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.

1 files changed, 5 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CodeGenModule.h b/clang/lib/CodeGen/CodeGenModule.h
index 1b67d43..cb013fe 100644
--- a/clang/lib/CodeGen/CodeGenModule.h
+++ b/clang/lib/CodeGen/CodeGenModule.h
@@ -678,6 +678,11 @@ private:
 
   AtomicOptions AtomicOpts;
 
+  // A set of functions which should be hot-patched; see
+  // -fms-hotpatch-functions-file (and -list). This will nearly always be empty.
+  // The list is sorted for binary-searching.
+  std::vector<std::string> MSHotPatchFunctions;
+
 public:
   CodeGenModule(ASTContext &C, IntrusiveRefCntPtr<llvm::vfs::FileSystem> FS,
                 const HeaderSearchOptions &headersearchopts,