[compiler-rt][fuzzer] Make fuzzer interface header public

[tomilov]

Summary

Make FuzzerInterface.h publicly available to ease writing custom structure-aware fuzzers.

[tomilov]

Currently it is just a copy of compiler-rt/lib/fuzzer/FuzzerInterface.h with s/dllexport/dllimport, because I want to start a discussion. Really it would be convenient to be able just to include such interface header.

[kcc]

This header is intentionally private, so that the fuzz targets remain engine-neutral.

[tomilov]

When function LLVMFuzzerTestOneInput is defined, one immediately get dependence on libFuzzer because of LLVM prefix, isn't it?

compiler-rt/include/CMakeLists.txt

set(SANITIZER_HEADERS
     sanitizer/netbsd_syscall_hooks.h
     sanitizer/scudo_interface.h
     sanitizer/tsan_interface.h
     sanitizer/tsan_interface_atomic.h
     sanitizer/ubsan_interface.h
     )
   set(FUZZER_HEADERS
     fuzzer/FuzzedDataProvider.h
+    fuzzer/FuzzerInterface.h
     )
 endif(COMPILER_RT_BUILD_SANITIZERS)
 
 if (COMPILER_RT_BUILD_MEMPROF)
   set(MEMPROF_HEADERS
     sanitizer/memprof_interface.h
     )
 endif(COMPILER_RT_BUILD_MEMPROF)

compiler-rt/include/fuzzer/FuzzerInterface.h

+//===- FuzzerInterface.h - Interface header for the Fuzzer ------*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Define the interface between libFuzzer and the library being tested.
+//===----------------------------------------------------------------------===//
+
+// NOTE: the libFuzzer interface is thin and in the majority of cases
+// you should not include this file into your target. In 95% of cases
+// all you need is to define the following function in your file:
+// extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size);
+
+// WARNING: keep the interface in C.
+// WARNING: keep in sync with compiler-rt/lib/fuzzer/FuzzerInterface.h.
+
+#ifndef LLVM_FUZZER_INTERFACE_H
+#define LLVM_FUZZER_INTERFACE_H
+
+#include <stddef.h>
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif  // __cplusplus
+
+// Define FUZZER_INTERFACE_VISIBILITY to set default visibility in a way that
+// doesn't break MSVC.
+#if defined(_WIN32)
+#define FUZZER_INTERFACE_VISIBILITY __declspec(dllimport)
+#else
+#define FUZZER_INTERFACE_VISIBILITY __attribute__((visibility("default")))
+#endif
+
+// Mandatory user-provided target function.
+// Executes the code under test with [Data, Data+Size) as the input.
+// libFuzzer will invoke this function *many* times with different inputs.
+// Must return 0.
+FUZZER_INTERFACE_VISIBILITY int
+LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size);
+
+// Optional user-provided initialization function.
+// If provided, this function will be called by libFuzzer once at startup.
+// It may read and modify argc/argv.
+// Must return 0.
+FUZZER_INTERFACE_VISIBILITY int LLVMFuzzerInitialize(int *argc, char ***argv);
+
+// Optional user-provided custom mutator.
+// Mutates raw data in [Data, Data+Size) inplace.
+// Returns the new size, which is not greater than MaxSize.
+// Given the same Seed produces the same mutation.
+FUZZER_INTERFACE_VISIBILITY size_t
+LLVMFuzzerCustomMutator(uint8_t *Data, size_t Size, size_t MaxSize,
+                        unsigned int Seed);
+
+// Optional user-provided custom cross-over function.
+// Combines pieces of Data1 & Data2 together into Out.
+// Returns the new size, which is not greater than MaxOutSize.
+// Should produce the same mutation given the same Seed.
+FUZZER_INTERFACE_VISIBILITY size_t
+LLVMFuzzerCustomCrossOver(const uint8_t *Data1, size_t Size1,
+                          const uint8_t *Data2, size_t Size2, uint8_t *Out,
+                          size_t MaxOutSize, unsigned int Seed);
+
+// Experimental, may go away in future.
+// libFuzzer-provided function to be used inside LLVMFuzzerCustomMutator.
+// Mutates raw data in [Data, Data+Size) inplace.
+// Returns the new size, which is not greater than MaxSize.
+FUZZER_INTERFACE_VISIBILITY size_t
+LLVMFuzzerMutate(uint8_t *Data, size_t Size, size_t MaxSize);
+
+#undef FUZZER_INTERFACE_VISIBILITY
+
+#ifdef __cplusplus
+}  // extern "C"
+#endif  // __cplusplus
+
+#endif  // LLVM_FUZZER_INTERFACE_H

compiler-rt/lib/fuzzer/FuzzerInterface.h

 //===- FuzzerInterface.h - Interface header for the Fuzzer ------*- C++ -* ===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 // Define the interface between libFuzzer and the library being tested.
 //===----------------------------------------------------------------------===//
 
 // NOTE: the libFuzzer interface is thin and in the majority of cases
 // you should not include this file into your target. In 95% of cases
 // all you need is to define the following function in your file:
 // extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size);
 
 // WARNING: keep the interface in C.
+// WARNING: keep in sync with compiler-rt/include/fuzzer/FuzzerInterface.h.
 
 #ifndef LLVM_FUZZER_INTERFACE_H
 #define LLVM_FUZZER_INTERFACE_H
 
 #include <stddef.h>
 #include <stdint.h>
 
 #ifdef __cplusplus