selftests/bpf: Restore original comm in test_overhead

test_overhead changes task comm in order to estimate BPF trampoline
overhead but never sets the comm back to the original one.
We have the tests (like core_reloc.c) that have 'test_progs'
as hard-coded expected comm, so let's try to preserve the
original comm.

Currently, everything works because the order of execution is:
first core_recloc, then test_overhead; but let's make it a bit
future-proof.

Other related changes: use 'test_overhead' as new comm instead of
'test' to make it easy to debug and drop '\n' at the end.
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Petar Penkov <ppenkov@google.com>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200108192132.189221-1-sdf@google.com

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

@@ -2,6 +2,7 @@
 /* Copyright (c) 2019 Facebook */
 #define _GNU_SOURCE
 #include <sched.h>
+#include <sys/prctl.h>
 #include <test_progs.h>
 
 #define MAX_CNT 100000
@@ -17,7 +18,7 @@ static __u64 time_get_ns(void)
 static int test_task_rename(const char *prog)
 {
 	int i, fd, duration = 0, err;
-	char buf[] = "test\n";
+	char buf[] = "test_overhead";
 	__u64 start_time;
 
 	fd = open("/proc/self/comm", O_WRONLY|O_TRUNC);
@@ -66,6 +67,10 @@ void test_test_overhead(void)
 	struct bpf_object *obj;
 	struct bpf_link *link;
 	int err, duration = 0;
+	char comm[16] = {};
+
+	if (CHECK_FAIL(prctl(PR_GET_NAME, comm, 0L, 0L, 0L)))
+		return;
 
 	obj = bpf_object__open_file("./test_overhead.o", NULL);
 	if (CHECK(IS_ERR(obj), "obj_open_file", "err %ld\n", PTR_ERR(obj)))
@@ -138,5 +143,6 @@ void test_test_overhead(void)
 	test_run("fexit");
 	bpf_link__destroy(link);
 cleanup:
+	prctl(PR_SET_NAME, comm, 0L, 0L, 0L);
 	bpf_object__close(obj);
 }