selftests: harness: Detect illegal mixing of kselftest and harness functionality

Users may accidentally use the kselftest_test_result_*() functions in
their harness tests. If ksft_finished() is not used, the results
reported in this way are silently ignored.

Detect such false-positive cases and fail the test.

A more correct test would be to reject *any* usage of the ksft APIs but
that would force code churn on users.

Correct usages, which do use ksft_finished() will not trigger this
validation as the test will exit before it.

Reported-by: Yuwen Chen <ywen.chen@foxmail.com>
Link: https://lore.kernel.org/lkml/tencent_56D79AF3D23CEFAF882E83A2196EC1F12107@qq.com/
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Link: https://lore.kernel.org/r/20260302-kselftest-harness-v2-4-3143aa41d989@linutronix.de
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Author: t-8ch

tools/testing/selftests/kselftest_harness.h

@@ -1222,7 +1222,16 @@ static void __run_test(struct __fixture_metadata *f,
 		t->exit_code = KSFT_FAIL;
 	} else if (child == 0) {
 		setpgrp();
+
+		/* Reset state inherited from the harness */
+		ksft_reset_state();
+
 		t->fn(t, variant);
+
+		if (__test_passed(t) && (ksft_get_fail_cnt() || ksft_get_error_cnt())) {
+			ksft_print_msg("Illegal usage of low-level ksft APIs in harness test\n");
+			t->exit_code = KSFT_FAIL;
+		}
 		_exit(t->exit_code);
 	} else {
 		t->pid = child;