[PATCH v2] perf top: expand the range of multithreaded phase

[Hangliang Lai]

In __cmd_top, perf_set_multithreaded is used to enable pthread_rwlock, thus
donw_read and down_write can work to handle concurrency problems. Then top
use perf_set_singlethreaded and switch to single threaded phase, assuming
that no thread concurrency will happen later.
However, a UAF problem could occur in perf top in single threaded phase,
The concurrent procedure is like this:
display_thread                              process_thread
--------------                              --------------
thread__comm_len
  -> thread__comm_str
    -> __thread__comm_str(thread)
                                            thread__delete
                                             -> comm__free
                                              -> comm_str__put
                                               -> zfree(&cs->str)
    -> thread->comm_len = strlen(comm);
Since in single thread phase, perf_singlethreaded is true, down_read and
down_write can not work to avoid concurrency problems.
This patch put perf_set_singlethreaded to the function tail to expand the
multithreaded phase range, make display_thread and process_thread run
safe.

Signed-off-by: Hangliang Lai  <laihangliang1@xxxxxxxxxx>
Reviewed-by: Yunfeng Ye <yeyunfeng@xxxxxxxxxx>
---
v1 -> v2
 - Since perf top is always multi-threaded, remove top->nr_threads_synthesize judgment.

 tools/perf/builtin-top.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
index d4b5b02bab73..a18db1ee87fa 100644
--- a/tools/perf/builtin-top.c
+++ b/tools/perf/builtin-top.c
@@ -1242,8 +1242,7 @@ static int __cmd_top(struct perf_top *top)
 	if (perf_session__register_idle_thread(top->session) < 0)
 		return ret;
 
-	if (top->nr_threads_synthesize > 1)
-		perf_set_multithreaded();
+	perf_set_multithreaded();
 
 	init_process_thread(top);
 
@@ -1273,9 +1272,6 @@ static int __cmd_top(struct perf_top *top)
 				    top->evlist->core.threads, true, false,
 				    top->nr_threads_synthesize);
 
-	if (top->nr_threads_synthesize > 1)
-		perf_set_singlethreaded();
-
 	if (perf_hpp_list.socket) {
 		ret = perf_env__read_cpu_topology_map(&perf_env);
 		if (ret < 0) {
@@ -1352,6 +1348,9 @@ static int __cmd_top(struct perf_top *top)
 out_join_thread:
 	cond_signal(&top->qe.cond);
 	pthread_join(thread_process, NULL);
+
+	perf_set_singlethreaded();
+
 	return ret;
 }
 
-- 
2.33.0