bpf_dsl: decouple PolicyCompiler from Syscall

sandbox/linux/bpf_dsl/policy_compiler.h

Index: sandbox/linux/bpf_dsl/policy_compiler.h
diff --git a/sandbox/linux/bpf_dsl/policy_compiler.h b/sandbox/linux/bpf_dsl/policy_compiler.h
index faf6be5c36257eef1723621fedee40430b9e9f99..4a062e27f1e7aab2a97829ff3b5257bd62848617 100644
--- a/sandbox/linux/bpf_dsl/policy_compiler.h
+++ b/sandbox/linux/bpf_dsl/policy_compiler.h
@@ -27,7 +27,9 @@ class Policy;
 // Linux kernel.
 class SANDBOX_EXPORT PolicyCompiler {
  public:
-  PolicyCompiler(const Policy* policy, TrapRegistry* registry);
+  PolicyCompiler(const Policy* policy,
+                 TrapRegistry* registry,
+                 uint64_t escapepc = 0);

[jln (very slow on Chromium), 2015/02/19 19:49:14]

I'm worried about making it too easy to inject a backdoor.

How about:

1. Instead of a constructor parameter, we require calling a clear
DangerousSetBackdoorEscapePC() API?

2. CHECK that kSandboxDebuggingEnv is set before calling
DangerousSetBackdoorEscapePC().

3. I would even add the same CHECK to MaybeAddEscapeHatch() for good measure.

[mdempsky, 2015/02/20 03:20:32]

FWIW, this just lets the "user" control what PC is used for the escape hatch. 
All of the same safe guards to prevent unsafe traps in production still existed,
even if someone accidentally passes a bogus PC value here.
Done.
Done.
I added an extra call to registry_->EnableUnsafeTraps(), since I don't want
bpf_dsl::PolicyCompiler to depend on the specifics of kSandboxDebuggingEnv.

   ~PolicyCompiler();
 
   // Compile registers any trap handlers needed by the policy and
@@ -88,7 +90,7 @@ class SANDBOX_EXPORT PolicyCompiler {
   CodeGen::Node CheckArch(CodeGen::Node passed);
 
   // If |has_unsafe_traps_| is true, returns an instruction sequence
-  // that allows all system calls from Syscall::Call(), and otherwise
+  // that allows all system calls from |escapepc_|, and otherwise
   // passes control to |rest|. Otherwise, simply returns |rest|.
   CodeGen::Node MaybeAddEscapeHatch(CodeGen::Node rest);
 
@@ -140,6 +142,7 @@ class SANDBOX_EXPORT PolicyCompiler {
 
   const Policy* policy_;
   TrapRegistry* registry_;
+  uint64_t escapepc_;
 
   Conds conds_;
   CodeGen gen_;