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[ARM] Add mapping symbols to PLT entries
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Make PLT entry atoms represent mapping symbols in the Release mode,
while in the Debug mode they are still function-like symbols
with regular names.
It's legal that mapping symbols denote unnamed parts of code,
and PLT entries are not required to have function-like names.

Differential Revision: http://reviews.llvm.org/D8819

llvm-svn: 234301
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Denis Protivensky committed Apr 7, 2015
1 parent 9dedc77 commit 6c50e88
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Showing 2 changed files with 184 additions and 19 deletions.
92 changes: 73 additions & 19 deletions lld/lib/ReaderWriter/ELF/ARM/ARMRelocationPass.cpp
View file
Expand Up @@ -62,6 +62,21 @@ static const uint8_t ARMPltVeneerAtomContent[4] = {
0x00, 0x00 // nop
};

// Determine proper names for mapping symbols.
static std::string getMappingAtomName(DefinedAtom::CodeModel model,
const std::string &part) {
switch (model) {
case DefinedAtom::codeARM_a:
return part.empty() ? "$a" : "$a." + part;
case DefinedAtom::codeARM_d:
return part.empty() ? "$d" : "$d." + part;
case DefinedAtom::codeARM_t:
return part.empty() ? "$t" : "$t." + part;
default:
llvm_unreachable("Wrong code model of mapping atom");
}
}

/// \brief Atoms that hold veneer code.
class VeneerAtom : public SimpleELFDefinedAtom {
StringRef _section;
Expand Down Expand Up @@ -141,30 +156,74 @@ class ARMGOTPLTAtom : public ARMGOTAtom {
ARMGOTPLTAtom(const File &f) : ARMGOTAtom(f, ".got.plt") {}
};

/// \brief PLT entry atom.
/// Serves as a mapping symbol in the release mode.
class ARMPLTAtom : public PLTAtom {
public:
ARMPLTAtom(const File &f) : PLTAtom(f, ".plt") {}
ARMPLTAtom(const File &f, const std::string &name)
: PLTAtom(f, ".plt") {
#ifndef NDEBUG
_name = name;
#else
// Don't move the code to any base classes since
// virtual codeModel method would return wrong value.
_name = getMappingAtomName(codeModel(), name);
#endif
}

DefinedAtom::CodeModel codeModel() const override {
#ifndef NDEBUG
return DefinedAtom::codeNA;
#else
return DefinedAtom::codeARM_a;
#endif
}

ArrayRef<uint8_t> rawContent() const override {
return llvm::makeArrayRef(ARMPltAtomContent);
}

Alignment alignment() const override { return 4; }

StringRef name() const override { return _name; }

private:
std::string _name;
};

/// \brief Veneer atom for PLT entry.
/// Serves as a mapping symbol in the release mode.
class ARMPLTVeneerAtom : public PLTAtom {
public:
ARMPLTVeneerAtom(const File &f) : PLTAtom(f, ".plt") {}
ARMPLTVeneerAtom(const File &f, const std::string &name)
: PLTAtom(f, ".plt") {
#ifndef NDEBUG
_name = name;
#else
// Don't move the code to any base classes since
// virtual codeModel method would return wrong value.
_name = getMappingAtomName(codeModel(), name);
#endif
}

DefinedAtom::CodeModel codeModel() const override {
#ifndef NDEBUG
return DefinedAtom::codeARMThumb;
#else
return DefinedAtom::codeARM_t;
#endif
}

ArrayRef<uint8_t> rawContent() const override {
return llvm::makeArrayRef(ARMPltVeneerAtomContent);
}

Alignment alignment() const override { return 4; }

StringRef name() const override { return _name; }

private:
std::string _name;
};

class ELFPassFile : public SimpleFile {
Expand Down Expand Up @@ -295,14 +354,14 @@ template <class Derived> class ARMRelocationPass : public Pass {
/// \brief get a veneer for a PLT entry.
const PLTAtom *getPLTVeneer(const DefinedAtom *da, PLTAtom *pa,
StringRef source) {
auto va = new (_file._alloc) ARMPLTVeneerAtom(_file);
std::string name = "__plt_from_thumb";
name += source;
name += da->name();
// Create veneer for PLT entry.
auto va = new (_file._alloc) ARMPLTVeneerAtom(_file, name);
// Fake reference to show connection between veneer and PLT entry.
va->addReferenceELF_ARM(R_ARM_NONE, 0, pa, 0);
#ifndef NDEBUG
va->_name = "__plt_from_thumb";
va->_name += source;
va->_name += da->name();
#endif

_pltAtoms[da] = PLTWithVeneer(pa, va);
return va;
}
Expand Down Expand Up @@ -335,16 +394,15 @@ template <class Derived> class ARMRelocationPass : public Pass {
assert(ga->customSectionName() == ".got.plt" &&
"GOT entry should be in a special section");

std::string name = "__plt";
name += source;
name += da->name();
// Create PLT entry for the GOT entry.
auto pa = new (_file._alloc) ARMPLTAtom(_file);
auto pa = new (_file._alloc) ARMPLTAtom(_file, name);
pa->addReferenceELF_ARM(R_ARM_ALU_PC_G0_NC, 0, ga, -8);
pa->addReferenceELF_ARM(R_ARM_ALU_PC_G1_NC, 4, ga, -4);
pa->addReferenceELF_ARM(R_ARM_LDR_PC_G2, 8, ga, 0);
#ifndef NDEBUG
pa->_name = "__plt";
pa->_name += source;
pa->_name += da->name();
#endif

// Since all PLT entries are in ARM code, Thumb to ARM
// switching should be added if the relocated place contais Thumb code.
if (fromThumb)
Expand All @@ -371,11 +429,7 @@ template <class Derived> class ARMRelocationPass : public Pass {

/// \brief get the PLT entry for a given IFUNC Atom.
const PLTAtom *getIFUNCPLTEntry(const DefinedAtom *da, bool fromThumb) {
StringRef source;
#ifndef NDEBUG
source = "_ifunc_";
#endif
return getPLTEntry(da, fromThumb, &Derived::createIFUNCGOTEntry, source);
return getPLTEntry(da, fromThumb, &Derived::createIFUNCGOTEntry, "_ifunc_");
}

/// \brief Redirect the call to the PLT stub for the target IFUNC.
Expand Down
111 changes: 111 additions & 0 deletions lld/test/elf/ARM/plt-mapping.test
View file
@@ -0,0 +1,111 @@
# Check that mapping symbols are properly generated for PLT entries.

# RUN: yaml2obj -format=elf -docnum 1 %s > %t-arm.o
# RUN: yaml2obj -format=elf -docnum 2 %s > %t-thm.o
# RUN: lld -flavor gnu -target arm-linux-gnu \
# RUN: -Bstatic --noinhibit-exec %t-arm.o %t-thm.o -o %t
# RUN: llvm-readobj -symbols %t | FileCheck %s

# CHECK: Name: {{[$]?[at]?[.]?}}__plt_ifunc_f1
# CHECK: Name: {{[$]?[at]?[.]?}}__plt_from_thumb_ifunc_f2
# CHECK: Name: {{[$]?[at]?[.]?}}__plt_ifunc_f2

# arm.o
---
FileHeader:
Class: ELFCLASS32
Data: ELFDATA2LSB
Type: ET_REL
Machine: EM_ARM
Flags: [ EF_ARM_EABI_VER5 ]
Sections:
- Name: .text
Type: SHT_PROGBITS
Flags: [ SHF_ALLOC, SHF_EXECINSTR ]
AddressAlign: 0x0000000000000004
Content: 04B02DE500B08DE2003000E3003040E30300A0E100D04BE204B09DE41EFF2FE100482DE904B08DE2FEFFFFEB0030A0E10300A0E10088BDE8
- Name: .rel.text
Type: SHT_REL
Link: .symtab
AddressAlign: 0x0000000000000004
Info: .text
Relocations:
- Offset: 0x0000000000000008
Symbol: main
Type: R_ARM_MOVW_ABS_NC
- Offset: 0x000000000000000C
Symbol: main
Type: R_ARM_MOVT_ABS
- Offset: 0x0000000000000028
Symbol: f1
Type: R_ARM_CALL
- Name: .data
Type: SHT_PROGBITS
Flags: [ SHF_WRITE, SHF_ALLOC ]
AddressAlign: 0x0000000000000001
Content: ''
- Name: .bss
Type: SHT_NOBITS
Flags: [ SHF_WRITE, SHF_ALLOC ]
AddressAlign: 0x0000000000000001
Content: ''
Symbols:
Global:
- Name: f1
Type: STT_GNU_IFUNC
Section: .text
- Name: main
Type: STT_FUNC
Section: .text
Value: 0x0000000000000020

# thm.o
---
FileHeader:
Class: ELFCLASS32
Data: ELFDATA2LSB
Type: ET_REL
Machine: EM_ARM
Flags: [ EF_ARM_EABI_VER5 ]
Sections:
- Name: .text
Type: SHT_PROGBITS
Flags: [ SHF_ALLOC, SHF_EXECINSTR ]
AddressAlign: 0x0000000000000004
Content: 80B400AF40F20003C0F200031846BD465DF8047B704700BF80B500AFFFF7FEFF0346184680BD00BF
- Name: .rel.text
Type: SHT_REL
Link: .symtab
AddressAlign: 0x0000000000000004
Info: .text
Relocations:
- Offset: 0x0000000000000004
Symbol: f
Type: R_ARM_THM_MOVW_ABS_NC
- Offset: 0x0000000000000008
Symbol: f
Type: R_ARM_THM_MOVT_ABS
- Offset: 0x000000000000001C
Symbol: f2
Type: R_ARM_THM_CALL
- Name: .data
Type: SHT_PROGBITS
Flags: [ SHF_WRITE, SHF_ALLOC ]
AddressAlign: 0x0000000000000001
Content: ''
- Name: .bss
Type: SHT_NOBITS
Flags: [ SHF_WRITE, SHF_ALLOC ]
AddressAlign: 0x0000000000000001
Content: ''
Symbols:
Global:
- Name: f2
Type: STT_GNU_IFUNC
Section: .text
Value: 0x0000000000000001
- Name: f
Type: STT_FUNC
Section: .text
Value: 0x0000000000000019
...

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