Fix UB in LLVM FFI when passing zero or >1 bundle

Rust passes a *const &OperandBundleDef to these APIs, usually from a
Vec<&OperandBundleDef> or so. Previously we were dereferencing that
pointer and passing it to the ArrayRef constructor with some length (N).

This meant that if the length was 0, we were dereferencing a pointer to
nowhere, and if the length was >1 then loading the *second* element
somewhere in LLVM would've been reading past the end.

Since Rust can't hold OperandBundleDef by-value we're forced to indirect
through a vector that copies out the OperandBundleDefs from the
by-reference list on the Rust side in order to match the LLVM expected
API.

Author: Mark-Simulacrum

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -1524,13 +1524,21 @@ extern "C" void LLVMRustFreeOperandBundleDef(OperandBundleDef *Bundle) {
 
 extern "C" LLVMValueRef LLVMRustBuildCall(LLVMBuilderRef B, LLVMTypeRef Ty, LLVMValueRef Fn,
                                           LLVMValueRef *Args, unsigned NumArgs,
-                                          OperandBundleDef **OpBundles,
+                                          OperandBundleDef **OpBundlesIndirect,
                                           unsigned NumOpBundles) {
   Value *Callee = unwrap(Fn);
   FunctionType *FTy = unwrap<FunctionType>(Ty);
+
+  // FIXME: Is there a way around this?
+  SmallVector<OperandBundleDef> OpBundles;
+  OpBundles.reserve(NumOpBundles);
+  for (unsigned i = 0; i < NumOpBundles; ++i) {
+    OpBundles.push_back(*OpBundlesIndirect[i]);
+  }
+
   return wrap(unwrap(B)->CreateCall(
       FTy, Callee, ArrayRef<Value*>(unwrap(Args), NumArgs),
-      ArrayRef<OperandBundleDef>(*OpBundles, NumOpBundles)));
+      ArrayRef<OperandBundleDef>(OpBundles)));
 }
 
 extern "C" LLVMValueRef LLVMRustGetInstrProfIncrementIntrinsic(LLVMModuleRef M) {
@@ -1570,13 +1578,21 @@ extern "C" LLVMValueRef
 LLVMRustBuildInvoke(LLVMBuilderRef B, LLVMTypeRef Ty, LLVMValueRef Fn,
                     LLVMValueRef *Args, unsigned NumArgs,
                     LLVMBasicBlockRef Then, LLVMBasicBlockRef Catch,
-                    OperandBundleDef **OpBundles, unsigned NumOpBundles,
+                    OperandBundleDef **OpBundlesIndirect, unsigned NumOpBundles,
                     const char *Name) {
   Value *Callee = unwrap(Fn);
   FunctionType *FTy = unwrap<FunctionType>(Ty);
+
+  // FIXME: Is there a way around this?
+  SmallVector<OperandBundleDef> OpBundles;
+  OpBundles.reserve(NumOpBundles);
+  for (unsigned i = 0; i < NumOpBundles; ++i) {
+    OpBundles.push_back(*OpBundlesIndirect[i]);
+  }
+
   return wrap(unwrap(B)->CreateInvoke(FTy, Callee, unwrap(Then), unwrap(Catch),
                                       ArrayRef<Value*>(unwrap(Args), NumArgs),
-                                      ArrayRef<OperandBundleDef>(*OpBundles, NumOpBundles),
+                                      ArrayRef<OperandBundleDef>(OpBundles),
                                       Name));
 }
 
@@ -1585,7 +1601,7 @@ LLVMRustBuildCallBr(LLVMBuilderRef B, LLVMTypeRef Ty, LLVMValueRef Fn,
                     LLVMBasicBlockRef DefaultDest,
                     LLVMBasicBlockRef *IndirectDests, unsigned NumIndirectDests,
                     LLVMValueRef *Args, unsigned NumArgs,
-                    OperandBundleDef **OpBundles, unsigned NumOpBundles,
+                    OperandBundleDef **OpBundlesIndirect, unsigned NumOpBundles,
                     const char *Name) {
   Value *Callee = unwrap(Fn);
   FunctionType *FTy = unwrap<FunctionType>(Ty);
@@ -1597,11 +1613,18 @@ LLVMRustBuildCallBr(LLVMBuilderRef B, LLVMTypeRef Ty, LLVMValueRef Fn,
     IndirectDestsUnwrapped.push_back(unwrap(IndirectDests[i]));
   }
 
+  // FIXME: Is there a way around this?
+  SmallVector<OperandBundleDef> OpBundles;
+  OpBundles.reserve(NumOpBundles);
+  for (unsigned i = 0; i < NumOpBundles; ++i) {
+    OpBundles.push_back(*OpBundlesIndirect[i]);
+  }
+
   return wrap(unwrap(B)->CreateCallBr(
       FTy, Callee, unwrap(DefaultDest),
       ArrayRef<BasicBlock*>(IndirectDestsUnwrapped),
       ArrayRef<Value*>(unwrap(Args), NumArgs),
-      ArrayRef<OperandBundleDef>(*OpBundles, NumOpBundles),
+      ArrayRef<OperandBundleDef>(OpBundles),
       Name));
 }