linux-user: Fix waitid return of siginfo_t and rusage

The copy back to siginfo_t should be conditional only on arg3,
not the specific values that might have been written.
The copy back to rusage was missing entirely.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2262
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Alex Fan <alex.fan.q@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>

linux-user/syscall.c

@@ -9272,14 +9272,24 @@ static abi_long do_syscall1(CPUArchState *cpu_env, int num, abi_long arg1,
 #ifdef TARGET_NR_waitid
     case TARGET_NR_waitid:
         {
+            struct rusage ru;
             siginfo_t info;
-            info.si_pid = 0;
-            ret = get_errno(safe_waitid(arg1, arg2, &info, arg4, NULL));
-            if (!is_error(ret) && arg3 && info.si_pid != 0) {
-                if (!(p = lock_user(VERIFY_WRITE, arg3, sizeof(target_siginfo_t), 0)))
+
+            ret = get_errno(safe_waitid(arg1, arg2, (arg3 ? &info : NULL),
+                                        arg4, (arg5 ? &ru : NULL)));
+            if (!is_error(ret)) {
+                if (arg3) {
+                    p = lock_user(VERIFY_WRITE, arg3,
+                                  sizeof(target_siginfo_t), 0);
+                    if (!p) {
+                        return -TARGET_EFAULT;
+                    }
+                    host_to_target_siginfo(p, &info);
+                    unlock_user(p, arg3, sizeof(target_siginfo_t));
+                }
+                if (arg5 && host_to_target_rusage(arg5, &ru)) {
                     return -TARGET_EFAULT;
-                host_to_target_siginfo(p, &info);
-                unlock_user(p, arg3, sizeof(target_siginfo_t));
+                }
             }
         }
         return ret;