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The initial .text section generated in object files was missing the

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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
This commit is contained in:
Eric Christopher 2018-09-06 22:09:31 +00:00
parent 47a041af4c
commit c2b29240e0
13 changed files with 127 additions and 3 deletions
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3
include/llvm/MC/MCELFObjectWriter.h
View File

@ -13,6 +13,7 @@
#include "llvm/ADT/Triple.h"
#include "llvm/BinaryFormat/ELF.h"
#include "llvm/MC/MCObjectWriter.h"
#include "llvm/MC/MCSectionELF.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/raw_ostream.h"
#include <cstdint>
@ -92,6 +93,8 @@ public:
virtual void sortRelocs(const MCAssembler &Asm,
std::vector<ELFRelocationEntry> &Relocs);
virtual void addTargetSectionFlags(MCContext &Ctx, MCSectionELF &Sec);
/// \name Accessors
/// @{
uint8_t getOSABI() const { return OSABI; }

7
include/llvm/MC/MCSection.h
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@ -78,6 +78,10 @@ private:
/// Whether this section has had instructions emitted into it.
bool HasInstructions : 1;
/// Whether this section has had data emitted into it.
/// Right now this is only used by the ARM backend.
bool HasData : 1;
bool IsRegistered : 1;
MCDummyFragment DummyFragment;
@ -137,6 +141,9 @@ public:
bool hasInstructions() const { return HasInstructions; }
void setHasInstructions(bool Value) { HasInstructions = Value; }
bool hasData() const { return HasData; }
void setHasData(bool Value) { HasData = Value; }
bool isRegistered() const { return IsRegistered; }
void setIsRegistered(bool Value) { IsRegistered = Value; }

3
lib/MC/ELFObjectWriter.cpp
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@ -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;

3
lib/MC/MCELFObjectTargetWriter.cpp
View File

@ -26,3 +26,6 @@ void
MCELFObjectTargetWriter::sortRelocs(const MCAssembler &Asm,
std::vector<ELFRelocationEntry> &Relocs) {
}
void MCELFObjectTargetWriter::addTargetSectionFlags(MCContext &Ctx,
MCSectionELF &Sec) {}

6
lib/MC/MCObjectStreamer.cpp
View File

@ -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,

3
lib/MC/MCSection.cpp
View File

@ -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)

14
lib/Target/ARM/ARMTargetObjectFile.cpp
View File

@ -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(

18
lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
View File

@ -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);

3
test/CodeGen/ARM/execute-only.ll
View File

@ -2,6 +2,9 @@
; RUN: llc -mtriple=thumbv7m-eabi -mattr=+execute-only %s -o - | FileCheck --check-prefix=CHECK --check-prefix=CHECK-T2 %s
; RUN: llc -mtriple=thumbv8m.main-eabi -mattr=+execute-only %s -o - | FileCheck --check-prefix=CHECK --check-prefix=CHECK-T2 %s
; CHECK-NOT: {{^ *}}.text{{$}}
; CHECK: .section .text,"axy",%progbits,unique,0
@var = global i32 0
define i32 @global() minsize {

13
test/MC/ARM/elf-execute-only-section.ll Normal file
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@ -0,0 +1,13 @@
; RUN: llc < %s -mtriple=thumbv8m.base-eabi -mattr=+execute-only -filetype=obj %s -o - | \
; RUN: llvm-readelf -s | FileCheck %s
; RUN: llc < %s -mtriple=thumbv8m.main-eabi -mattr=+execute-only -filetype=obj %s -o - | \
; RUN: llvm-readelf -s | FileCheck %s
; RUN: llc < %s -mtriple=thumbv7m-eabi -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
}

27
test/MC/ELF/ARM/execute-only-populated-text-section.s Normal file
View File

@ -0,0 +1,27 @@
// RUN: llvm-mc -filetype=obj -triple thumbv7m-arm-linux-gnu %s -o - \
// RUN: | llvm-readobj -s -t | FileCheck %s
.text
bx lr
.section .text.foo,"axy"
bx lr
// CHECK: Section {
// CHECK: Name: .text
// CHECK-NEXT: Type: SHT_PROGBITS (0x1)
// CHECK-NEXT: Flags [ (0x6)
// CHECK-NEXT: SHF_ALLOC (0x2)
// CHECK-NEXT: SHF_EXECINSTR (0x4)
// CHECK-NEXT: ]
// CHECK: }
// CHECK: Section {
// CHECK: Name: .text.foo
// CHECK-NEXT: Type: SHT_PROGBITS (0x1)
// CHECK-NEXT: Flags [ (0x20000006)
// CHECK-NEXT: SHF_ALLOC (0x2)
// CHECK-NEXT: SHF_ARM_PURECODE (0x20000000)
// CHECK-NEXT: SHF_EXECINSTR (0x4)
// CHECK-NEXT: ]
// CHECK: }

3
test/MC/ELF/ARM/execute-only-section.s
View File

@ -20,8 +20,9 @@ foo:
// CHECK: Section {
// CHECK: Name: .text (16)
// CHECK-NEXT: Type: SHT_PROGBITS (0x1)
// CHECK-NEXT: Flags [ (0x6)
// CHECK-NEXT: Flags [ (0x20000006)
// CHECK-NEXT: SHF_ALLOC (0x2)
// CHECK-NEXT: SHF_ARM_PURECODE (0x20000000)
// CHECK-NEXT: SHF_EXECINSTR (0x4)
// CHECK-NEXT: ]
// CHECK: Size: 0

27
test/MC/ELF/ARM/execute-only-text-section-data.s Normal file
View File

@ -0,0 +1,27 @@
// RUN: llvm-mc -filetype=obj -triple thumbv7m-arm-linux-gnu %s -o - \
// RUN: | llvm-readobj -s -t | FileCheck %s
.text
.ascii "test"
.section .text.foo,"axy"
bx lr
// CHECK: Section {
// CHECK: Name: .text
// CHECK-NEXT: Type: SHT_PROGBITS (0x1)
// CHECK-NEXT: Flags [ (0x6)
// CHECK-NEXT: SHF_ALLOC (0x2)
// CHECK-NEXT: SHF_EXECINSTR (0x4)
// CHECK-NEXT: ]
// CHECK: }
// CHECK: Section {
// CHECK: Name: .text.foo
// CHECK-NEXT: Type: SHT_PROGBITS (0x1)
// CHECK-NEXT: Flags [ (0x20000006)
// CHECK-NEXT: SHF_ALLOC (0x2)
// CHECK-NEXT: SHF_ARM_PURECODE (0x20000000)
// CHECK-NEXT: SHF_EXECINSTR (0x4)
// CHECK-NEXT: ]
// CHECK: }