[AArch64] Set execute-only flags in .text.

[ivanlozano]

Summary

The initial .text section generated in object files was missing the SHF_ARM_PURECODE flag when being built with the -mexecute-only flag. All code sections of an ELF must have the flag set for the final .text section to be execute-only, otherwise the flag gets removed.

[ivanlozano]

Alternative solution committed: rL338271

lib/Target/AArch64/AArch64TargetObjectFile.cpp

 //===-- AArch64TargetObjectFile.cpp - AArch64 Object Info -----------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #include "AArch64TargetObjectFile.h"
 #include "AArch64TargetMachine.h"
 #include "llvm/BinaryFormat/Dwarf.h"
+#include "llvm/BinaryFormat/ELF.h"
 #include "llvm/IR/Mangler.h"
 #include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCExpr.h"
 #include "llvm/MC/MCSectionELF.h"
 #include "llvm/MC/MCStreamer.h"
 #include "llvm/MC/MCValue.h"
 using namespace llvm;
 using namespace dwarf;
 
 void AArch64_ELFTargetObjectFile::Initialize(MCContext &Ctx,
                                              const TargetMachine &TM) {
   TargetLoweringObjectFileELF::Initialize(Ctx, TM);
   InitializeELF(TM.Options.UseInitArray);
+
+  bool genExecuteOnly = TM.getMCSubtargetInfo()->hasFeature(AArch64::FeatureExecuteOnly);
+
+  // Make code section unreadable when in execute-only mode
+  if (genExecuteOnly) {
+    unsigned  Type = ELF::SHT_PROGBITS;
+    unsigned Flags = ELF::SHF_EXECINSTR | ELF::SHF_ALLOC | ELF::SHF_ARM_PURECODE;
+    // Since we cannot modify flags for an existing section, we create a new
+    // section with the right flags, and use 0 as the unique ID for
+    // execute-only text
+    TextSection = Ctx.getELFSection(".text", Type, Flags, 0, "", 0U);
+  }
 }
 
 AArch64_MachoTargetObjectFile::AArch64_MachoTargetObjectFile()
   : TargetLoweringObjectFileMachO() {
   SupportGOTPCRelWithOffset = false;
 }
 
 const MCExpr *AArch64_MachoTargetObjectFile::getTTypeGlobalReference(

test/MC/AArch64/elf-execute-only-section.ll

+; RUN: llc < %s -mtriple=aarch64 -mattr=+execute-only -filetype=obj %s -o - |\
+; RUN: llvm-readelf -s | FileCheck %s
+
+; CHECK-NOT: {{.text[ ]+PROGBITS[ ]+[0-9]+ [0-9]+ [0-9]+ [0-9]+ AX[^p]}}
+; CHECK: {{.text[ ]+PROGBITS[ ]+[0-9]+ [0-9]+ [0-9]+ [0-9]+ AXp}}
+define void @test_func() {
+entry:
+  ret void
+}