Implement Gas Metering for pre-instrumented modules. Closes #127

platforms/app/main.c

@@ -11,11 +11,15 @@
 #include <ctype.h>
 
 #include "wasm3.h"
+#include "m3_api_defs.h"
 #include "m3_api_wasi.h"
 #include "m3_api_libc.h"
 #include "m3_api_tracer.h"
 
-#define FATAL(msg, ...) { printf("Error: [Fatal] " msg "\n", ##__VA_ARGS__); goto _onfatal; }
+// Gas metering/limit only applies to pre-instrumented modules
+#define GAS_LIMIT   2000000000000
+
+#define FATAL(msg, ...) { fprintf(stderr, "Error: [Fatal] " msg "\n", ##__VA_ARGS__); goto _onfatal; }
 
 #if defined(d_m3HasWASI) || defined(d_m3HasMetaWASI) || defined(d_m3HasUVWASI)
 #define LINK_WASI
@@ -24,6 +28,26 @@
 IM3Environment env;
 IM3Runtime runtime;
 
+#if defined(GAS_LIMIT)
+
+static int64_t current_gas = GAS_LIMIT;
+static bool is_gas_metered = false;
+
+m3ApiRawFunction(metering_usegas)
+{
+    m3ApiGetArg     (int32_t, gas)
+
+    current_gas -= gas;
+
+    if (UNLIKELY(current_gas < 0)) {
+        m3ApiTrap("[trap] Out of gas");
+    }
+    m3ApiSuccess();
+}
+
+#endif // GAS_LIMIT
+
+
 M3Result link_all  (IM3Module module)
 {
     M3Result res;
@@ -43,6 +67,15 @@ M3Result link_all  (IM3Module module)
     if (res) return res;
 #endif
 
+#if defined(GAS_LIMIT)
+    res = m3_LinkRawFunction (module, "metering", "usegas", "v(i)", &metering_usegas);
+    if (!res) {
+        fprintf(stderr, "Warning: Gas is limited to %0.4f\n", (double)(current_gas)/10000);
+        is_gas_metered = true;
+    }
+    if (res == m3Err_functionLookupFailed) { res = NULL; }
+#endif
+
     return res;
 }
 
@@ -199,6 +232,13 @@ M3Result repl_call  (const char* name, int argc, const char* argv[])
     }
 
     result = m3_CallArgv (func, argc, argv);
+
+#if defined(GAS_LIMIT)
+    if (is_gas_metered) {
+        fprintf(stderr, "Gas used: %0.4f\n", (double)(GAS_LIMIT - current_gas)/10000);
+    }
+#endif
+
     if (result) return result;
 
     static uint64_t    valbuff[128];