[PATCH] 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>
Reported-by: Wenyu Liu <liuwenyu7@xxxxxxxxxx>
Reviewed-by: Yunfeng Ye <yeyunfeng@xxxxxxxxxx>
---
 tools/perf/builtin-top.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
index 7c6413447..74239940b 100644
--- a/tools/perf/builtin-top.c
+++ b/tools/perf/builtin-top.c
@@ -1280,9 +1280,6 @@ static int __cmd_top(struct perf_top *top)
 				    top->evlist->core.threads, 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) {
@@ -1359,6 +1356,10 @@ out_join:
 out_join_thread:
 	pthread_cond_signal(&top->qe.cond);
 	pthread_join(thread_process, NULL);
+
+	if (top->nr_threads_synthesize > 1)
+		perf_set_singlethreaded();
+
 	return ret;
 }
 
-- 
2.33.0