From 26d318e4a9437f95b6a2e7abace5f2b867c88a3e Mon Sep 17 00:00:00 2001
From: sivadeilra <ardavis@microsoft.com>
Date: Tue, 24 Jun 2025 09:22:38 -0700
Subject: Add support for Windows Secure Hot-Patching (#138972)

This PR adds some of the support needed for Windows hot-patching.

Windows implements a form of hot-patching. This allows patches to be
applied to Windows apps, drivers, and the kernel, without rebooting or
restarting any of these components. Hot-patching is a complex technology
and requires coordination between the OS, compilers, linkers, and
additional tools.

This PR adds support to Clang and LLVM for part of the hot-patching
process. It enables LLVM to generate the required code changes and to
generate CodeView symbols which identify hot-patched functions. The PR
provides new command-line arguments to Clang which allow developers to
identify the list of functions that need to be hot-patched. This PR also
allows LLVM to directly receive the list of functions to be modified, so
that language front-ends which have not yet been modified (such as Rust)
can still make use of hot-patching.

This PR:

* Adds a `MarkedForWindowsHotPatching` LLVM function attribute. This
attribute indicates that a function should be _hot-patched_. This
generates a new CodeView symbol, `S_HOTPATCHFUNC`, which identifies any
function that has been hot-patched. This attribute also causes accesses
to global variables to be indirected through a `_ref_*` global variable.
This allows hot-patched functions to access the correct version of a
global variable; the hot-patched code needs to access the variable in
the _original_ image, not the patch image.
* Adds a `AllowDirectAccessInHotPatchFunction` LLVM attribute. This
attribute may be placed on global variable declarations. It indicates
that the variable may be safely accessed without the `_ref_*`
indirection.
* Adds two Clang command-line parameters: `-fms-hotpatch-functions-file`
and `-fms-hotpatch-functions-list`. The `-file` flag may point to a text
file, which contains a list of functions to be hot-patched (one function
name per line). The `-list` flag simply directly identifies functions to
be patched, using a comma-separated list. These two command-line
parameters may also be combined; the final set of functions to be
hot-patched is the union of the two sets.
* Adds similar LLVM command-line parameters:
`--ms-hotpatch-functions-file` and `--ms-hotpatch-functions-list`.
* Adds integration tests for both LLVM and Clang.
* Adds support for dumping the new `S_HOTPATCHFUNC` CodeView symbol.

Although the flags are redundant between Clang and LLVM, this allows
additional languages (such as Rust) to take advantage of hot-patching
support before they have been modified to generate the required
attributes.

Credit to @dpaoliello, who wrote the original form of this patch.
---
 clang/lib/CodeGen/CodeGenModule.cpp | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

(limited to 'clang/lib/CodeGen/CodeGenModule.cpp')

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 1668881..96fdab2 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -458,6 +458,35 @@ CodeGenModule::CodeGenModule(ASTContext &C,
   if (Context.getTargetInfo().getTriple().getArch() == llvm::Triple::x86)
     getModule().addModuleFlag(llvm::Module::Error, "NumRegisterParameters",
                               CodeGenOpts.NumRegisterParameters);
+
+  // If there are any functions that are marked for Windows secure hot-patching,
+  // then build the list of functions now.
+  if (!CGO.MSSecureHotPatchFunctionsFile.empty() ||
+      !CGO.MSSecureHotPatchFunctionsList.empty()) {
+    if (!CGO.MSSecureHotPatchFunctionsFile.empty()) {
+      auto BufOrErr =
+          llvm::MemoryBuffer::getFile(CGO.MSSecureHotPatchFunctionsFile);
+      if (BufOrErr) {
+        const llvm::MemoryBuffer &FileBuffer = **BufOrErr;
+        for (llvm::line_iterator I(FileBuffer.getMemBufferRef(), true), E;
+             I != E; ++I)
+          this->MSHotPatchFunctions.push_back(std::string{*I});
+      } else {
+        auto &DE = Context.getDiagnostics();
+        unsigned DiagID =
+            DE.getCustomDiagID(DiagnosticsEngine::Error,
+                               "failed to open hotpatch functions file "
+                               "(-fms-hotpatch-functions-file): %0 : %1");
+        DE.Report(DiagID) << CGO.MSSecureHotPatchFunctionsFile
+                          << BufOrErr.getError().message();
+      }
+    }
+
+    for (const auto &FuncName : CGO.MSSecureHotPatchFunctionsList)
+      this->MSHotPatchFunctions.push_back(FuncName);
+
+    llvm::sort(this->MSHotPatchFunctions);
+  }
 }
 
 CodeGenModule::~CodeGenModule() {}
-- 
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