Tidy BrokerProcess::SIGSYS_Handler()

Stop trying to match which syscalls are present based on compiler
flags since we already know this via which symbols are defined in
the system_headers/*_syscalls.h file.

Add a macro for msan poisoning to reduce nesting of ifdefs.
Use early returns to avoid "else considered harmful".

Change-Id: Ic37cd41037bba11ebd25c8326d8e3db115b58254
Reviewed-on: https://chromium-review.googlesource.com/783332Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518411}

sandbox/linux/syscall_broker/broker_process.cc

@@ -114,40 +114,44 @@ int BrokerProcess::Open(const char* pathname, int flags) const {
   return broker_client_->Open(pathname, flags);
 }
 
+#if defined(MEMORY_SANITIZER)
+#define BROKER_UNPOISON_STRING(x) __msan_unpoison_string(x)
+#else
+#define BROKER_UNPOISON_STRING(x)
+#endif
+
 // static
 intptr_t BrokerProcess::SIGSYS_Handler(const sandbox::arch_seccomp_data& args,
                                        void* aux_broker_process) {
   RAW_CHECK(aux_broker_process);
   auto* broker_process = static_cast<BrokerProcess*>(aux_broker_process);
   switch (args.nr) {
-#if !defined(__aarch64__)
+#if defined(__NR_access)
     case __NR_access:
       return broker_process->Access(reinterpret_cast<const char*>(args.args[0]),
                                     static_cast<int>(args.args[1]));
+#endif
+#if defined(__NR_open)
     case __NR_open:
-#if defined(MEMORY_SANITIZER)
       // http://crbug.com/372840
-      __msan_unpoison_string(reinterpret_cast<const char*>(args.args[0]));
-#endif
+      BROKER_UNPOISON_STRING(reinterpret_cast<const char*>(args.args[0]));
       return broker_process->Open(reinterpret_cast<const char*>(args.args[0]),
                                   static_cast<int>(args.args[1]));
-#endif  // !defined(__aarch64__)
+#endif
+#if defined(__NR_faccessat)
     case __NR_faccessat:
-      if (static_cast<int>(args.args[0]) == AT_FDCWD) {
-        return broker_process->Access(
-            reinterpret_cast<const char*>(args.args[1]),
-            static_cast<int>(args.args[2]));
-      } else {
+      if (static_cast<int>(args.args[0]) != AT_FDCWD)
         return -EPERM;
-      }
-    case __NR_openat:
-      // Allow using openat() as open().
-      if (static_cast<int>(args.args[0]) == AT_FDCWD) {
-        return broker_process->Open(reinterpret_cast<const char*>(args.args[1]),
+      return broker_process->Access(reinterpret_cast<const char*>(args.args[1]),
                                     static_cast<int>(args.args[2]));
-      } else {
+#endif
+#if defined(__NR_openat)
+    case __NR_openat:
+      if (static_cast<int>(args.args[0]) != AT_FDCWD)
         return -EPERM;
-      }
+      return broker_process->Open(reinterpret_cast<const char*>(args.args[1]),
+                                  static_cast<int>(args.args[2]));
+#endif
     default:
       RAW_CHECK(false);
       return -ENOSYS;