[cmake] Unconditionally set the force flag when codesigning

[vsk]

Summary

The Darwin linker now defaults to ad hoc signing binaries when targeting
Apple Silicon. This creates a problem when configuring targets that must
be built with entitlements: we either need to add -Wl,-no_adhoc_codesign
when building the target, or sign with the force flag set to allow
replacing a pre-existing signature.

Unconditionally force-signing is the more convenient solution. This
doesn't require a ld64 version check, and it's a much less invasive
cmake change.

Patch by Fred Riss!

rdar://70237254

[kastiglione]

This doesn't require a ld64 version check, and it's a much less invasive cmake change.

Was there consideration about using an arch check, and only doing this for Apple Silicon targets? I think this is fine too, just wanting to cover the bases.

[vsk]

In D89343#2328892, @kastiglione wrote:

This doesn't require a ld64 version check, and it's a much less invasive cmake change.

Was there consideration about using an arch check, and only doing this for Apple Silicon targets? I think this is fine too, just wanting to cover the bases.

I actually hadn't considered that. We do have <TARGET_FILE:${name}>, so we could conceivably derive the target architecture from there and avoid setting force_flag on Intel. I'm not sure the added complexity is warranted though.

llvm/cmake/modules/AddLLVM.cmake

elseif(APPLE AND CMAKE_HOST_SYSTEM_NAME MATCHES Darwin)
     if(DEFINED ARG_ENTITLEMENTS)
       set(pass_entitlements --entitlements ${ARG_ENTITLEMENTS})
     endif()
 
     if (NOT ARG_BUNDLE_PATH)
       set(ARG_BUNDLE_PATH $<TARGET_FILE:${name}>)
     endif()
 
-    if(ARG_FORCE)
-      set(force_flag "-f")
-    endif()
+    # ld64 now always codesigns the binaries it creates. Apply the force arg
+    # unconditionally so that we can - for example - add entitlements to the
+    # targets that need it.
 
     add_custom_command(
       TARGET ${name} POST_BUILD
       COMMAND ${CMAKE_COMMAND} -E
               env CODESIGN_ALLOCATE=${CMAKE_CODESIGN_ALLOCATE}
               ${CMAKE_CODESIGN} -s ${LLVM_CODESIGNING_IDENTITY}
               ${pass_entitlements} ${force_flag} ${ARG_BUNDLE_PATH}
     )