Introduce sys_sigprocmask and sys_sigaction.

sandbox/linux/services/syscall_wrappers.h

Index: sandbox/linux/services/syscall_wrappers.h
diff --git a/sandbox/linux/services/syscall_wrappers.h b/sandbox/linux/services/syscall_wrappers.h
index 4558adff2bb647f212cf523cb4784eefcfe7217a..29988a3d0f6e29d235786818ab63d69deb747693 100644
--- a/sandbox/linux/services/syscall_wrappers.h
+++ b/sandbox/linux/services/syscall_wrappers.h
@@ -5,6 +5,7 @@
 #ifndef SANDBOX_LINUX_SERVICES_SYSCALL_WRAPPERS_H_
 #define SANDBOX_LINUX_SERVICES_SYSCALL_WRAPPERS_H_
 
+#include <signal.h>
 #include <stdint.h>
 #include <sys/types.h>
 
@@ -66,6 +67,15 @@ SANDBOX_EXPORT int sys_chroot(const char* path);
 // Some libcs do not expose a unshare wrapper.
 SANDBOX_EXPORT int sys_unshare(int flags);
 
+// Some libcs do not expose a sigprocmask. Note that oldset must be a nullptr,
+// because of some ABI gap between toolchain's and Linux's.
+SANDBOX_EXPORT int sys_sigprocmask(
+    int how, const sigset_t* set, decltype(nullptr) oldset);
+
+// Some libcs do not expose a sigaction().
+SANDBOX_EXPORT int sys_sigaction(int signum, const struct sigaction* act,
+                                 struct sigaction* oldact);
+
 }  // namespace sandbox
 
 #endif  // SANDBOX_LINUX_SERVICES_SYSCALL_WRAPPERS_H_