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) {

[Markus (顧孟勤), 2013/09/05 22:23:25]

SIG_IGN is probably also OK, but we can hold off on that. Let's see which
reports we get back.

On that note, how are we getting any of these reports? Users don't typically
look at the output from LOG(ERROR) and tell us about it.

[jln (very slow on Chromium), 2013/09/05 22:29:05]

It'll mostly be for the spurious SIGSYS bug report(s) for the time being.

Hopefully, soon we can DCHECK and "lock this in".

+    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 = { };

[Markus (顧孟勤), 2013/09/05 22:23:25]

Why do you need to zero out this structure? I know, just a nitpick.

[jln (very slow on Chromium), 2013/09/05 22:29:05]

Done.

+  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";

[Markus (顧孟勤), 2013/09/05 22:23:25]

It would be nice, if we could actually log what the handler points to. But
that's a lot more complicated.

[jln (very slow on Chromium), 2013/09/05 22:29:05]

Yeah, and in the spurious SIGSYS case, it's likely that we won't have symbols
anyways (binary driver), even if we implemented symbolization support.

+  }
+
   // 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