Add a linker script to version LLVM symbols

[sylvestre.ledru]

Summary

This patch adds a very simple linker script to version the lib's symbols
and thus trying to avoid crashes if an application loads two different
LLVM versions (as long as they do not share data between them).

Note that we deliberately *don't* make LLVM_5.0 depend on LLVM_4.0:
they're incompatible and the whole point of this patch is
to tell the linker that.

Avoid unexpected crashes when two LLVM versions are used in the same process.

Author: Rebecca N. Palmer <[e-mail]>
Author: Lisandro Damían Nicanor Pérez Meyer <[e-mail]>
Author: Sylvestre Ledru <[e-mail]>
Bug-Debian: https://bugs.debian.org/848368

[sylvestre.ledru]

Programs using these libraries WONT have to recompile anything.

[rnk]

cmake/modules/AddLLVM.cmake

85

Is this correct for DSOs that aren't libLLVM.so.N.M? This function is called from LLDB and elsewhere.

[sylvestre.ledru]

cmake/modules/AddLLVM.cmake

85

It doesn't have too. libLLVM.so.N.M is for the SONAME.
However, I can rename it, no worries

[sylvestre.ledru]

ping? (FYI, Debian packages are shipping with this change, so will Ubuntu)

[rnk]

Note that we deliberately *don't* make LLVM_5.0 depend on LLVM_5.0:

Can you elaborate on what you mean by this? Do you mean LLVM_5.0 doesn't depend on LLVM_4.0?

cmake/modules/AddLLVM.cmake

85

So, this doesn't refer to LLVM the DSO, it refers to LLVM the project. After this change, libclang.so will have symbols with LLVM_N.M as their version, right?

[sylvestre.ledru]

Can you elaborate on what you mean by this? Do you mean LLVM_5.0 doesn't depend on LLVM_4.0?

Yeah, sorry. I will fix that in the commit

cmake/modules/AddLLVM.cmake

85

Yes, I can rename it to LLVM_TOOLCHAIN_N.M if you prefer but this is a bit long.

[rnk]

cmake/modules/AddLLVM.cmake

85

No, it's fine. Maybe add a comment, since I don't think ELF version script details are common knowledge.

cmake/modules/AddLLVM.cmake

     set(native_export_file "${target_name}.exports")
     # FIXME: Don't write the "local:" line on OpenBSD.
     add_custom_command(OUTPUT ${native_export_file}
-      COMMAND echo "{" > ${native_export_file}
+      COMMAND echo "LLVM_${LLVM_VERSION_MAJOR}.${LLVM_VERSION_MINOR} {" > ${native_export_file}
       COMMAND grep -q "[[:alnum:]]" ${export_file} && echo "  global:" >> ${native_export_file} || :
       COMMAND sed -e "s/$/;/" -e "s/^/    /" < ${export_file} >> ${native_export_file}
       COMMAND echo "  local: *;" >> ${native_export_file}

tools/llvm-shlib/CMakeLists.txt

 
 list(REMOVE_DUPLICATES LIB_NAMES)
 if(("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux") OR (MINGW) OR ("${CMAKE_SYSTEM_NAME}" STREQUAL "FreeBSD") OR ("${CMAKE_SYSTEM_NAME}" STREQUAL "DragonFly")) # FIXME: It should be "GNU ld for elf"
+  configure_file(
+  ${CMAKE_CURRENT_SOURCE_DIR}/simple_version_script.map.in
+  ${LLVM_LIBRARY_DIR}/tools/llvm-shlib/simple_version_script.map)
+
   # GNU ld doesn't resolve symbols in the version script.
-  set(LIB_NAMES -Wl,--whole-archive ${LIB_NAMES} -Wl,--no-whole-archive)
+  set(LIB_NAMES -Wl,--version-script,${LLVM_LIBRARY_DIR}/tools/llvm-shlib/simple_version_script.map -Wl,--whole-archive ${LIB_NAMES} -Wl,--no-whole-archive)
 elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "Darwin")
   set(LIB_NAMES -Wl,-all_load ${LIB_NAMES})
 endif()

tools/llvm-shlib/simple_version_script.map.in

+LLVM_@LLVM_VERSION_MAJOR@.@LLVM_VERSION_MINOR@ { global: *; };