[PAC] Implement function pointer type discrimination (#96992)

Give users an option (-fptrauth-function-pointer-type-discrimination) to
sign a function pointer using a non-zero discriminator based on the
function type.

The discriminator is computed by first translating the function type to
a string and then computing the hash value of the string. Two function
types that are compatible in C must be translated to the same string
with the exception of function types that use typedefs of anonymous
structs in their return type or parameter types.

This patch doesn't have the code to resign function pointers, which is
needed when a function pointer is converted to a different function
type. That will be implemented in another patch.

Co-authored-by: John McCall <rjmccall@apple.com>

---------

Co-authored-by: John McCall <rjmccall@apple.com>

1 files changed, 23 insertions, 9 deletions

diff --git a/clang/lib/CodeGen/CGExprConstant.cpp b/clang/lib/CodeGen/CGExprConstant.cpp
index 1fec587..00a5a7e 100644
--- a/clang/lib/CodeGen/CGExprConstant.cpp
+++ b/clang/lib/CodeGen/CGExprConstant.cpp
@@ -1542,9 +1542,11 @@ ConstantEmitter::emitAbstract(const Expr *E, QualType destType) {
 
 llvm::Constant *
 ConstantEmitter::emitAbstract(SourceLocation loc, const APValue &value,
-                              QualType destType) {
+                              QualType destType,
+                              bool EnablePtrAuthFunctionTypeDiscrimination) {
   auto state = pushAbstract();
-  auto C = tryEmitPrivate(value, destType);
+  auto C =
+      tryEmitPrivate(value, destType, EnablePtrAuthFunctionTypeDiscrimination);
   C = validateAndPopAbstract(C, state);
   if (!C) {
     CGM.Error(loc,
@@ -1938,14 +1940,18 @@ class ConstantLValueEmitter : public ConstStmtVisitor<ConstantLValueEmitter,
   ConstantEmitter &Emitter;
   const APValue &Value;
   QualType DestType;
+  bool EnablePtrAuthFunctionTypeDiscrimination;
 
   // Befriend StmtVisitorBase so that we don't have to expose Visit*.
   friend StmtVisitorBase;
 
 public:
   ConstantLValueEmitter(ConstantEmitter &emitter, const APValue &value,
-                        QualType destType)
-    : CGM(emitter.CGM), Emitter(emitter), Value(value), DestType(destType) {}
+                        QualType destType,
+                        bool EnablePtrAuthFunctionTypeDiscrimination = true)
+      : CGM(emitter.CGM), Emitter(emitter), Value(value), DestType(destType),
+        EnablePtrAuthFunctionTypeDiscrimination(
+            EnablePtrAuthFunctionTypeDiscrimination) {}
 
   llvm::Constant *tryEmit();
 
@@ -2069,7 +2075,10 @@ ConstantLValueEmitter::tryEmitBase(const APValue::LValueBase &base) {
       return CGM.GetWeakRefReference(D).getPointer();
 
     auto PtrAuthSign = [&](llvm::Constant *C) {
-      CGPointerAuthInfo AuthInfo = CGM.getFunctionPointerAuthInfo(DestType);
+      CGPointerAuthInfo AuthInfo;
+
+      if (EnablePtrAuthFunctionTypeDiscrimination)
+        AuthInfo = CGM.getFunctionPointerAuthInfo(DestType);
 
       if (AuthInfo) {
         if (hasNonZeroOffset())
@@ -2220,8 +2229,10 @@ llvm::Constant *ConstantLValueEmitter::emitPointerAuthPointer(const Expr *E) {
 
   // The assertions here are all checked by Sema.
   assert(Result.Val.isLValue());
+  if (isa<FunctionDecl>(Result.Val.getLValueBase().get<const ValueDecl *>()))
+    assert(Result.Val.getLValueOffset().isZero());
   return ConstantEmitter(CGM, Emitter.CGF)
-      .emitAbstract(E->getExprLoc(), Result.Val, E->getType());
+      .emitAbstract(E->getExprLoc(), Result.Val, E->getType(), false);
 }
 
 unsigned ConstantLValueEmitter::emitPointerAuthKey(const Expr *E) {
@@ -2278,15 +2289,18 @@ ConstantLValueEmitter::VisitMaterializeTemporaryExpr(
   return CGM.GetAddrOfGlobalTemporary(E, Inner);
 }
 
-llvm::Constant *ConstantEmitter::tryEmitPrivate(const APValue &Value,
-                                                QualType DestType) {
+llvm::Constant *
+ConstantEmitter::tryEmitPrivate(const APValue &Value, QualType DestType,
+                                bool EnablePtrAuthFunctionTypeDiscrimination) {
   switch (Value.getKind()) {
   case APValue::None:
   case APValue::Indeterminate:
     // Out-of-lifetime and indeterminate values can be modeled as 'undef'.
     return llvm::UndefValue::get(CGM.getTypes().ConvertType(DestType));
   case APValue::LValue:
-    return ConstantLValueEmitter(*this, Value, DestType).tryEmit();
+    return ConstantLValueEmitter(*this, Value, DestType,
+                                 EnablePtrAuthFunctionTypeDiscrimination)
+        .tryEmit();
   case APValue::Int:
     return llvm::ConstantInt::get(CGM.getLLVMContext(), Value.getInt());
   case APValue::FixedPoint: