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.

Author: sivadeilra

llvm/lib/CodeGen/WindowsSecureHotPatching.cpp

@@ -369,6 +369,19 @@ static GlobalVariable *getOrCreateRefVariable(
                          AddrOfOldGV, Twine("__ref_").concat(GV->getName()),
                          nullptr, GlobalVariable::NotThreadLocal);
 
+  // RefGV is created with isConstant = false, but we want to place RefGV into
+  // .rdata, not .data.  It is important that the GlobalVariable be mutable
+  // from the compiler's point of view, so that the optimizer does not remove
+  // the global variable entirely and replace all references to it with its
+  // initial value.
+  //
+  // When the Windows hot-patch loader applies a hot-patch, it maps the
+  // pages of .rdata as read/write so that it can set each __ref_* variable
+  // to point to the original variable in the base image. Afterward, pages in
+  // .rdata are remapped as read-only. This protects the __ref_* variables from
+  // being overwritten during execution.
+  RefGV->setSection(".rdata");
+
   // Create debug info for the replacement global variable.
   DataLayout Layout = M->getDataLayout();
   DIType *DebugType = DebugInfo.createPointerType(