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.
---
 llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

(limited to 'llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp')

diff --git a/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
index ea57a8f..5e1b313 100644
--- a/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
@@ -669,6 +669,8 @@ void CodeViewDebug::endModule() {
   if (!Asm)
     return;
 
+  emitSecureHotPatchInformation();
+
   emitInlineeLinesSubsection();
 
   // Emit per-function debug information.
@@ -823,6 +825,28 @@ void CodeViewDebug::emitObjName() {
   endSymbolRecord(CompilerEnd);
 }
 
+void CodeViewDebug::emitSecureHotPatchInformation() {
+  MCSymbol *hotPatchInfo = nullptr;
+
+  for (const auto &F : MMI->getModule()->functions()) {
+    if (!F.isDeclarationForLinker() &&
+        F.hasFnAttribute("marked_for_windows_hot_patching")) {
+      if (hotPatchInfo == nullptr)
+        hotPatchInfo = beginCVSubsection(DebugSubsectionKind::Symbols);
+      MCSymbol *HotPatchEnd = beginSymbolRecord(SymbolKind::S_HOTPATCHFUNC);
+      auto *SP = F.getSubprogram();
+      OS.AddComment("Function");
+      OS.emitInt32(getFuncIdForSubprogram(SP).getIndex());
+      OS.AddComment("Name");
+      emitNullTerminatedSymbolName(OS, F.getName());
+      endSymbolRecord(HotPatchEnd);
+    }
+  }
+
+  if (hotPatchInfo != nullptr)
+    endCVSubsection(hotPatchInfo);
+}
+
 namespace {
 struct Version {
   int Part[4];
-- 
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