[llvm][StackProtector] Add noreturn to __stack_chk_fail aliassee

It's possible for __stack_chk_fail to be an alias when using CrossDSOCFI
since it will make a jump table entry for this function and replace it
with an alias. StackProtector can crash since it always expects this to
be a regular function. I think it should be safe to continue treating
this as an alias since we only call into it, and we can apply the
noreturn attr to the underlying function if it is an alias.

Author: PiJoules

llvm/lib/CodeGen/StackProtector.cpp

@@ -725,8 +725,8 @@ BasicBlock *CreateFailBB(Function *F, const Triple &Trip) {
     StackChkFail =
         M->getOrInsertFunction("__stack_chk_fail", Type::getVoidTy(Context));
   }
-  cast<Function>(StackChkFail.getCallee())->addFnAttr(Attribute::NoReturn);
-  B.CreateCall(StackChkFail, Args);
+  CallInst *Call = B.CreateCall(StackChkFail, Args);
+  Call->addFnAttr(Attribute::NoReturn);
   B.CreateUnreachable();
   return FailBB;
 }

llvm/test/Transforms/StackProtector/stack-chk-fail-alias.ll

@@ -0,0 +1,21 @@
+;; __stack_chk_fail should have the noreturn attr even if it is an alias
+; RUN: opt -mtriple=x86_64-pc-linux-gnu %s -passes=stack-protector -S | FileCheck %s
+
+define hidden void @__stack_chk_fail_impl() {
+  unreachable
+}
+
+@__stack_chk_fail = hidden alias void (), ptr @__stack_chk_fail_impl
+
+; CHECK-LABEL: @store_captures(
+; CHECK:       CallStackCheckFailBlk:
+; CHECK-NEXT:      call void @__stack_chk_fail() [[ATTRS:#.*]]
+define void @store_captures() sspstrong {
+entry:
+  %a = alloca i32, align 4
+  %j = alloca ptr, align 8
+  store ptr %a, ptr %j, align 8
+  ret void
+}
+
+; CHECK: attributes [[ATTRS]] = { noreturn }