feat: add a fuse to speed up the case where no resource is present (#59)

Fixes: #58
Signed-off-by: Darshan Sen <[email protected]>

README.markdown

@@ -75,6 +75,15 @@ ability to inject the resources into pre-built executables, with the
 goal that the end result should be as close as possible to what is
 obtained by embedding them at build-time.
 
+Note: Other runtime injection implementers should search the binary
+compiled with `postject-api.h` for the
+`POSTJECT_SENTINEL_fce680ab2cc467b6e072b8b5df1996b2:0` fuse and
+flip the last character to `1` to indicate that a resource has been
+injected. A different fuse can also be used by defining the
+`POSTJECT_SENTINEL_FUSE` macro before including `postject-api.h` and
+passing the same string to postject with
+`--sentinel-fuse <sentinel_fuse>`.
+
 ### Windows
 
 For PE executables, the resources are added into the `.rsrc` section,

postject-api.h

@@ -1,6 +1,7 @@
 #ifndef POSTJECT_API_H_
 #define POSTJECT_API_H_
 
+#include <stdbool.h>
 #include <stddef.h>
 #include <stdlib.h>
 #include <string.h>
@@ -17,6 +18,11 @@
 #include <windows.h>
 #endif
 
+#ifndef POSTJECT_SENTINEL_FUSE
+#define POSTJECT_SENTINEL_FUSE \
+  "POSTJECT_SENTINEL_fce680ab2cc467b6e072b8b5df1996b2"
+#endif
+
 struct postject_options {
   const char* elf_section_name;
   const char* macho_framework_name;
@@ -33,6 +39,11 @@ inline void postject_options_init(struct postject_options* options) {
   options->pe_resource_name = NULL;
 }
 
+static inline bool postject_has_resource() {
+  static const volatile char* sentinel = POSTJECT_SENTINEL_FUSE ":0";
+  return sentinel[sizeof(POSTJECT_SENTINEL_FUSE)] == '1';
+}
+
 static const void* postject_find_resource(
     const char* name,
     size_t* size,

src/api.js

@@ -6,6 +6,9 @@ const loadPostjectModule = require("./postject.js");
 async function inject(filename, resourceName, resourceData, options) {
   const machoSegmentName = options?.machoSegmentName || "__POSTJECT";
   const overwrite = options?.overwrite || false;
+  let sentinelFuse =
+    options?.sentinelFuse ||
+    "POSTJECT_SENTINEL_fce680ab2cc467b6e072b8b5df1996b2";
 
   if (!Buffer.isBuffer(resourceData)) {
     throw new TypeError("resourceData must be a buffer");
@@ -112,8 +115,46 @@ async function inject(filename, resourceName, resourceData, options) {
     throw new Error("Error when injecting resource");
   }
 
+  const buffer = Buffer.from(data.buffer);
+  const firstSentinel = buffer.indexOf(sentinelFuse);
+
+  if (firstSentinel === -1) {
+    throw new Error(
+      `Could not find the sentinel ${sentinelFuse} in the binary`
+    );
+  }
+
+  const lastSentinel = buffer.lastIndexOf(sentinelFuse);
+
+  if (firstSentinel !== lastSentinel) {
+    throw new Error(
+      `Multiple occurences of sentinel "${sentinelFuse}" found in the binary`
+    );
+  }
+
+  const colonIndex = firstSentinel + sentinelFuse.length;
+  if (buffer[colonIndex] !== ":".charCodeAt(0)) {
+    throw new Error(
+      `Value at index ${colonIndex} must be ':' but '${buffer[
+        colonIndex
+      ].charCodeAt(0)}' was found`
+    );
+  }
+
+  const hasResourceIndex = firstSentinel + sentinelFuse.length + 1;
+  const hasResourceValue = buffer[hasResourceIndex];
+  if (hasResourceValue === "0".charCodeAt(0)) {
+    buffer[hasResourceIndex] = "1".charCodeAt(0);
+  } else if (hasResourceValue != "1".charCodeAt(0)) {
+    throw new Error(
+      `Value at index ${hasResourceIndex} must be '0' or '1' but '${hasResourceValue.charCodeAt(
+        0
+      )}' was found`
+    );
+  }
+
   try {
-    await fs.writeFile(filename, data);
+    await fs.writeFile(filename, buffer);
   } catch {
     throw new Error("Couldn't write executable");
   }

src/cli.js

@@ -28,6 +28,7 @@ async function main(filename, resourceName, resource, options) {
     await inject(filename, resourceName, resourceData, {
       machoSegmentName: options.machoSegmentName,
       overwrite: options.overwrite,
+      sentinelFuse: options.sentinelFuse,
     });
   } catch (err) {
     console.log(err.message);
@@ -52,6 +53,11 @@ if (require.main === module) {
       "Name for the Mach-O segment",
       "__POSTJECT"
     )
+    .option(
+      "--sentinel-fuse <sentinel_fuse>",
+      "Sentinel fuse for resource presence detection",
+      "POSTJECT_SENTINEL_fce680ab2cc467b6e072b8b5df1996b2"
+    )
     .option("--output-api-header", "Output the API header to stdout")
     .option("--overwrite", "Overwrite the resource if it already exists")
     .action(main)

test/cli.mjs

@@ -75,7 +75,14 @@ describe("postject CLI", () => {
     {
       const { status, stdout, stderr } = spawnSync(
         "node",
-        ["./dist/cli.js", filename, "foobar", resourceFilename],
+        [
+          "./dist/cli.js",
+          filename,
+          "foobar",
+          resourceFilename,
+          "--sentinel-fuse",
+          "NODE_JS_FUSE_fce680ab2cc467b6e072b8b5df1996b2",
+        ],
         { encoding: "utf-8" }
       );
       // TODO(dsanders11) - Enable this once we squelch LIEF warnings
@@ -153,7 +160,9 @@ describe("postject API", () => {
 
     {
       const resourceData = await fs.readFile(resourceFilename);
-      await inject(filename, "foobar", resourceData);
+      await inject(filename, "foobar", resourceData, {
+        sentinelFuse: "NODE_JS_FUSE_fce680ab2cc467b6e072b8b5df1996b2",
+      });
     }
 
     // Verifying code signing using a self-signed certificate.

test/test.c

@@ -4,10 +4,18 @@
 #include "../dist/postject-api.h"
 
 int main() {
-  size_t size;
-  const void* ptr = postject_find_resource("foobar", &size, NULL);
+  size_t size = 0;
 
-  if (ptr && size > 0) {
+  if (postject_has_resource()) {
+    const void* ptr = postject_find_resource("foobar", &size, NULL);
+    if (ptr == NULL) {
+      fprintf(stderr, "ptr must not be NULL.\n");
+      exit(1);
+    }
+    if (size == 0) {
+      fprintf(stderr, "size must not be 0.\n");
+      exit(1);
+    }
     char* str = (char*)malloc(size + 1);
     memset(str, 0, size + 1);
 #if defined(_WIN32)
@@ -17,6 +25,15 @@ int main() {
 #endif
     printf("%s\n", str);
   } else {
+    const void* ptr = postject_find_resource("foobar", &size, NULL);
+    if (ptr != NULL) {
+      fprintf(stderr, "ptr must be NULL.\n");
+      exit(1);
+    }
+    if (size > 0) {
+      fprintf(stderr, "size must not be greater than 0.\n");
+      exit(1);
+    }
     printf("Hello world\n");
   }
 

test/test.cpp

@@ -1,15 +1,33 @@
 #include <iostream>
 #include <string>
 
+#define POSTJECT_SENTINEL_FUSE "NODE_JS_FUSE_fce680ab2cc467b6e072b8b5df1996b2"
 #include "../dist/postject-api.h"
 
 int main() {
-  size_t size;
-  const void* ptr = postject_find_resource("foobar", &size, nullptr);
+  size_t size = 0;
 
-  if (ptr && size > 0) {
+  if (postject_has_resource()) {
+    const void* ptr = postject_find_resource("foobar", &size, nullptr);
+    if (ptr == NULL) {
+      std::cerr << "ptr must not be NULL." << std::endl;
+      exit(1);
+    }
+    if (size == 0) {
+      std::cerr << "size must not be 0." << std::endl;
+      exit(1);
+    }
     std::cout << std::string(static_cast<const char*>(ptr), size) << std::endl;
   } else {
+    const void* ptr = postject_find_resource("foobar", &size, nullptr);
+    if (ptr != nullptr) {
+      std::cerr << "ptr must be nullptr." << std::endl;
+      exit(1);
+    }
+    if (size > 0) {
+      std::cerr << "size must not be greater than 0." << std::endl;
+      exit(1);
+    }
     std::cout << "Hello world" << std::endl;
   }