[scudo] Try to fix standalone build on armv7

[rovka]

Summary

When linking scudo standalone on armv7, it can't find symbols related to
unwinding (e.g. __aeabi_unwind_cpp_pr0). This is probably because it
is passing --unwindlib=none. This patch hacks around the issue by adding
COMPILER_RT_UNWINDER_LINK_LIBS if we're using the LLVM unwinder.

I don't know anything about scudo, so I'm not sure what the original
intention was.

This is a release blocker.
See also https://github.com/llvm/llvm-project/issues/56900

[phosek]

This looks like a correct fix to me, thank you for looking into it.

compiler-rt/lib/scudo/standalone/CMakeLists.txt

138–140

You should be able to set this unconditionally since COMPILER_RT_UNWINDER_LINK_LIBS is already set (or unset) based on the content of COMPILER_RT_USE_LLVM_UNWINDER (in the future it might also expand to -lgcc on other platforms).

[rovka]

compiler-rt/lib/scudo/standalone/CMakeLists.txt

138–140

Makes sense, thanks for the quick review!

compiler-rt/lib/scudo/standalone/CMakeLists.txt

if (COMPILER_RT_HAS_GWP_ASAN)
 
   append_list_if(COMPILER_RT_HAS_OMIT_FRAME_POINTER_FLAG -fno-omit-frame-pointer
                                                          -mno-omit-leaf-frame-pointer
                  SCUDO_CFLAGS)
   list(APPEND SCUDO_CFLAGS -DGWP_ASAN_HOOKS)
 
 endif()
 
-set(SCUDO_LINK_LIBS)
+set(SCUDO_LINK_LIBS ${COMPILER_RT_UNWINDER_LINK_LIBS})
 
 append_list_if(COMPILER_RT_HAS_LIBPTHREAD -pthread SCUDO_LINK_FLAGS)
 
 append_list_if(FUCHSIA zircon SCUDO_LINK_LIBS)
 
 if(COMPILER_RT_DEFAULT_TARGET_ARCH MATCHES "mips|mips64|mipsel|mips64el")
   list(APPEND SCUDO_LINK_LIBS atomic)
 endif()
 
 if(COMPILER_RT_HAS_SCUDO_STANDALONE)