Hot-patch __ref_* variables should be placed in .rdata, not .data #151008
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dpaoliello
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sivadeilra
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…vm#151008) This is a refinment of llvm#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.
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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
.rdatasection, 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
.rdatasection, not.data.However, it is still important that the LLVM
GlobalVariablethat is created for the ref variable be created withisConstant = false. This prevents LLVM from optimizing accesses to theGlobalVariable, 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, soisConstant = falseis still the correct value for these ref variables.