[KernelAddressSanitizer] Make globals constructors compatible with kernel

Summary:
This makes -fsanitize=kernel-address emit the correct globals
constructors for the kernel. We had to do the following:

- Disable generation of constructors that rely on linker features such
  as dead-global elimination.

- Only emit constructors for globals *not* in explicit sections. The
  kernel uses sections for special globals, which we should not touch.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=203493

Tested:
1. With 'clang/test/CodeGen/asan-globals.cpp'.
2. With test_kasan.ko, we can see:

  	BUG: KASAN: global-out-of-bounds in kasan_global_oob+0xb3/0xba [test_kasan]

Reviewers: glider, andreyknvl

Reviewed By: glider

Subscribers: cfe-commits, nickdesaulniers, hiraditya, llvm-commits

Tags: #llvm, #clang

Differential Revision: https://reviews.llvm.org/D80805

1 files changed, 11 insertions, 5 deletions

diff --git a/llvm/lib/Transforms/Utils/ModuleUtils.cpp b/llvm/lib/Transforms/Utils/ModuleUtils.cpp
index b2e6d98..ef9f18a 100644
--- a/llvm/lib/Transforms/Utils/ModuleUtils.cpp
+++ b/llvm/lib/Transforms/Utils/ModuleUtils.cpp
@@ -119,6 +119,15 @@ llvm::declareSanitizerInitFunction(Module &M, StringRef InitName,
       AttributeList());
 }
 
+Function *llvm::createSanitizerCtor(Module &M, StringRef CtorName) {
+  Function *Ctor = Function::Create(
+      FunctionType::get(Type::getVoidTy(M.getContext()), false),
+      GlobalValue::InternalLinkage, CtorName, &M);
+  BasicBlock *CtorBB = BasicBlock::Create(M.getContext(), "", Ctor);
+  ReturnInst::Create(M.getContext(), CtorBB);
+  return Ctor;
+}
+
 std::pair<Function *, FunctionCallee> llvm::createSanitizerCtorAndInitFunctions(
     Module &M, StringRef CtorName, StringRef InitName,
     ArrayRef<Type *> InitArgTypes, ArrayRef<Value *> InitArgs,
@@ -128,11 +137,8 @@ std::pair<Function *, FunctionCallee> llvm::createSanitizerCtorAndInitFunctions(
          "Sanitizer's init function expects different number of arguments");
   FunctionCallee InitFunction =
       declareSanitizerInitFunction(M, InitName, InitArgTypes);
-  Function *Ctor = Function::Create(
-      FunctionType::get(Type::getVoidTy(M.getContext()), false),
-      GlobalValue::InternalLinkage, CtorName, &M);
-  BasicBlock *CtorBB = BasicBlock::Create(M.getContext(), "", Ctor);
-  IRBuilder<> IRB(ReturnInst::Create(M.getContext(), CtorBB));
+  Function *Ctor = createSanitizerCtor(M, CtorName);
+  IRBuilder<> IRB(Ctor->getEntryBlock().getTerminator());
   IRB.CreateCall(InitFunction, InitArgs);
   if (!VersionCheckName.empty()) {
     FunctionCallee VersionCheckFunction = M.getOrInsertFunction(