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

sandbox/linux/seccomp-bpf/trap.cc

Index: sandbox/linux/seccomp-bpf/trap.cc
diff --git a/sandbox/linux/seccomp-bpf/trap.cc b/sandbox/linux/seccomp-bpf/trap.cc
index 3c30de35546d89768d79ddb490cfba7635aa6409..40ee61a6f64c78ff8a6f9e8b3d6ee748ecb819a7 100644
--- a/sandbox/linux/seccomp-bpf/trap.cc
+++ b/sandbox/linux/seccomp-bpf/trap.cc
@@ -61,6 +61,14 @@ void SetIsInSigHandler() {
   }
 }
 
+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 {
@@ -74,10 +82,16 @@ Trap::Trap()
   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) ||