Thunks: Add support for building with clang

Fairly straightforward, just requires enabling lld in this case since
cross-compiling doesn't work well with gnu linker.

Also lld doesn't understand the linker script program header symbolic
names for read/write/execute. So we need to use the raw number there.

Works around an issue where GCC 11 generates broken `init_array` section
and also plt sections that glibc doesn't understand.

CMakeLists.txt

@@ -7,6 +7,7 @@ CHECK_INCLUDE_FILES ("gdb/jit-reader.h" HAVE_GDB_JIT_READER_H)
 option(BUILD_TESTS "Build unit tests to ensure sanity" TRUE)
 option(BUILD_FEX_LINUX_TESTS "Build FEXLinuxTests, requires x86 compiler" FALSE)
 option(BUILD_THUNKS "Build thunks" FALSE)
+option(BUILD_CLANG_THUNKS "Build thunks with clang" FALSE)
 option(ENABLE_CLANG_FORMAT "Run clang format over the source" FALSE)
 option(ENABLE_IWYU "Enables include what you use program" FALSE)
 option(ENABLE_LTO "Enable LTO with compilation" TRUE)
@@ -415,6 +416,7 @@ if (BUILD_THUNKS)
     CMAKE_ARGS
       "-DBITNESS=64"
       "-DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}"
+      "-DENABLE_CLANG_THUNKS=${ENABLE_CLANG_THUNKS}"
       "-DCMAKE_TOOLCHAIN_FILE:FILEPATH=${X86_64_TOOLCHAIN_FILE}"
       "-DCMAKE_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX}"
       "-DSTRUCT_VERIFIER=${CMAKE_SOURCE_DIR}/Scripts/StructPackVerifier.py"
@@ -432,6 +434,7 @@ if (BUILD_THUNKS)
     CMAKE_ARGS
       "-DBITNESS=32"
       "-DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}"
+      "-DENABLE_CLANG_THUNKS=${ENABLE_CLANG_THUNKS}"
       "-DCMAKE_TOOLCHAIN_FILE:FILEPATH=${X86_32_TOOLCHAIN_FILE}"
       "-DCMAKE_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX}"
       "-DSTRUCT_VERIFIER=${CMAKE_SOURCE_DIR}/Scripts/StructPackVerifier.py"

ThunkLibs/GuestLibs/CMakeLists.txt

@@ -2,6 +2,12 @@ cmake_minimum_required(VERSION 3.14)
 project(guest-thunks)
 
 option(BITNESS "Which bitness the thunks are building for" 64)
+option(ENABLE_CLANG_THUNKS "Enable building thunks with clang" FALSE)
+
+if (ENABLE_CLANG_THUNKS)
+  set (LD_OVERRIDE "-fuse-ld=lld")
+  add_link_options(${LD_OVERRIDE})
+endif()
 
 if (CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR)
   # We've been included using ExternalProject_add, so set up the actual thunk libraries to be cross-compiled

ThunkLibs/HostLibs/CMakeLists.txt

@@ -3,6 +3,12 @@ project(host-thunks)
 
 set(CMAKE_CXX_STANDARD 17)
 set (HOSTLIBS_DATA_DIRECTORY "${CMAKE_INSTALL_PREFIX}/lib/fex-emu" CACHE PATH "global data directory")
+option(ENABLE_CLANG_THUNKS "Enable building thunks with clang" FALSE)
+
+if (ENABLE_CLANG_THUNKS)
+  set (LD_OVERRIDE "-fuse-ld=lld")
+  add_link_options(${LD_OVERRIDE})
+endif()
 
 # Syntax: generate(libxyz libxyz-interface.cpp)
 # This defines two targets and a custom command:

ThunkLibs/include/common/Guest.h

@@ -7,8 +7,12 @@
 #if __SIZEOF_POINTER__ == 8
 #define THUNK_ABI
 #else
+#ifdef __clang__
+#define THUNK_ABI __fastcall
+#else
 #define THUNK_ABI [[gnu::fastcall]]
 #endif
+#endif
 
 template<typename signature>
 THUNK_ABI

ThunkLibs/libVDSO/libVDSO_Guest.lds

@@ -29,9 +29,9 @@ SECTIONS {
 }
 
 PHDRS {
-  text PT_LOAD FLAGS(PF_R | PF_X) FILEHDR PHDRS;
-  dynamic PT_DYNAMIC FLAGS(PF_R);
-  note PT_NOTE FLAGS(PF_R);
+  text PT_LOAD FLAGS(4 | 1) FILEHDR PHDRS;
+  dynamic PT_DYNAMIC FLAGS(4);
+  note PT_NOTE FLAGS(4);
 }
 
 VERSION {

ThunkLibs/libVDSO/libVDSO_Guest_32.lds

@@ -29,9 +29,9 @@ SECTIONS {
 }
 
 PHDRS {
-  text PT_LOAD FLAGS(PF_R | PF_X) FILEHDR PHDRS;
-  dynamic PT_DYNAMIC FLAGS(PF_R);
-  note PT_NOTE FLAGS(PF_R);
+  text PT_LOAD FLAGS(4 | 1) FILEHDR PHDRS;
+  dynamic PT_DYNAMIC FLAGS(4);
+  note PT_NOTE FLAGS(4);
 }
 
 VERSION {

toolchain_x86_32.cmake

@@ -1,4 +1,20 @@
+option(ENABLE_CLANG_THUNKS "Enable building thunks with clang" FALSE)
+
 set(CMAKE_SYSTEM_PROCESSOR i686)
 
-set(CMAKE_C_COMPILER x86_64-linux-gnu-gcc -m32)
-set(CMAKE_CXX_COMPILER x86_64-linux-gnu-g++ -m32)
+if (ENABLE_CLANG_THUNKS)
+  message(STATUS "Enabling thunk clang building. Force enabling LLD as well")
+
+  set(CMAKE_EXE_LINKER_FLAGS_INIT "-fuse-ld=lld")
+  set(CMAKE_MODULE_LINKER_FLAGS_INIT "-fuse-ld=lld")
+  set(CMAKE_SHARED_LINKER_FLAGS_INIT "-fuse-ld=lld")
+  set(CMAKE_C_COMPILER clang)
+  set(CMAKE_CXX_COMPILER clang++)
+  set(CLANG_FLAGS "-target i686-linux-gnu -msse2 -mfpmath=sse")
+
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${CLANG_FLAGS}")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${CLANG_FLAGS}")
+else()
+  set(CMAKE_C_COMPILER x86_64-linux-gnu-gcc -m32)
+  set(CMAKE_CXX_COMPILER x86_64-linux-gnu-g++ -m32)
+endif()

toolchain_x86_64.cmake

@@ -1,4 +1,20 @@
+option(ENABLE_CLANG_THUNKS "Enable building thunks with clang" FALSE)
+
 set(CMAKE_SYSTEM_PROCESSOR x86_64)
 
-set(CMAKE_C_COMPILER x86_64-linux-gnu-gcc)
-set(CMAKE_CXX_COMPILER x86_64-linux-gnu-g++)
+if (ENABLE_CLANG_THUNKS)
+  message(STATUS "Enabling thunk clang building. Force enabling LLD as well")
+
+  set(CMAKE_EXE_LINKER_FLAGS_INIT "-fuse-ld=lld")
+  set(CMAKE_MODULE_LINKER_FLAGS_INIT "-fuse-ld=lld")
+  set(CMAKE_SHARED_LINKER_FLAGS_INIT "-fuse-ld=lld")
+  set(CMAKE_C_COMPILER clang)
+  set(CMAKE_CXX_COMPILER clang++)
+  set(CLANG_FLAGS "-target x86_64-linux-gnu")
+
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${CLANG_FLAGS}")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${CLANG_FLAGS}")
+else()
+  set(CMAKE_C_COMPILER x86_64-linux-gnu-gcc)
+  set(CMAKE_CXX_COMPILER x86_64-linux-gnu-g++)
+endif()