Do not set __guard_local to hidden for OpenBSD SSP

[sisnkemp]

Summary

\_\_guard_local is defined as long on OpenBSD. If the source file contains
a definition of \_\_guard_local, it mismatches with the int8 pointer type
used in LLVM. In that case, Module::getOrInsertGlobal() returns a
cast operation instead of a GlobalVariable. Trying to set the
visibility on the cast operation leads to random segfaults (seen when
compiling the OpenBSD kernel, which also runs with stack protection).

In the kernel, the hidden attribute does not matter. For userspace code,
\_\_guard_local is defined as hidden in the startup code. If a program
re-defines \_\_guard_local, the definition from the startup code will
either win or the linker complains about multiple definitions
(depending on whether the re-defined \_\_guard_local is placed in the
common segment or not).

It also matches what gcc on OpenBSD does.

[timshen]

• Do you have a reduced testcase that produces the crash?
• Would it be better to call M.getGlobalVariable(), assert on the returned value, and then create an IR cast on the returned GlobalValue?

[timshen]

In D23674#519922, @timshen wrote:

• Would it be better to call M.getGlobalVariable(), assert on the returned value, and then create an IR cast on the returned GlobalValue?

Never mind, I thought __guard_local is already declared by the time getIRStackGuard is called, but it seems not the case.

llvm/trunk/lib/CodeGen/TargetLoweringBase.cpp

 //===----------------------------------------------------------------------===//
 
 // For OpenBSD return its special guard variable. Otherwise return nullptr,
 // so that SelectionDAG handle SSP.
 Value *TargetLoweringBase::getIRStackGuard(IRBuilder<> &IRB) const {
   if (getTargetMachine().getTargetTriple().isOSOpenBSD()) {
     Module &M = *IRB.GetInsertBlock()->getParent()->getParent();
     PointerType *PtrTy = Type::getInt8PtrTy(M.getContext());
-    auto Guard = cast<GlobalValue>(M.getOrInsertGlobal("__guard_local", PtrTy));
-    Guard->setVisibility(GlobalValue::HiddenVisibility);
-    return Guard;
+    return M.getOrInsertGlobal("__guard_local", PtrTy);
   }
   return nullptr;
 }
 
 // Currently only support "standard" __stack_chk_guard.
 // TODO: add LOAD_STACK_GUARD support.
 void TargetLoweringBase::insertSSPDeclarations(Module &M) const {
   M.getOrInsertGlobal("__stack_chk_guard", Type::getInt8PtrTy(M.getContext()));