================ @@ -0,0 +1,599 @@ +//===------ WindowsHotPatch.cpp - Support for Windows hotpatching ---------===// +// +// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. +// See https://llvm.org/LICENSE.txt for license information. +// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception +// +//===----------------------------------------------------------------------===// +// +// Provides support for the Windows "Secure Hot-Patching" feature. +// +// Windows contains technology, called "Secure Hot-Patching" (SHP), for securely +// applying hot-patches to a running system. Hot-patches may be applied to the +// kernel, kernel-mode components, device drivers, user-mode system services, +// etc. +// +// SHP relies on integration between many tools, including compiler, linker, +// hot-patch generation tools, and the Windows kernel. This file implements that +// part of the workflow needed in compilers / code generators. +// +// SHP is not intended for productivity scenarios such as Edit-and-Continue or +// interactive development. SHP is intended to minimize downtime during +// installation of Windows OS patches. +// +// In order to work with SHP, LLVM must do all of the following: +// +// * On some architectures (X86, AMD64), the function prolog must begin with +// hot-patchable instructions. This is handled by the MSVC `/hotpatch` option +// and the equivalent `-fms-hotpatch` function. This is necessary because we +// generally cannot anticipate which functions will need to be patched in the +// future. This option ensures that a function can be hot-patched in the +// future, but does not actually generate any hot-patch for it. +// +// * For a selected set of functions that are being hot-patched (which are +// identified using command-line options), LLVM must generate the +// `S_HOTPATCHFUNC` CodeView record (symbol). This record indicates that a +// function was compiled with hot-patching enabled. +// +// This implementation uses the `MarkedForWindowsHotPatching` attribute to +// annotate those functions that were marked for hot-patching by command-line +// parameters. The attribute may be specified by a language front-end by +// setting an attribute when a function is created in LLVM IR, or it may be +// set by passing LLVM arguments. +// +// * For those functions that are hot-patched, LLVM must rewrite references to +// global variables so that they are indirected through a `__ref_*` pointer +// variable. For each global variable, that is accessed by a hot-patched +// function, e.g. `FOO`, a `__ref_FOO` global pointer variable is created and +// all references to the original `FOO` are rewritten as dereferences of the +// `__ref_FOO` pointer. +// +// Some globals do not need `__ref_*` indirection. The pointer indirection +// behavior can be disabled for these globals by marking them with the +// `AllowDirectAccessInHotPatchFunction`. +// +// References +// +// * "Hotpatching on Windows": +// https://techcommunity.microsoft.com/blog/windowsosplatform/hotpatching-on-windows/2959541 +// +// * "Hotpatch for Windows client now available": +// https://techcommunity.microsoft.com/blog/windows-itpro-blog/hotpatch-for-windows-client-now-available/4399808 +// +// * "Get hotpatching for Windows Server": +// https://www.microsoft.com/en-us/windows-server/blog/2025/04/24/tired-of-all-the-restarts-get-hotpatching-for-windows-server/ +// +//===----------------------------------------------------------------------===// + +#include "llvm/ADT/SmallSet.h" +#include "llvm/CodeGen/Passes.h" +#include "llvm/IR/Attributes.h" +#include "llvm/IR/DIBuilder.h" +#include "llvm/IR/DiagnosticInfo.h" +#include "llvm/IR/Function.h" +#include "llvm/IR/IRBuilder.h" +#include "llvm/IR/InstIterator.h" +#include "llvm/IR/Module.h" +#include "llvm/InitializePasses.h" +#include "llvm/Pass.h" +#include "llvm/Support/CommandLine.h" +#include "llvm/Support/LineIterator.h" +#include "llvm/Support/MemoryBuffer.h" + +using namespace llvm; + +#define DEBUG_TYPE "windows-secure-hot-patch" + +// A file containing list of mangled function names to mark for hot patching. +static cl::opt<std::string> LLVMMSSecureHotPatchFunctionsFile( + "ms-secure-hotpatch-functions-file", cl::value_desc("filename"), + cl::desc("A file containing list of mangled function names to mark for " + "Windows Secure Hot-Patching")); + +// A list of mangled function names to mark for hot patching. +static cl::list<std::string> LLVMMSSecureHotPatchFunctionsList( + "ms-secure-hotpatch-functions-list", cl::value_desc("list"), + cl::desc("A list of mangled function names to mark for Windows Secure " + "Hot-Patching"), + cl::CommaSeparated); + +namespace { + +struct GlobalVariableUse { + // GlobalVariable *GV; + Instruction *User; + unsigned Op; +}; + +class WindowsSecureHotPatching : public ModulePass { +public: + static char ID; + + WindowsSecureHotPatching() : ModulePass(ID) { + initializeWindowsSecureHotPatchingPass(*PassRegistry::getPassRegistry()); + } + + void getAnalysisUsage(AnalysisUsage &AU) const override { + AU.setPreservesCFG(); + } + + bool doInitialization(Module &) override; + bool runOnModule(Module &M) override { return false; } + +private: + bool + runOnFunction(Function &F, + SmallDenseMap<GlobalVariable *, GlobalVariable *> &RefMapping); +}; + +} // end anonymous namespace + +char WindowsSecureHotPatching::ID = 0; + +INITIALIZE_PASS(WindowsSecureHotPatching, "windows-secure-hot-patch", + "Mark functions for Windows hot patch support", false, false) +ModulePass *llvm::createWindowsSecureHotPatchingPass() { + return new WindowsSecureHotPatching(); +} + +// Find functions marked with Attribute::MarkedForWindowsHotPatching and modify +// their code (if necessary) to account for accesses to global variables. +// +// This runs during doInitialization() instead of runOnModule() because it needs +// to run before CodeViewDebug::collectGlobalVariableInfo(). +bool WindowsSecureHotPatching::doInitialization(Module &M) { + // The front end may have already marked functions for hot-patching. However, + // we also allow marking functions by passing -ms-hotpatch-functions-file or + // -ms-hotpatch-functions-list directly to LLVM. This allows hot-patching to + // work with languages that have not yet updated their front-ends. + if (!LLVMMSSecureHotPatchFunctionsFile.empty() || + !LLVMMSSecureHotPatchFunctionsList.empty()) { + std::vector<std::string> HotPatchFunctionsList; + + if (!LLVMMSSecureHotPatchFunctionsFile.empty()) { + auto BufOrErr = MemoryBuffer::getFile(LLVMMSSecureHotPatchFunctionsFile); + if (BufOrErr) { + const MemoryBuffer &FileBuffer = **BufOrErr; + for (line_iterator I(FileBuffer.getMemBufferRef(), true), E; I != E; + ++I) + HotPatchFunctionsList.push_back(std::string{*I}); + } else { + M.getContext().diagnose(DiagnosticInfoGeneric{ + Twine("failed to open hotpatch functions file " + "(--ms-hotpatch-functions-file): ") + + LLVMMSSecureHotPatchFunctionsFile + Twine(" : ") + + BufOrErr.getError().message()}); + } + } + + if (!LLVMMSSecureHotPatchFunctionsList.empty()) + for (const auto &FuncName : LLVMMSSecureHotPatchFunctionsList) + HotPatchFunctionsList.push_back(FuncName); + + // Build a set for quick lookups. This points into HotPatchFunctionsList, so + // HotPatchFunctionsList must live longer than HotPatchFunctionsSet. + SmallSet<StringRef, 16> HotPatchFunctionsSet; + for (const auto &FuncName : HotPatchFunctionsList) + HotPatchFunctionsSet.insert(StringRef{FuncName}); + + // Iterate through all of the functions and check whether they need to be + // marked for hotpatching using the list provided directly to LLVM. + for (auto &F : M.functions()) { + // Ignore declarations that are not definitions. + if (F.isDeclarationForLinker()) + continue; + + if (HotPatchFunctionsSet.contains(F.getName())) + F.addFnAttr(Attribute::MarkedForWindowsHotPatching); + } + } + + SmallDenseMap<GlobalVariable *, GlobalVariable *> RefMapping; + bool MadeChanges = false; + for (auto &F : M.functions()) { + if (F.hasFnAttribute(Attribute::MarkedForWindowsHotPatching)) { + if (runOnFunction(F, RefMapping)) + MadeChanges = true; + } + } + return MadeChanges; +} + +static bool TypeContainsPointers(Type *ty) { + switch (ty->getTypeID()) { + case Type::PointerTyID: + return true; + + case Type::ArrayTyID: + return TypeContainsPointers(ty->getArrayElementType()); + + case Type::StructTyID: { + unsigned NumElements = ty->getStructNumElements(); + for (unsigned I = 0; I < NumElements; ++I) { + if (TypeContainsPointers(ty->getStructElementType(I))) { + return true; + } + } + return false; + } + + default: + return false; + } +} + +// Returns true if GV needs redirection through a __ref_* variable. +static bool globalVariableNeedsRedirect(GlobalVariable *GV) { + // If a global variable is explictly marked as allowing access in hot-patched + // functions, then do not redirect it. + if (GV->hasAttribute(Attribute::AllowDirectAccessInHotPatchFunction)) { + return false; + } + + // If the global variable is not a constant, then we want to redirect it. + if (!GV->isConstant()) { + if (GV->getName().starts_with("??_R")) { + // This is the name mangling prefix that MSVC uses for RTTI data. + // Clang is currently generating RTTI data that is marked non-constant. + // We override that and treat it like it is constant. + return false; + } + + // In general, if a global variable is not a constant, then redirect it. + return true; + } + + // If the type of GV cannot contain pointers, then it cannot point to + // other global variables. In this case, there is no need for redirects. + // For example, string literals do not contain pointers. + return TypeContainsPointers(GV->getValueType()); +} + +/* + +Rewriting references to global variables has some complexity. + +For ordinary instructions that reference GlobalVariables, we rewrite the +operand of the instruction to a Load of the __ref_* variable. + +For constant expressions, we have to convert the constant expression (and +transitively all constant expressions in its parent chain) to non-constant +expressions, i.e. to a sequence of instructions. + +Pass 1: + * Enumerate all instructions in all basic blocks. + + * If an instruction references a GlobalVariable (and it is not marked + as being ignored), then we create (if necessary) the __ref_* variable + for the GlobalVariable reference. However, we do not yet modify the + Instruction. + + * If an instruction has an operand that is a ConstantExpr and the + ConstantExpression tree contains a reference to a GlobalVariable, then + we similarly create __ref_*. Similarly, we do not yet modify the Instruction + or the ConstantExpr tree. + +After Pass 1 completes, we will know whether we found any references to +globals in this pass. If the function does not use any globals (and most +functions do not use any globals), then we return immediately. + +If a function does reference globals, then we iterate the list of globals +used by this function and we generate Load instructions for each (unique) +global. + +Next, we do another pass over all instructions: + +Pass 2: + * Re-visit the instructions that were found in Pass 1. + + * If an instruction operand is a GlobalVariable, then look up the replacement + __ref_* global variable and the Value that came from the Load instruction + for it. Replace the operand of the GlobalVariable with the Load Value. + + * If an instruction operand is a ConstantExpr, then recursively examine the + operands of all instructions in the ConstantExpr tree. If an operand is + a GlobalVariable, then replace the operand with the result of the load + *and* convert the ConstantExpr to a non-constant instruction. This + instruction will need to be inserted into the BB of the instruction whose + operand is being modified, ideally immediately before the instruction + being modified. +*/ + +// Get or create a new global variable that points to the old one and whose +// name begins with `__ref_`. +// +// In hot-patched images, the __ref_* variables point to global variables in +// the original (unpatched) image. Hot-patched functions in the hot-patch +// image use these __ref_* variables to access global variables. This ensures +// that all code (both unpatched and patched) is using the same instances of +// global variables. +// +// The Windows hot-patch infrastructure handles initializing these __ref_* +// variables. By default, they are initialized with pointers to the equivalent +// global variables, so when a hot-patch module is loaded *as* a base image +// (such as after a system reboot), hot-patch functions will access the +// instances of global variables that are compiled into the hot-patch image. +// This is the desired outcome, since in this situation (normal boot) the +// hot-patch image *is* the base image. +// +// When we create the GlobalVariable for the __ref_* variable, we must create +// it as a *non-constant* global variable. The __ref_* pointers will not change +// during the runtime of the program, so it is tempting to think that they +// should be constant. However, they still need to be updateable by the +// hot-patching infrastructure. Also, if the GlobalVariable is created as a +// constant, then the LLVM optimizer will assume that it can dereference the +// definition of the __ref_* variable at compile time, which defeats the +// purpose of the indirection (pointer). +// +// The RefMapping table spans the entire module, not just a single function. +static GlobalVariable *getOrCreateRefVariable( + Function &F, SmallDenseMap<GlobalVariable *, GlobalVariable *> &RefMapping, + GlobalVariable *GV) { + GlobalVariable *&ReplaceWithRefGV = RefMapping.try_emplace(GV).first->second; + if (ReplaceWithRefGV != nullptr) { + // We have already created a __ref_* pointer for this GlobalVariable. + return ReplaceWithRefGV; + } + + Module *M = F.getParent(); + + const DISubprogram *Subprogram = F.getSubprogram(); + DICompileUnit *Unit = Subprogram != nullptr ? Subprogram->getUnit() : nullptr; + DIFile *File = Subprogram != nullptr ? Subprogram->getFile() : nullptr; + DIBuilder DebugInfo{*F.getParent(), true, Unit}; + + auto PtrTy = PointerType::get(M->getContext(), 0); + + Constant *AddrOfOldGV = + ConstantExpr::getGetElementPtr(PtrTy, GV, ArrayRef<Value *>{}); + + GlobalVariable *RefGV = + new GlobalVariable(*M, PtrTy, false, GlobalValue::LinkOnceAnyLinkage, + AddrOfOldGV, Twine("__ref_").concat(GV->getName()), + nullptr, GlobalVariable::NotThreadLocal); + + // Create debug info for the replacement global variable. + DataLayout Layout = M->getDataLayout(); + DIType *DebugType = DebugInfo.createPointerType( + nullptr, Layout.getTypeSizeInBits(GV->getValueType())); + DIGlobalVariableExpression *GVE = DebugInfo.createGlobalVariableExpression( + Unit, RefGV->getName(), StringRef{}, File, + /*LineNo*/ 0, DebugType, + /*IsLocalToUnit*/ false); + RefGV->addDebugInfo(GVE); + + // Store the __ref_* in RefMapping so that future calls use the same RefGV. + ReplaceWithRefGV = RefGV; + + return RefGV; +} + +// Given a ConstantExpr, this searches for GlobalVariable references within +// the expression tree. If found, it will generate instructions and will +// return a non-null Value* that points to the new root instruction. +// +// If C does not contain any GlobalVariable references, this returns nullptr. +// +// If this function creates new instructions, then it will insert them +// before InsertionPoint. +static Value *rewriteGlobalVariablesInConstant( + Constant *C, Instruction *InsertionPoint, + SmallDenseMap<GlobalVariable *, Value *> &GVLoadMap) { + if (C->getValueID() == Value::GlobalVariableVal) { + GlobalVariable *GV = cast<GlobalVariable>(C); + if (globalVariableNeedsRedirect(GV)) { + return GVLoadMap.at(GV); + } else { + return nullptr; + } + } + + // Scan the operands of this expression. + + SmallVector<Value *, 8> ReplacedValues; + bool ReplacedAnyOperands = false; + + unsigned NumOperands = C->getNumOperands(); + for (unsigned OpIndex = 0; OpIndex < NumOperands; ++OpIndex) { + Value *OldValue = C->getOperand(OpIndex); + Value *ReplacedValue = nullptr; + if (Constant *OldConstant = dyn_cast<Constant>(OldValue)) { + ReplacedValue = rewriteGlobalVariablesInConstant( + OldConstant, InsertionPoint, GVLoadMap); + } + // Do not use short-circuiting, here. We need to traverse the whole tree. + ReplacedAnyOperands |= ReplacedValue != nullptr; + ReplacedValues.push_back(ReplacedValue); + } + + // If none of our operands were replaced, then don't rewrite this expression. + if (!ReplacedAnyOperands) { + return nullptr; + } + + // We need to rewrite this expression. Convert this constant expression + // to an instruction, then replace any operands as needed. + Instruction *NewInst = cast<ConstantExpr>(C)->getAsInstruction(); + for (unsigned OpIndex = 0; OpIndex < NumOperands; ++OpIndex) { + Value *ReplacedValue = ReplacedValues[OpIndex]; + if (ReplacedValue != nullptr) { + NewInst->setOperand(OpIndex, ReplacedValue); + } + } + + // Insert the new instruction before the reference instruction. + IRBuilder<> Builder(InsertionPoint); + Builder.Insert(NewInst); + + return NewInst; +} + +// Processes a function that is marked for hot-patching. ---------------- dpaoliello wrote:
This comment seems out of place https://github.com/llvm/llvm-project/pull/138972 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
