selftests/harness: Fix fixture teardown

Make sure fixture teardowns are run when test cases failed, including
when _metadata->teardown_parent is set to true.

Make sure only one fixture teardown is run per test case, handling the
case where the test child forks.

Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Shengyu Li <shengyu.li.evgeny@gmail.com>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Fixes: 72d7cb5 ("selftests/harness: Prevent infinite loop due to Assert in FIXTURE_TEARDOWN")
Fixes: 0710a1a ("selftests/harness: Merge TEST_F_FORK() into TEST_F()")
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20240511171445.904356-4-mic@digikod.net
Rule: add
Link: https://lore.kernel.org/stable/20240506165518.474504-4-mic%40digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>

Author: l0kod

tools/testing/selftests/kselftest_harness.h

@@ -382,7 +382,10 @@
 		FIXTURE_DATA(fixture_name) self; \
 		pid_t child = 1; \
 		int status = 0; \
-		bool jmp = false; \
+		/* Makes sure there is only one teardown, even when child forks again. */ \
+		bool *teardown = mmap(NULL, sizeof(*teardown), \
+			PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0); \
+		*teardown = false; \
 		memset(&self, 0, sizeof(FIXTURE_DATA(fixture_name))); \
 		if (setjmp(_metadata->env) == 0) { \
 			/* Use the same _metadata. */ \
@@ -399,15 +402,16 @@
 				_metadata->exit_code = KSFT_FAIL; \
 			} \
 		} \
-		else \
-			jmp = true; \
 		if (child == 0) { \
-			if (_metadata->setup_completed && !_metadata->teardown_parent && !jmp) \
+			if (_metadata->setup_completed && !_metadata->teardown_parent && \
+					__sync_bool_compare_and_swap(teardown, false, true)) \
 				fixture_name##_teardown(_metadata, &self, variant->data); \
 			_exit(0); \
 		} \
-		if (_metadata->setup_completed && _metadata->teardown_parent) \
+		if (_metadata->setup_completed && _metadata->teardown_parent && \
+				__sync_bool_compare_and_swap(teardown, false, true)) \
 			fixture_name##_teardown(_metadata, &self, variant->data); \
+		munmap(teardown, sizeof(*teardown)); \
 		if (!WIFEXITED(status) && WIFSIGNALED(status)) \
 			/* Forward signal to __wait_for_test(). */ \
 			kill(getpid(), WTERMSIG(status)); \