Fix SGX simulation mode broken by the interrupt subsystem

The new interrupt subsystem breaks the simulation mode in two ways:

1. The signal 64 is not handled by Intel SGX SDK in simulation mode. A
handled real-time signal crashes the process.

2. The newly-enabled test case exit_group depends on interrupts. But
enclave interrupts, like enclave exceptions, are not supported in
simulation mode.

This commit ensures signal 64 is ignored by default and exit_group test
case is not enabled in simulation mode.

src/pal/src/pal_api.c

@@ -4,6 +4,7 @@
 #include "pal_error.h"
 #include "pal_interrupt_thread.h"
 #include "pal_log.h"
+#include "pal_sig_handler.h"
 #include "pal_syscall.h"
 #include "pal_thread_counter.h"
 #include "errno2str.h"
@@ -29,6 +30,10 @@ int occlum_pal_init(const struct occlum_pal_attr *attr) {
         return -1;
     }
 
+    if (pal_register_sig_handlers() < 0) {
+        return -1;
+    }
+
     if (pal_init_enclave(attr->instance_dir) < 0) {
         return -1;
     }

src/pal/src/pal_sig_handler.c

@@ -0,0 +1,19 @@
+#include "pal_sig_handler.h"
+#include "pal_log.h"
+#include <signal.h>
+#include <string.h>
+
+// Signal 64 is used to notify interrupts
+#define SIGRT_INTERRUPT     64
+
+int pal_register_sig_handlers(void) {
+    struct sigaction action;
+    action.sa_handler = SIG_IGN;
+    memset(&action.sa_mask, 0, sizeof(action.sa_mask));
+    action.sa_flags = 0;
+    if (sigaction(SIGRT_INTERRUPT, &action, NULL) < 0) {
+        PAL_ERROR("Failed to regiter signal handlers");
+        return -1;
+    }
+    return 0;
+}

src/pal/src/pal_sig_handler.h

@@ -0,0 +1,15 @@
+#ifndef __PAL_SIG_HANDLER_H__
+#define __PAL_SIG_HANDLER_H__
+
+// Register signal handlers for PAL.
+//
+// Currently, there is only one signal number that needs to be covered: signal
+// 64, which is used to notify interrupts (see LibOS code for more info). For
+// a hardware-mode enclave, the signal is handled by the signal handlers
+// registered by Intel SGX SDK. So we are ok in this case. But for a
+// simulation-mode enclave, there is no signal handler registered by Intel SGX
+// SDK. Without a signal handler, the delivery of the signal will kill the
+// process. This crash can be prevented by this API.
+int pal_register_sig_handlers(void);
+
+#endif /* __PAL_SIG_HANDLER_H__ */

test/exit_group/main.c

@@ -38,6 +38,11 @@ static void *futex_wait_thread_func(void *_) {
 
 // exit_group syscall should terminate all threads in a thread group.
 int test_exit_group_to_force_threads_terminate(void) {
+#ifdef SGX_MODE_SIM
+    printf("WARNING: Skip this test case as we do not support "
+           "enclave interruption in SGX simulation mode\n");
+    return 0;
+#else
     // Create three types of threads that will not exit voluntarily
     pthread_t busyloop_thread;
     if (pthread_create(&busyloop_thread, NULL, busyloop_thread_func, NULL) < 0) {
@@ -63,6 +68,7 @@ int test_exit_group_to_force_threads_terminate(void) {
     // main function returns. If Occlum can terminate normally, this means
     // exit_group syscall taking effect.
     return 0;
+#endif
 }
 
 // ============================================================================