selftests/bpf: Restore the netns after flow dissector reattach test

flow_dissector_reattach test changes the netns we run in but does not
restore it to the one we started in when finished. This interferes with
tests that run after it. Fix it by restoring the netns when done.

Fixes: f97eea17 ("selftests/bpf: Check that flow dissector can be re-attached")
Reported-by: NAlexei Starovoitov <ast@kernel.org>
Reported-by: NAndrii Nakryiko <andriin@fb.com>
Signed-off-by: NJakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>
Acked-by: NMartin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20191017083752.30999-1-jakub@cloudflare.com

tools/testing/selftests/bpf/prog_tests/flow_dissector_reattach.c

@@ -91,12 +91,18 @@ static void do_flow_dissector_reattach(void)
 
 void test_flow_dissector_reattach(void)
 {
-	int init_net, err;
+	int init_net, self_net, err;
+
+	self_net = open("/proc/self/ns/net", O_RDONLY);
+	if (CHECK_FAIL(self_net < 0)) {
+		perror("open(/proc/self/ns/net");
+		return;
+	}
 
 	init_net = open("/proc/1/ns/net", O_RDONLY);
 	if (CHECK_FAIL(init_net < 0)) {
 		perror("open(/proc/1/ns/net)");
-		return;
+		goto out_close;
 	}
 
 	err = setns(init_net, CLONE_NEWNET);
@@ -108,7 +114,7 @@ void test_flow_dissector_reattach(void)
 	if (is_attached(init_net)) {
 		test__skip();
 		printf("Can't test with flow dissector attached to init_net\n");
-		return;
+		goto out_setns;
 	}
 
 	/* First run tests in root network namespace */
@@ -118,10 +124,17 @@ void test_flow_dissector_reattach(void)
 	err = unshare(CLONE_NEWNET);
 	if (CHECK_FAIL(err)) {
 		perror("unshare(CLONE_NEWNET)");
-		goto out_close;
+		goto out_setns;
 	}
 	do_flow_dissector_reattach();
 
+out_setns:
+	/* Move back to netns we started in. */
+	err = setns(self_net, CLONE_NEWNET);
+	if (CHECK_FAIL(err))
+		perror("setns(/proc/self/ns/net)");
+
 out_close:
 	close(init_net);
+	close(self_net);
 }