test: verify fdinfo for pidfd of reaped process

Test that the fdinfo field of a pidfd referring to a dead process
correctly shows Pid: -1 and NSpid: -1.

Cc: Christian Kellner <[e-mail]>
Cc: [e-mail]
Reviewed-by: Christian Kellner <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

tools/testing/selftests/pidfd/pidfd_fdinfo_test.c

···
 	return ret;
 }
 
+static inline void child_close(struct child *child)
+{
+	close(child->fd);
+}
+
 static inline int child_join(struct child *child, struct error *err)
 {
 	int r;
 
-	(void)close(child->fd);
 	r = wait_for_pid(child->pid);
 	if (r < 0)
 		error_set(err, PIDFD_ERROR, "waitpid failed (ret %d, errno %d)",
···
 	return r;
 }
 
+static inline int child_join_close(struct child *child, struct error *err)
+{
+	child_close(child);
+	return child_join(child, err);
+}
+
 static inline void trim_newline(char *str)
 {
 	char *pos = strrchr(str, '\n');
···
 		*pos = '\0';
 }
 
-static int verify_fdinfo_nspid(int pidfd, struct error *err,
-			       const char *expect, ...)
+static int verify_fdinfo(int pidfd, struct error *err, const char *prefix,
+			 size_t prefix_len, const char *expect, ...)
 {
 	char buffer[512] = {0, };
 	char path[512] = {0, };
···
 				 pidfd);
 
 	while (getline(&line, &n, f) != -1) {
-		if (strncmp(line, "NSpid:", 6))
+		char *val;
+
+		if (strncmp(line, prefix, prefix_len))
 			continue;
 
 		found = 1;
 
-		r = strcmp(line + 6, buffer);
+		val = line + prefix_len;
+		r = strcmp(val, buffer);
 		if (r != 0) {
 			trim_newline(line);
 			trim_newline(buffer);
-			error_set(err, PIDFD_FAIL, "NSpid: '%s' != '%s'",
-				  line + 6, buffer);
+			error_set(err, PIDFD_FAIL, "%s '%s' != '%s'",
+				  prefix, val, buffer);
 		}
 		break;
 	}
···
 	fclose(f);
 
 	if (found == 0)
-		return error_set(err, PIDFD_FAIL, "NSpid not found for fd %d",
-				 pidfd);
+		return error_set(err, PIDFD_FAIL, "%s not found for fd %d",
+				 prefix, pidfd);
 
 	return PIDFD_PASS;
 }
···
 	}
 
 	pidfd = *(int *)args;
-	r = verify_fdinfo_nspid(pidfd, &err, "\t0\n");
+	r = verify_fdinfo(pidfd, &err, "NSpid:", 6, "\t0\n");
 
 	if (r != PIDFD_PASS)
 		ksft_print_msg("NSpid fdinfo check failed: %s\n", err.msg);
···
 	/* The children will have pid 1 in the new pid namespace,
 	 * so the line must be 'NSPid:\t<pid>\t1'.
 	 */
-	verify_fdinfo_nspid(a.fd, &err, "\t%d\t%d\n", a.pid, 1);
-	verify_fdinfo_nspid(b.fd, &err, "\t%d\t%d\n", b.pid, 1);
+	verify_fdinfo(a.fd, &err, "NSpid:", 6, "\t%d\t%d\n", a.pid, 1);
+	verify_fdinfo(b.fd, &err, "NSpid:", 6, "\t%d\t%d\n", b.pid, 1);
 
 	/* wait for the process, check the exit status and set
 	 * 'err' accordingly, if it is not already set.
 	 */
-	child_join(&a, &err);
-	child_join(&b, &err);
+	child_join_close(&a, &err);
+	child_join_close(&b, &err);
 
+	error_report(&err, test_name);
+}
+
+static void test_pidfd_dead_fdinfo(void)
+{
+	struct child a;
+	struct error err = {0, };
+	const char *test_name = "pidfd check fdinfo for dead process";
+
+	/* Create a new child in a new pid and mount namespace */
+	a = clone_newns(child_fdinfo_nspid_test, NULL, &err);
+	error_check(&err, test_name);
+	child_join(&a, &err);
+
+	verify_fdinfo(a.fd, &err, "Pid:", 4, "\t-1\n");
+	verify_fdinfo(a.fd, &err, "NSpid:", 6, "\t-1\n");
+	child_close(&a);
 	error_report(&err, test_name);
 }
 
 int main(int argc, char **argv)
 {
 	ksft_print_header();
-	ksft_set_plan(1);
+	ksft_set_plan(2);
 
 	test_pidfd_fdinfo_nspid();
+	test_pidfd_dead_fdinfo();
 
 	return ksft_exit_pass();
 }