llvm-project

8ecbfa66 - Hot-patch __ref_* variables should be placed in .rdata, not .data (#151008)

Hot-patch __ref_* variables should be placed in .rdata, not .data (#151008) This is a refinment of #145565 . That PR added support for "Windows Secure Hot-patching". In this design, functions that are compiled for hot-patching need to be modified when they access mutable global variables. The modification is to insert a level of indirection, the so-called `__ref_*` variables. Ref variables are supposed to be inserted into the `.rdata` section, not `.data`. This provides a degree of protection against modification (accidental or malicious) of ref variables during program execution. When the Windows hot-patch subsystem loads a module as a hot-patch, it finds all ref variables and changes the page protections for the pages containing them to read/write. Then it sets the ref variables to point to the real variable locations within the base image. Then it changes page protections back to read-only. This relies on the variables being placed in the `.rdata` section, not `.data`. However, it is still important that the LLVM `GlobalVariable` that is created for the ref variable be created with `isConstant = false`. This prevents LLVM from optimizing accesses to the `GlobalVariable`, i.e. assuming that the variable can never change and thus inlining its value into expressions that would ordinarily dereference it. That optimization would defeat the purpose of hot-patching, so `isConstant = false` is still the correct value for these ref variables.