bpf: handle implicit declaration of function gettid in bpf_iter.c

commit 42602e3 upstream.

As we can see from the title, when I compiled the selftests/bpf, I
saw the error:
implicit declaration of function ‘gettid’ ; did you mean ‘getgid’? [-Werror=implicit-function-declaration]
  skel->bss->tid = gettid();
                   ^~~~~~
                   getgid

Directly call the syscall solves this issue.

Signed-off-by: Jason Xing <[email protected]>
Reviewed-by: Alan Maguire <[email protected]>
Tested-by: Alan Maguire <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Martin KaFai Lau <[email protected]>
Cc: Alan Maguire <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: JasonXing

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

@@ -226,7 +226,7 @@ static void test_task_common_nocheck(struct bpf_iter_attach_opts *opts,
 	ASSERT_OK(pthread_create(&thread_id, NULL, &do_nothing_wait, NULL),
 		  "pthread_create");
 
-	skel->bss->tid = gettid();
+	skel->bss->tid = syscall(SYS_gettid);
 
 	do_dummy_read_opts(skel->progs.dump_task, opts);
 
@@ -255,10 +255,10 @@ static void *run_test_task_tid(void *arg)
 	union bpf_iter_link_info linfo;
 	int num_unknown_tid, num_known_tid;
 
-	ASSERT_NEQ(getpid(), gettid(), "check_new_thread_id");
+	ASSERT_NEQ(getpid(), syscall(SYS_gettid), "check_new_thread_id");
 
 	memset(&linfo, 0, sizeof(linfo));
-	linfo.task.tid = gettid();
+	linfo.task.tid = syscall(SYS_gettid);
 	opts.link_info = &linfo;
 	opts.link_info_len = sizeof(linfo);
 	test_task_common(&opts, 0, 1);