[ARM] Implement execute-only support in CodeGen

This implements execute-only support for ARM code generation, which
prevents the compiler from generating data accesses to code sections.
The following changes are involved:

* Add the CodeGen option "-arm-execute-only" to the ARM code generator.
* Add the clang flag "-mexecute-only" as well as the GCC-compatible
  alias "-mpure-code" to enable this option.
* When enabled, literal pools are replaced with MOVW/MOVT instructions,
  with VMOV used in addition for floating-point literals. As the MOVT
  instruction is required, execute-only support is only available in
  Thumb mode for targets supporting ARMv8-M baseline or Thumb2.
* Jump tables are placed in data sections when in execute-only mode.
* The execute-only text section is assigned section ID 0, and is
  marked as unreadable with the SHF_ARM_PURECODE flag with symbol 'y'.
  This also overrides selection of ELF sections for globals.

llvm-svn: 289784

1 files changed, 34 insertions, 2 deletions

diff --git a/llvm/lib/Target/ARM/ARMTargetObjectFile.cpp b/llvm/lib/Target/ARM/ARMTargetObjectFile.cpp
index ec364e8..625c428 100644
--- a/llvm/lib/Target/ARM/ARMTargetObjectFile.cpp
+++ b/llvm/lib/Target/ARM/ARMTargetObjectFile.cpp
@@ -27,8 +27,10 @@ using namespace dwarf;
 
 void ARMElfTargetObjectFile::Initialize(MCContext &Ctx,
                                         const TargetMachine &TM) {
-  bool isAAPCS_ABI = static_cast<const ARMTargetMachine &>(TM).TargetABI ==
-                     ARMTargetMachine::ARMABI::ARM_ABI_AAPCS;
+  const ARMTargetMachine &ARM_TM = static_cast<const ARMTargetMachine &>(TM);
+  bool isAAPCS_ABI = ARM_TM.TargetABI == ARMTargetMachine::ARMABI::ARM_ABI_AAPCS;
+  genExecuteOnly = ARM_TM.getSubtargetImpl()->genExecuteOnly();
+
   TargetLoweringObjectFileELF::Initialize(Ctx, TM);
   InitializeELF(isAAPCS_ABI);
 
@@ -38,6 +40,16 @@ void ARMElfTargetObjectFile::Initialize(MCContext &Ctx,
 
   AttributesSection =
       getContext().getELFSection(".ARM.attributes", ELF::SHT_ARM_ATTRIBUTES, 0);
+
+  // 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);
+  }
 }
 
 const MCExpr *ARMElfTargetObjectFile::getTTypeGlobalReference(
@@ -58,3 +70,23 @@ getDebugThreadLocalSymbol(const MCSymbol *Sym) const {
   return MCSymbolRefExpr::create(Sym, MCSymbolRefExpr::VK_ARM_TLSLDO,
                                  getContext());
 }
+
+MCSection *
+ARMElfTargetObjectFile::getExplicitSectionGlobal(const GlobalObject *GO,
+                                                 SectionKind SK, const TargetMachine &TM) const {
+  // Set execute-only access for the explicit section
+  if (genExecuteOnly && SK.isText())
+    SK = SectionKind::getExecuteOnly();
+
+  return TargetLoweringObjectFileELF::getExplicitSectionGlobal(GO, SK, TM);
+}
+
+MCSection *
+ARMElfTargetObjectFile::SelectSectionForGlobal(const GlobalObject *GO,
+                                               SectionKind SK, const TargetMachine &TM) const {
+  // Place the global in the execute-only text section
+  if (genExecuteOnly && SK.isText())
+    SK = SectionKind::getExecuteOnly();
+
+  return TargetLoweringObjectFileELF::SelectSectionForGlobal(GO, SK, TM);
+}