bpf_dsl: support arbitrary (arg & mask) == val expressions

sandbox/linux/bpf_dsl/bpf_dsl.h

Index: sandbox/linux/bpf_dsl/bpf_dsl.h
diff --git a/sandbox/linux/bpf_dsl/bpf_dsl.h b/sandbox/linux/bpf_dsl/bpf_dsl.h
index da90efd421805467a64ee10311d4024d3f21aa65..b46d1adb113afa0b60fd51b0e17f26331d124663 100644
--- a/sandbox/linux/bpf_dsl/bpf_dsl.h
+++ b/sandbox/linux/bpf_dsl/bpf_dsl.h
@@ -7,7 +7,6 @@
 
 #include <stdint.h>
 
-#include <limits>
 #include <utility>
 
 #include "base/macros.h"
@@ -62,7 +61,7 @@ class SandboxBPF;
 //
 //   result = Allow() | Error(errno) | Trap(trap_func, arg)
 //          | If(bool, result)[.ElseIf(bool, result)].Else(result)
-//   bool   = arg == val | (arg & mask) == mask | (arg & mask) == 0
+//   bool   = arg == val | (arg & mask) == val
 //          | !bool | bool && bool | bool || bool
 //
 // The semantics of each function and operator are intended to be
@@ -135,8 +134,7 @@ class SANDBOX_EXPORT Arg {
  public:
   // Initializes the Arg to represent the |num|th system call
   // argument (indexed from 0), which is of type |T|.
-  explicit Arg(int num)
-      : num_(num), mask_(std::numeric_limits<uint64_t>::max()) {}
+  explicit Arg(int num);
 
   Arg(const Arg& arg) : num_(arg.num_), mask_(arg.mask_) {}
 
@@ -211,6 +209,9 @@ namespace internal {
 SANDBOX_EXPORT BoolExpr
     ArgEq(int num, size_t size, uint64_t mask, uint64_t val);
 
+// Returns the default mask for a system call argument of the specified size.
+SANDBOX_EXPORT uint64_t DefaultMask(size_t size);
+
 // Internal interface implemented by BoolExpr implementations.
 class SANDBOX_EXPORT BoolExprImpl : public base::RefCounted<BoolExprImpl> {
  public:
@@ -243,6 +244,11 @@ class SANDBOX_EXPORT ResultExprImpl : public base::RefCounted<ResultExprImpl> {
 
 }  // namespace internal
 
+template <typename T>
+Arg<T>::Arg(int num)
+    : num_(num), mask_(internal::DefaultMask(sizeof(T))) {
+}
+
 // Definition requires ArgEq to have been declared.  Moved out-of-line
 // to minimize how much internal clutter users have to ignore while
 // reading the header documentation.