c9c7ef4e5..ca0f12fb5fd3 100644
--- a/tools/perf/util/header.c
+++ b/tools/perf/util/header.c
@@ -503,24 +503,31 @@ static void free_cpu_topo(struct cpu_topo *tp)
 
 static struct cpu_topo *build_cpu_topology(void)
 {
-	struct cpu_topo *tp;
+	struct cpu_topo *tp = NULL;
 	void *addr;
 	u32 nr, i;
 	size_t sz;
 	long ncpus;
 	int ret = -1;
+	struct cpu_map *map;
 
 	ncpus = sysconf(_SC_NPROCESSORS_CONF);
 	if (ncpus < 0)
 		return NULL;
 
+	/* build online CPU map */
+	map = cpu_map__new(NULL);
+	if (map == NULL) {
+		pr_debug("failed to get system cpumap\n");
+		return NULL;
+	}
+
 	nr = (u32)(ncpus & UINT_MAX);
 
 	sz = nr * sizeof(char *);
-
 	addr = calloc(1, sizeof(*tp) + 2 * sz);
 	if (!addr)
-		return NULL;
+		goto out_free;
 
 	tp = addr;
 	tp->cpu_nr = nr;
@@ -530,10 +537,16 @@ static struct cpu_topo *build_cpu_topology(void)
 	tp->thread_siblings = addr;
 
 	for (i = 0; i < nr; i++) {
+		if (!cpu_map__has(map, i))
+			continue;
+
 		ret = build_cpu_topo(tp, i);
 		if (ret < 0)
 			break;
 	}
+
+out_free:
+	cpu_map__put(map);
 	if (ret) {
 		free_cpu_topo(tp);
 		tp = NULL;
-- 
1.8.3.1