[libfuzzer] Don't try to build tests with the just built compiler unless it's usable.

[hintonda]

Summary

Add test for COMPILER_RT_CAN_EXECUTE_TESTS before using
the just built compiler.

[phosek]

I don't think this is correct, compiling libFuzzer should be unrelated to whether we can execute tests or not. Your just built compiler should be capable of targeting Linux if that's your target platform.

[hintonda]

In D42286#982173, @phosek wrote:

I don't think this is correct, compiling libFuzzer should be unrelated to whether we can execute tests or not. Your just built compiler should be capable of targeting Linux if that's your target platform.

The problem here is that you are using the just built compiler, which may not run on the host if you are cross-compiling. If you take a look at the implementation of add_custom_libcxx, you'll see that it uses the just-built compiler -- -DCMAKE_CXX_COMPILER=${COMPILER_RT_TEST_CXX_COMPILER}.

In my case:

CMAKE_HOST_SYSTEM_NAME=Darwin
CMAKE_SYSTEM_NAME=Linux

With the just built compiler built like this: --target=x86_64-unknown-linux-gnu, so it won't run on Darwin.

[phosek]

In that case, I think we should modify add_custom_libcxx and add a new argument USE_TEST_COMPILER, if not set it should use the host compiler (i.e. CMAKE_C_COMPILER and CMAKE_CXX_COMPILER). @vitalybuka WDYT?

[bogner]

In D42286#982281, @phosek wrote:

In that case, I think we should modify add_custom_libcxx and add a new argument USE_TEST_COMPILER, if not set it should use the host compiler (i.e. CMAKE_C_COMPILER and CMAKE_CXX_COMPILER). @vitalybuka WDYT?

Running libFuzzer's tests with the host compiler is a very different thing than running libFuzzer's tests with the just built compiler. I'm pretty sure using CMAKE_C_COMPILER for the tests will more or less just test the fuzzer and sanitizers from your host system, which really isn't useful for testing that the libFuzzer you built works.

[hintonda]

Btw, shouldn't this all be hidden behind a test target, e.g., check-libfuzzer or something like that? That way, the just-built compiler is only used when you actually run tests.

[hintonda]

With D42330, it's no longer using the just-built compiler, but still fails when cross compiling (host=Darwin, target=Linux).

Temporary workaround: -DBOOTSTRAP_COMPILER_RT_BUILD_LIBFUZZER=OFF

lib/fuzzer/CMakeLists.txt

CHECK_CXX_SOURCE_COMPILES("
   static thread_local int blah;
   int main() {
   return 0;
   }
   " HAS_THREAD_LOCAL)
 
 set(LIBFUZZER_CFLAGS ${SANITIZER_COMMON_CFLAGS})
 
-if("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux" AND COMPILER_RT_LIBCXX_PATH)
+if("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux" AND COMPILER_RT_LIBCXX_PATH AND COMPILER_RT_CAN_EXECUTE_TESTS)
   list(APPEND LIBFUZZER_CFLAGS -nostdinc++ -D_LIBCPP_ABI_VERSION=__Fuzzer)
 endif()
 
 append_list_if(COMPILER_RT_HAS_OMIT_FRAME_POINTER_FLAG -fno-omit-frame-pointer LIBFUZZER_CFLAGS)
 
 if (CMAKE_CXX_FLAGS MATCHES "fsanitize-coverage")
   list(APPEND LIBFUZZER_CFLAGS -fno-sanitize-coverage=trace-pc-guard,edge,trace-cmp,indirect-calls,8bit-counters)
 endif()

 add_compiler_rt_runtime(clang_rt.fuzzer_no_main
   STATIC
   OS ${FUZZER_SUPPORTED_OS}
   ARCHS ${FUZZER_SUPPORTED_ARCH}
   OBJECT_LIBS RTfuzzer
   CFLAGS ${LIBFUZZER_CFLAGS}
   PARENT_TARGET fuzzer)
 
-if("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux" AND COMPILER_RT_LIBCXX_PATH)
+if("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux" AND COMPILER_RT_LIBCXX_PATH AND COMPILER_RT_CAN_EXECUTE_TESTS)
   macro(partially_link_libcxx name dir arch)
     set(cxx_${arch}_merge_dir "${CMAKE_CURRENT_BINARY_DIR}/cxx_${arch}_merge.dir")
     file(MAKE_DIRECTORY ${cxx_${arch}_merge_dir})
     add_custom_command(TARGET clang_rt.${name}-${arch} POST_BUILD
       COMMAND ${CMAKE_LINKER} --whole-archive "$<TARGET_LINKER_FILE:clang_rt.${name}-${arch}>" --no-whole-archive ${dir}/lib/libc++.a -r -o ${name}.o
       COMMAND ${CMAKE_OBJCOPY} --localize-hidden ${name}.o
       COMMAND ${CMAKE_COMMAND} -E remove "$<TARGET_LINKER_FILE:clang_rt.${name}-${arch}>"
       COMMAND ${CMAKE_AR} qcs "$<TARGET_LINKER_FILE:clang_rt.${name}-${arch}>" ${name}.o

lib/fuzzer/tests/CMakeLists.txt

 list(APPEND LIBFUZZER_UNITTEST_LINK_FLAGS --driver-mode=g++)
 
 if(APPLE OR CMAKE_SYSTEM_NAME STREQUAL "FreeBSD")
   list(APPEND LIBFUZZER_UNITTEST_LINK_FLAGS -lc++ -lpthread)
 else()
   list(APPEND LIBFUZZER_UNITTEST_LINK_FLAGS -lstdc++ -lpthread)
 endif()
 
-if("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux" AND COMPILER_RT_LIBCXX_PATH)
+if("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux" AND COMPILER_RT_LIBCXX_PATH AND COMPILER_RT_CAN_EXECUTE_TESTS)
   list(APPEND LIBFUZZER_UNITTEST_CFLAGS -nostdinc++ -D_LIBCPP_ABI_VERSION=__Fuzzer)
 endif()
 
 foreach(arch ${FUZZER_SUPPORTED_ARCH})
   set(LIBFUZZER_TEST_RUNTIME RTFuzzerTest.${arch})
   if(APPLE)
     set(LIBFUZZER_TEST_RUNTIME_OBJECTS
       $<TARGET_OBJECTS:RTfuzzer.osx>)
   else()
     set(LIBFUZZER_TEST_RUNTIME_OBJECTS
       $<TARGET_OBJECTS:RTfuzzer.${arch}>)
   endif()
   add_library(${LIBFUZZER_TEST_RUNTIME} STATIC
     ${LIBFUZZER_TEST_RUNTIME_OBJECTS})
   set_target_properties(${LIBFUZZER_TEST_RUNTIME} PROPERTIES
     ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
     FOLDER "Compiler-RT Runtime tests")
 
-  if("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux" AND COMPILER_RT_LIBCXX_PATH)
+  if("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux" AND COMPILER_RT_LIBCXX_PATH AND COMPILER_RT_CAN_EXECUTE_TESTS)
     set(LIBFUZZER_TEST_RUNTIME_DEPS libcxx_fuzzer_${arch})
     set(LIBFUZZER_TEST_RUNTIME_CFLAGS -isystem ${LIBCXX_${arch}_PREFIX}/include/c++/v1)
     set(LIBFUZZER_TEST_RUNTIME_LINK_FLAGS ${LIBCXX_${arch}_PREFIX}/lib/libc++.a)
   endif()
 
   set(FuzzerTestObjects)
   generate_compiler_rt_tests(FuzzerTestObjects
     FuzzerUnitTests "Fuzzer-${arch}-Test" ${arch}
     SOURCES FuzzerUnittest.cpp ${COMPILER_RT_GTEST_SOURCE}
     RUNTIME ${LIBFUZZER_TEST_RUNTIME}
     DEPS gtest ${LIBFUZZER_TEST_RUNTIME_DEPS}
     CFLAGS ${LIBFUZZER_UNITTEST_CFLAGS} ${LIBFUZZER_TEST_RUNTIME_CFLAGS}
     LINK_FLAGS ${LIBFUZZER_UNITTEST_LINK_FLAGS} ${LIBFUZZER_TEST_RUNTIME_LINK_FLAGS})
   set_target_properties(FuzzerUnitTests PROPERTIES
     RUNTIME_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
 endforeach()