[x86][harden-sls] insert int3 trap after __x86_return_thunk jump

[nickdesaulniers]

Summary

This ensures that -mharden-sls=all -mfunction-return=thunk-extern
compose well.

llvm/lib/Target/X86/X86AsmPrinter.cpp

return MI.getDesc().isIndirectBranch() /*Make below code in a good shape*/ ||
          Opc == X86::TCRETURNri64 || Opc == X86::TCRETURNmi64 ||
          Opc == X86::TAILJMPr64_REX || Opc == X86::TAILJMPm64_REX;
 }
 
 void X86AsmPrinter::emitBasicBlockEnd(const MachineBasicBlock &MBB) {
   if (Subtarget->hardenSlsRet() || Subtarget->hardenSlsIJmp()) {
     auto I = MBB.getLastNonDebugInstr();
     if (I != MBB.end()) {
-      if ((Subtarget->hardenSlsRet() && isSimpleReturn(*I)) ||
+      if ((Subtarget->hardenSlsRet() &&
+           (isSimpleReturn(*I) ||
+            (I->getOpcode() == X86::TAILJMPd && I->getOperand(0).isSymbol() &&
+             !strcmp(I->getOperand(0).getSymbolName(),
+                     "__x86_return_thunk")))) ||
           (Subtarget->hardenSlsIJmp() && isIndirectBranchOrTailCall(*I))) {
         MCInst TmpInst;
         TmpInst.setOpcode(X86::INT3);
         EmitToStreamer(*OutStreamer, TmpInst);
       }
     }
   }
   AsmPrinter::emitBasicBlockEnd(MBB);