Replace fuzzer::FuzzerDriver's INTERFACE marking with new LLVMRunFuzzerDriver.

This adds a new extern "C" function that serves the same purpose. This removes the need for external users to depend on internal headers in order to use this feature. It also standardizes the interface in a way that other fuzzing engines will be able to match.

Patch By: IanPudney

Reviewed By: kcc

Differential Revision: https://reviews.llvm.org/D84561

Author: morehouse

compiler-rt/lib/fuzzer/FuzzerDriver.cpp

@@ -858,6 +858,12 @@ int FuzzerDriver(int *argc, char ***argv, UserCallback Callback) {
   exit(0);  // Don't let F destroy itself.
 }
 
+extern "C" ATTRIBUTE_INTERFACE int
+LLVMFuzzerRunDriver(int *argc, char ***argv,
+                    int (*UserCb)(const uint8_t *Data, size_t Size)) {
+  return FuzzerDriver(argc, argv, UserCb);
+}
+
 // Storage for global ExternalFunctions object.
 ExternalFunctions *EF = nullptr;
 

llvm/docs/LibFuzzer.rst

@@ -617,6 +617,35 @@ really need to access ``argv``/``argc``.
     return 0;
    }
 
+Using libFuzzer as a library
+----------------------------
+If the code being fuzzed must provide its own `main`, it's possible to
+invoke libFuzzer as a library. Be sure to pass ``-fsanitize=fuzzer-no-link``
+during compilation, and link your binary against the no-main version of
+libFuzzer. On Linux installations, this is typically located at:
+
+.. code-block:: bash
+
+  /usr/lib/<llvm-version>/lib/clang/<clang-version>/lib/linux/libclang_rt.fuzzer_no_main-<architecture>.a
+
+If building libFuzzer from source, this is located at the following path
+in the build output directory:
+
+.. code-block:: bash
+
+  lib/linux/libclang_rt.fuzzer_no_main-<architecture>.a
+
+From here, the code can do whatever setup it requires, and when it's ready
+to start fuzzing, it can call `LLVMFuzzerRunDriver`, passing in the program
+arguments and a callback. This callback is invoked just like
+`LLVMFuzzerTestOneInput`, and has the same signature.
+
+.. code-block:: c++
+
+  extern "C" int LLVMFuzzerRunDriver(int *argc, char ***argv,
+                    int (*UserCb)(const uint8_t *Data, size_t Size));
+
+
 
 Leaks
 -----