Start to use an unrecognised syscall to trigger snapshotting

This replaces use of SIGUSR1 in some cases.

ptracer.cc

@@ -347,6 +347,12 @@ int main(int argc, char **argv) {
           regs.orig_rax = -1;
           rc = ptrace(PTRACE_SETREGS, pid, 0, &regs);
           assert(rc == 0);
+        } else if (regs.orig_rax == (uintptr_t) -1) {
+          // Unrecognised syscall: trigger snapshotting.
+          // TODO: Whitelist syscalls instead of blacklisting this one.
+          ptracer.Dump();
+          ptracer.TerminateSubprocess();
+          break;
         }
       } else {
         ptracer.HandleSyscall(&regs);

tests/example_loader.c

@@ -14,13 +14,8 @@ static int my_errno;
 extern char code_start[], code_end[];
 asm("code_start:\n"
 
-    // Call getpid()
-    "movl $39, %eax\n"
-    "syscall\n"
-    // Call kill(getpid(), SIGUSR1)
-    "movl %eax, %edi\n" // arg1: result of getpid()
-    "movl $10, %esi\n" // arg2: SIGUSR1
-    "movl $62, %eax\n" // __NR_kill
+    // Do unhandled syscall -1 to trigger snapshotting
+    "movq $-1, %rax\n"
     "syscall\n"
 
     // Call write()

tests/example_prog2.c

@@ -2,12 +2,17 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include <signal.h>
+#include <assert.h>
+#include <errno.h>
 #include <stdio.h>
+#include <unistd.h>
 
 
 int main(void) {
-  raise(SIGUSR1);
+  // Do unhandled syscall -1 to trigger snapshotting.
+  int rc = syscall(-1);
+  assert(rc == -1);
+  assert(errno == ENOSYS);
 
   printf("In example_prog2.c!\n");
   return 0;

tests/save_restore_tests.cc

@@ -5,7 +5,6 @@
 #include <assert.h>
 #include <elf.h>
 #include <errno.h>
-#include <signal.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -19,7 +18,10 @@
 namespace {
 
 void do_snapshot() {
-  raise(SIGUSR1);
+  // Do unhandled syscall -1 to trigger snapshotting.
+  int rc = syscall(-1);
+  assert(rc == -1);
+  assert(errno == ENOSYS);
 }
 
 void test_munmap_whole_mapping() {