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.

A HasData flag is added to MCSection to aid in the determination that
the section is empty. A virtual setTargetSectionFlags is added to
MCELFObjectTargetWriter to allow subclasses to set target specific
section flags to be added to sections which we then use in the ARM
backend to set SHF_ARM_PURECODE.

Patch by Ivan Lozano!

Reviewed By: echristo

Differential Revision: https://reviews.llvm.org/D48792

llvm-svn: 341593

6 files changed, 45 insertions, 2 deletions

diff --git a/llvm/lib/MC/ELFObjectWriter.cpp b/llvm/lib/MC/ELFObjectWriter.cpp
index 358c036f2a7..5b8b013ad08 100644
--- a/llvm/lib/MC/ELFObjectWriter.cpp
+++ b/llvm/lib/MC/ELFObjectWriter.cpp
@@ -29,6 +29,7 @@
 #include "llvm/MC/MCFixup.h"
 #include "llvm/MC/MCFixupKindInfo.h"
 #include "llvm/MC/MCFragment.h"
+#include "llvm/MC/MCObjectFileInfo.h"
 #include "llvm/MC/MCObjectWriter.h"
 #include "llvm/MC/MCSection.h"
 #include "llvm/MC/MCSectionELF.h"
@@ -1107,6 +1108,8 @@ uint64_t ELFWriter::writeObject(MCAssembler &Asm, const MCAsmLayout &Layout) {
       SectionIndexMap[RelSection] = addToSectionTable(RelSection);
       Relocations.push_back(RelSection);
     }
+
+    OWriter.TargetObjectWriter->addTargetSectionFlags(Ctx, Section);
   }
 
   MCSectionELF *CGProfileSection = nullptr;
diff --git a/llvm/lib/MC/MCELFObjectTargetWriter.cpp b/llvm/lib/MC/MCELFObjectTargetWriter.cpp
index 483ee94c0db..ff53dd7299c 100644
--- a/llvm/lib/MC/MCELFObjectTargetWriter.cpp
+++ b/llvm/lib/MC/MCELFObjectTargetWriter.cpp
@@ -26,3 +26,6 @@ void
 MCELFObjectTargetWriter::sortRelocs(const MCAssembler &Asm,
                                     std::vector<ELFRelocationEntry> &Relocs) {
 }
+
+void MCELFObjectTargetWriter::addTargetSectionFlags(MCContext &Ctx,
+                                                    MCSectionELF &Sec) {}
diff --git a/llvm/lib/MC/MCObjectStreamer.cpp b/llvm/lib/MC/MCObjectStreamer.cpp
index 019b9348a4e..8c88db009bd 100644
--- a/llvm/lib/MC/MCObjectStreamer.cpp
+++ b/llvm/lib/MC/MCObjectStreamer.cpp
@@ -496,6 +496,12 @@ void MCObjectStreamer::EmitBytes(StringRef Data) {
   MCDataFragment *DF = getOrCreateDataFragment();
   flushPendingLabels(DF, DF->getContents().size());
   DF->getContents().append(Data.begin(), Data.end());
+
+  // EmitBytes might not cover all possible ways we emit data (or could be used
+  // to emit executable code in some cases), but is the best method we have
+  // right now for checking this.
+  MCSection *Sec = getCurrentSectionOnly();
+  Sec->setHasData(true);
 }
 
 void MCObjectStreamer::EmitValueToAlignment(unsigned ByteAlignment,
diff --git a/llvm/lib/MC/MCSection.cpp b/llvm/lib/MC/MCSection.cpp
index 97bc65387dd..d4f11d10136 100644
--- a/llvm/lib/MC/MCSection.cpp
+++ b/llvm/lib/MC/MCSection.cpp
@@ -23,7 +23,8 @@ using namespace llvm;
 
 MCSection::MCSection(SectionVariant V, SectionKind K, MCSymbol *Begin)
     : Begin(Begin), BundleGroupBeforeFirstInst(false), HasInstructions(false),
-      IsRegistered(false), DummyFragment(this), Variant(V), Kind(K) {}
+      HasData(false), IsRegistered(false), DummyFragment(this), Variant(V),
+      Kind(K) {}
 
 MCSymbol *MCSection::getEndSymbol(MCContext &Ctx) {
   if (!End)
diff --git a/llvm/lib/Target/ARM/ARMTargetObjectFile.cpp b/llvm/lib/Target/ARM/ARMTargetObjectFile.cpp
index d0620761ea9..9c13359cba7 100644
--- a/llvm/lib/Target/ARM/ARMTargetObjectFile.cpp
+++ b/llvm/lib/Target/ARM/ARMTargetObjectFile.cpp
@@ -32,7 +32,8 @@ void ARMElfTargetObjectFile::Initialize(MCContext &Ctx,
                                         const TargetMachine &TM) {
   const ARMBaseTargetMachine &ARM_TM = static_cast<const ARMBaseTargetMachine &>(TM);
   bool isAAPCS_ABI = ARM_TM.TargetABI == ARMBaseTargetMachine::ARMABI::ARM_ABI_AAPCS;
-  //  genExecuteOnly = ARM_TM.getSubtargetImpl()->genExecuteOnly();
+  bool genExecuteOnly =
+      ARM_TM.getMCSubtargetInfo()->hasFeature(ARM::FeatureExecuteOnly);
 
   TargetLoweringObjectFileELF::Initialize(Ctx, TM);
   InitializeELF(isAAPCS_ABI);
@@ -40,6 +41,17 @@ void ARMElfTargetObjectFile::Initialize(MCContext &Ctx,
   if (isAAPCS_ABI) {
     LSDASection = nullptr;
   }
+
+  // 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(
diff --git a/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp b/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
index 7d04c73fb3f..b8ba7584911 100644
--- a/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
+++ b/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
@@ -14,6 +14,7 @@
 #include "llvm/MC/MCELFObjectWriter.h"
 #include "llvm/MC/MCExpr.h"
 #include "llvm/MC/MCFixup.h"
+#include "llvm/MC/MCObjectFileInfo.h"
 #include "llvm/MC/MCObjectWriter.h"
 #include "llvm/MC/MCValue.h"
 #include "llvm/Support/ErrorHandling.h"
@@ -40,6 +41,8 @@ namespace {
 
     bool needsRelocateWithSymbol(const MCSymbol &Sym,
                                  unsigned Type) const override;
+
+    void addTargetSectionFlags(MCContext &Ctx, MCSectionELF &Sec) override;
   };
 
 } // end anonymous namespace
@@ -236,6 +239,21 @@ unsigned ARMELFObjectWriter::GetRelocTypeInner(const MCValue &Target,
   }
 }
 
+void ARMELFObjectWriter::addTargetSectionFlags(MCContext &Ctx,
+                                               MCSectionELF &Sec) {
+  // The mix of execute-only and non-execute-only at link time is
+  // non-execute-only. To avoid the empty implicitly created .text
+  // section from making the whole .text section non-execute-only, we
+  // mark it execute-only if it is empty and there is at least one
+  // execute-only section in the object.
+  MCSectionELF *TextSection =
+      static_cast<MCSectionELF *>(Ctx.getObjectFileInfo()->getTextSection());
+  if (Sec.getKind().isExecuteOnly() && !TextSection->hasInstructions() &&
+      !TextSection->hasData()) {
+    TextSection->setFlags(TextSection->getFlags() | ELF::SHF_ARM_PURECODE);
+  }
+}
+
 std::unique_ptr<MCObjectTargetWriter>
 llvm::createARMELFObjectWriter(uint8_t OSABI) {
   return llvm::make_unique<ARMELFObjectWriter>(OSABI);