Linux Sandbox: LOG error if a previous SIGSYS handler exists.

sandbox/linux/seccomp-bpf/trap.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <errno.h>
 #include <signal.h>
 #include <string.h>
 #include <sys/prctl.h>
 #include <sys/syscall.h>
 
@@ skipping 43 matching lines @@
 
 void SetIsInSigHandler() {
   sigset_t mask;
   if (sigemptyset(&mask) ||
       sigaddset(&mask, SIGBUS) ||
       sigprocmask(SIG_BLOCK, &mask, NULL)) {
     SANDBOX_DIE("Failed to block SIGBUS");
   }
 }
 
+bool IsDefaultSignalAction(const struct sigaction& sa) {
+  if (sa.sa_flags & SA_SIGINFO ||
+      sa.sa_handler != SIG_DFL) {
+    return false;
+  }
+  return true;
+}
+
 }  // namespace
 
 namespace playground2 {
 
 Trap::Trap()
     : trap_array_(NULL),
       trap_array_size_(0),
       trap_array_capacity_(0),
       has_unsafe_traps_(false) {
   // Set new SIGSYS handler
   struct sigaction sa = { };
   sa.sa_sigaction = SigSysAction;
   sa.sa_flags = SA_SIGINFO | SA_NODEFER;
-  if (sigaction(SIGSYS, &sa, NULL) < 0) {
+  struct sigaction old_sa;
+  if (sigaction(SIGSYS, &sa, &old_sa) < 0) {
     SANDBOX_DIE("Failed to configure SIGSYS handler");
   }
 
+  if (!IsDefaultSignalAction(old_sa)) {
+    // TODO(jln): make this FATAL, at least in DEBUG mode.
+    LOG(ERROR) << "Existing signal handler when trying to install SIGSYS";
+  }
+
   // Unmask SIGSYS
   sigset_t mask;
   if (sigemptyset(&mask) ||
       sigaddset(&mask, SIGSYS) ||
       sigprocmask(SIG_UNBLOCK, &mask, NULL)) {
     SANDBOX_DIE("Failed to configure SIGSYS handler");
   }
 }
 
 Trap *Trap::GetInstance() {
@@ skipping 247 matching lines @@
   if (global_trap_ && id > 0 && id <= global_trap_->trap_array_size_) {
     return global_trap_->trap_array_[id - 1];
   } else {
     return ErrorCode();
   }
 }
 
 Trap *Trap::global_trap_;
 
 }  // namespace playground2