[cmake] Allow building fuzzers with OSS-Fuzz flags.

[bogner]

llvm/cmake/modules/AddLLVM.cmake

897–902

How / where does this get set?

[morehouse]

llvm/cmake/modules/AddLLVM.cmake

897–902

In the CMake configure command. See https://github.com/google/oss-fuzz/pull/885.

[bogner]

llvm/cmake/modules/AddLLVM.cmake

897–902

Okay, if we're going to use a cmake variable for this we should do it right and define a cache variable with documentation in the top level CMakeLists.txt file. I'd probably add it next to where we add LLVM_USE_SANITIZER.

Also, we should namespace this appropriately. Something like LLVM_LIB_FUZZING_ENGINE or LLVM_FUZZING_ENGINE.

Optionally, we could also set it up so that we default that to libFuzzer in the LLVM_USE_SANITIZE_COVERAGE case so that we don't even need two cases here.

[morehouse]

• Add cache variable and rename to LLVM_LIB_FUZZING_ENGINE.

[morehouse]

llvm/cmake/modules/AddLLVM.cmake

897–902

Regarding switching the configuration under LLVM_USE_SANITIZE_COVERAGE, I don't think we want to do that. OSS-Fuzz uses its own instrumentation which we don't want to mix with -fsanitize=fuzzer. But having -fsanitize=fuzzer by default seems helpful for developers.

[kcc]

llvm/cmake/modules/AddLLVM.cmake

897–902

Right.
oss-fuzz uses *different* build modes (e.g. one instrumentation for AFL, another for libFuzzer).

[bogner]

LGTM

llvm/CMakeLists.txt

 if( LLVM_USE_OPROFILE )
   if( NOT CMAKE_SYSTEM_NAME MATCHES "Linux" )
     message(FATAL_ERROR "OProfile support is available on Linux only.")
   endif( NOT CMAKE_SYSTEM_NAME MATCHES "Linux" )
 endif( LLVM_USE_OPROFILE )
 
 set(LLVM_USE_SANITIZER "" CACHE STRING
   "Define the sanitizer used to build binaries and tests.")
+set(LLVM_LIB_FUZZING_ENGINE "" CACHE PATH
+  "Path to fuzzing library for linking with fuzz targets")
 
 option(LLVM_USE_SPLIT_DWARF
   "Use -gsplit-dwarf when compiling llvm." OFF)
 
 option(LLVM_POLLY_LINK_INTO_TOOLS "Statically link Polly into tools (if available)" ON)
 option(LLVM_POLLY_BUILD "Build LLVM with Polly" ON)
 
 if (EXISTS ${LLVM_MAIN_SRC_DIR}/tools/polly/CMakeLists.txt)

llvm/cmake/modules/AddLLVM.cmake

if (NOT CMAKE_CONFIGURATION_TYPES)
                                 -DCMAKE_INSTALL_COMPONENT=${name}
                                 -P "${CMAKE_BINARY_DIR}/cmake_install.cmake")
     endif()
   endif()
 endmacro(add_llvm_utility name)
 
 macro(add_llvm_fuzzer name)
   cmake_parse_arguments(ARG "" "DUMMY_MAIN" "" ${ARGN})
-  if( LLVM_USE_SANITIZE_COVERAGE )
+  if( LLVM_LIB_FUZZING_ENGINE )
+    set(LLVM_OPTIONAL_SOURCES ${ARG_DUMMY_MAIN})
+    add_llvm_executable(${name} ${ARG_UNPARSED_ARGUMENTS})
+    target_link_libraries(${name} ${LLVM_LIB_FUZZING_ENGINE})
+    set_target_properties(${name} PROPERTIES FOLDER "Fuzzers")
+  elseif( LLVM_USE_SANITIZE_COVERAGE )
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsanitize=fuzzer")
     set(LLVM_OPTIONAL_SOURCES ${ARG_DUMMY_MAIN})
     add_llvm_executable(${name} ${ARG_UNPARSED_ARGUMENTS})
     set_target_properties(${name} PROPERTIES FOLDER "Fuzzers")
   elseif( ARG_DUMMY_MAIN )
     add_llvm_executable(${name} ${ARG_DUMMY_MAIN} ${ARG_UNPARSED_ARGUMENTS})
     set_target_properties(${name} PROPERTIES FOLDER "Fuzzers")
   endif()
 endmacro()
 
 macro(add_llvm_target target_name)
   include_directories(BEFORE
     ${CMAKE_CURRENT_BINARY_DIR}
     ${CMAKE_CURRENT_SOURCE_DIR})
   add_llvm_library(LLVM${target_name} ${ARGN})
   set( CURRENT_LLVM_TARGET LLVM${target_name} )