Add support for __declspec(guard(nocf))

Summary:
Avoid using the `nocf_check` attribute with Control Flow Guard. Instead, use a
new `"guard_nocf"` function attribute to indicate that checks should not be
added on indirect calls within that function. Add support for
`__declspec(guard(nocf))` following the same syntax as MSVC.

Reviewers: rnk, dmajor, pcc, hans, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: aaron.ballman, tomrittervg, hiraditya, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D72167

2 files changed, 33 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CGCall.cpp b/clang/lib/CodeGen/CGCall.cpp
index b49b194..e4803fd 100644
--- a/clang/lib/CodeGen/CGCall.cpp
+++ b/clang/lib/CodeGen/CGCall.cpp
@@ -4415,6 +4415,17 @@ RValue CodeGenFunction::EmitCall(const CGFunctionInfo &CallInfo,
   if (callOrInvoke)
     *callOrInvoke = CI;
 
+  // If this is within a function that has the guard(nocf) attribute and is an
+  // indirect call, add the "guard_nocf" attribute to this call to indicate that
+  // Control Flow Guard checks should not be added, even if the call is inlined.
+  if (const auto *FD = dyn_cast_or_null<FunctionDecl>(CurFuncDecl)) {
+    if (const auto *A = FD->getAttr<CFGuardAttr>()) {
+      if (A->getGuard() == CFGuardAttr::GuardArg::nocf && !CI->getCalledFunction())
+        Attrs = Attrs.addAttribute(
+            getLLVMContext(), llvm::AttributeList::FunctionIndex, "guard_nocf");
+    }
+  }
+
   // Apply the attributes and calling convention.
   CI->setAttributes(Attrs);
   CI->setCallingConv(static_cast<llvm::CallingConv::ID>(CallingConv));
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index 7f1da40..142c6f1 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -6626,6 +6626,25 @@ static void handleHandleAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   D->addAttr(Attr::Create(S.Context, Argument, AL));
 }
 
+static void handleCFGuardAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
+  // The guard attribute takes a single identifier argument.
+
+  if (!AL.isArgIdent(0)) {
+    S.Diag(AL.getLoc(), diag::err_attribute_argument_type)
+        << AL << AANT_ArgumentIdentifier;
+    return;
+  }
+
+  CFGuardAttr::GuardArg Arg;
+  IdentifierInfo *II = AL.getArgAsIdent(0)->Ident;
+  if (!CFGuardAttr::ConvertStrToGuardArg(II->getName(), Arg)) {
+    S.Diag(AL.getLoc(), diag::warn_attribute_type_not_supported) << AL << II;
+    return;
+  }
+
+  D->addAttr(::new (S.Context) CFGuardAttr(S.Context, AL, Arg));
+}
+
 //===----------------------------------------------------------------------===//
 // Top Level Sema Entry Points
 //===----------------------------------------------------------------------===//
@@ -7254,6 +7273,9 @@ static void ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D,
   case ParsedAttr::AT_AbiTag:
     handleAbiTagAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_CFGuard:
+    handleCFGuardAttr(S, D, AL);
+    break;
 
   // Thread safety attributes:
   case ParsedAttr::AT_AssertExclusiveLock: