Bug 1299581 - Fail waitpid et al. with ECHILD in sandboxed content processes. r=gcp

MozReview-Commit-ID: 7Qjcnrd7KqK

--HG--
extra : rebase_source : 98e9bcb247edad657d8e45e30901861a9193f249

security/sandbox/linux/SandboxFilter.cpp

@@ -793,10 +793,16 @@ public:
 
     case __NR_umask:
     case __NR_kill:
+      return Allow();
+
     case __NR_wait4:
 #ifdef __NR_waitpid
     case __NR_waitpid:
 #endif
+      // NSPR will start a thread to wait for child processes even if
+      // fork() fails; see bug 227246 and bug 1299581.
+      return Error(ECHILD);
+
 #ifdef __NR_arch_prctl
     case __NR_arch_prctl:
 #endif