Introduce sys_sigprocmask and sys_sigaction.

sandbox/linux/seccomp-bpf/die.cc

Index: sandbox/linux/seccomp-bpf/die.cc
diff --git a/sandbox/linux/seccomp-bpf/die.cc b/sandbox/linux/seccomp-bpf/die.cc
index 777c9d1139298f46d78bb1ef0a97667b48414015..a014e7fbcf7bed796d3132884351ca000b936f57 100644
--- a/sandbox/linux/seccomp-bpf/die.cc
+++ b/sandbox/linux/seccomp-bpf/die.cc
@@ -16,6 +16,8 @@
 #include "base/logging.h"
 #include "base/posix/eintr_wrapper.h"
 #include "sandbox/linux/seccomp-bpf/syscall.h"
+#include "sandbox/linux/services/syscall_wrappers.h"
+#include "sandbox/linux/system_headers/linux_signal.h"
 
 namespace sandbox {
 
@@ -32,7 +34,11 @@ void Die::ExitGroup() {
   // to a defined state; but we have not way to verify whether we actually
   // succeeded in doing so. Nonetheless, triggering a fatal signal could help
   // us terminate.
-  signal(SIGSEGV, SIG_DFL);
+  struct sigaction sa = {};
+  sa.sa_handler = LINUX_SIG_DFL;
+  sa.sa_flags = LINUX_SA_RESTART;
+  sigaddset(&sa.sa_mask, LINUX_SIGSEGV);

[mdempsky, 2015/04/13 18:55:07]

With POSIX sigaction(), SIGSEGV would be masked automatically since you didn't
specify SA_NODEFER.  Does Linux's sigaction() system call push that
responsibility into userland?

[hidehiko, 2015/04/14 11:56:40]

IIUC, SIGSEGV is masked in the handler invocation (if we do not set SA_NODEFER),
right? So, I didn't think it is a problem here...?

[mdempsky, 2015/04/17 04:36:22]

Sorry, what I meant was:

  1. Setting sa_mask is only meaningful if you set a signal handler; I don't
think setting it is meaningful here because you're using SIG_DFL.
  2. Even if we were registering a handler here, then because we're not setting
SA_NODEFER, SIGSEGV would be masked anyway.

So I think this sigaddset is unnecessary (although not harmful either).

[hidehiko, 2015/04/17 06:18:41]

Oh, I misunderstood, and now fixed. Thanks!

+  sys_sigaction(LINUX_SIGSEGV, &sa, nullptr);
   Syscall::Call(__NR_prctl, PR_SET_DUMPABLE, (void*)0, (void*)0, (void*)0);
   if (*(volatile char*)0) {
   }