[ELF] - Define special symbols _etext and _edata

[grimar]

Summary

Description of symbols is avalable here:
https://docs.oracle.com/cd/E53394_01/html/E54766/u-etext-3c.html

It is said that:
_etext - The address of _etext is the first location after the last read-only loadable segment.
_edata - The address of _edata is the first location after the last read-write loadable segment.
_end - If the address of _edata is greater than the address of _etext, the address of _end is same as the address of _edata.

In real life _end and _edata has different values for that case.
Both gold/bfd set _edata to the end of the last non SHT_NOBITS section.
This patch do the same for consistency.

It should fix the https://llvm.org/bugs/show_bug.cgi?id=26729.

[• rafael]

test/ELF/edata-etext.s

4 ↗

(On Diff #49032)

You are not running FileCheck.

[• rafael]

test/ELF/edata-etext.s

79 ↗

(On Diff #49032)

I don't this this is correct. In this case they are the same because there is no padding before .bss.

To see the difference, add a

.align 4

after the .bss and before the .space in this test.

Also, is .bss really special? My guess would be that _edata actually points to the end of the last non NOBITS section.

[grimar]

Addressed Rafael's comments.

test/ELF/edata-etext.s

4 ↗

(On Diff #49032)

That keeps my nerves healthy..
Fixed, thanks.

79 ↗

(On Diff #49032)

That seems to be true. I was confused by output results which showed equality to .bss address in addition to such line of gold code:

{
  "_edata",			// name
  cgccpp::PT_LOAD,		// segment_type
  cgccpp::PF_W,		// segment_flags_set
  cgccpp::PF(0),		// segment_flags_clear
  0,				// value
  0,				// size
  cgccpp::STT_NOTYPE,		// type
  cgccpp::STB_GLOBAL,		// binding
  cgccpp::STV_DEFAULT,	// visibility
  0,				// nonvis
  Symbol::SEGMENT_BSS,	// offset_from_base <-------------------------------
  false			// only_if_ref
},

Fixed.

[ruiu]

ELF/Writer.cpp

906 ↗

(On Diff #49065)

Local variable names need to start with a uppercase letter. Maybe just Define should suffice.

914–921 ↗

(On Diff #49065)

Need to update this comment.

1357 ↗

(On Diff #49065)

Can you define the variables at once here?

bool Alloc = Sec->getFlags() & SHF_ALLOC;
bool NoBits = Sec->getFlags() & SHF_NOBITS;

1379–1382 ↗

(On Diff #49065)

Both symbols get the same address. Is this correct?

Is there any way to move this piece of code to fixAbsoluteSymbols? I don't want to add more code to this function.

[grimar]

• Addressed review comments.

ELF/Writer.cpp

906 ↗

(On Diff #49065)

Done.

914–921 ↗

(On Diff #49065)

Done.

1357 ↗

(On Diff #49065)

That code is not needed anymore as I moved it to
fixAbsoluteSymbols() after your hint, but
(just in case it will be returned back):

HasBits IMO better than NoBits:

bool HasBits = Sec->getType() != SHT_NOBITS;

because double negation not looking good:

if (!NoBits)
  FileOff = alignTo(FileOff, Align);
Sec->setFileOffset(FileOff);
if (!NoBits)
  FileOff += Sec->getSize();

1379–1382 ↗

(On Diff #49065)

Yes it is expected code here. Finally they will get different address. You can see that _etext and _edata are different in testcase.
Values for them will be updated on each iteration and stop updating at different moments.

// _etext points to location after the last read-only loadable segment.
// _edata points to the end of the last non SHT_NOBITS section.

moved that code to fixAbsoluteSymbols.

[ruiu]

LGTM

ELF/Writer.cpp

1453 ↗

(On Diff #49162)

if (Sec->getFlags() & SHF_ALLOC)
  continue;

1454–1455 ↗

(On Diff #49162)

Move this comment before this for loop.

lld/trunk/ELF/Symbols.h

 // Here we allocate such Elf_Sym symbols statically.
 template <class ELFT> struct ElfSym {
   typedef typename llvm::object::ELFFile<ELFT>::Elf_Sym Elf_Sym;
 
   // Used to represent an undefined symbol which we don't want to add to the
   // output file's symbol table. It has weak binding and can be substituted.
   static Elf_Sym Ignored;
 
+  // The content for _etext and etext symbols.
+  static Elf_Sym Etext;
+
+  // The content for _edata and edata symbols.
+  static Elf_Sym Edata;
+
   // The content for _end and end symbols.
   static Elf_Sym End;
 
   // The content for _gp symbol for MIPS target.
   static Elf_Sym MipsGp;
 
   // __rel_iplt_start/__rel_iplt_end for signaling
   // where R_[*]_IRELATIVE relocations do live.
   static Elf_Sym RelaIpltStart;
   static Elf_Sym RelaIpltEnd;
 };
 
 template <class ELFT> typename ElfSym<ELFT>::Elf_Sym ElfSym<ELFT>::Ignored;
+template <class ELFT> typename ElfSym<ELFT>::Elf_Sym ElfSym<ELFT>::Etext;
+template <class ELFT> typename ElfSym<ELFT>::Elf_Sym ElfSym<ELFT>::Edata;
 template <class ELFT> typename ElfSym<ELFT>::Elf_Sym ElfSym<ELFT>::End;
 template <class ELFT> typename ElfSym<ELFT>::Elf_Sym ElfSym<ELFT>::MipsGp;
 template <class ELFT>
 typename ElfSym<ELFT>::Elf_Sym ElfSym<ELFT>::RelaIpltStart;
 template <class ELFT> typename ElfSym<ELFT>::Elf_Sym ElfSym<ELFT>::RelaIpltEnd;
 
 } // namespace elf2
 } // namespace lld
 
 #endif

lld/trunk/ELF/Symbols.cpp

std::unique_ptr<InputFile> Lazy::getMember() {
   // getMember returns an empty buffer if the member was already
   // read from the library.
   if (MBRef.getBuffer().empty())
     return std::unique_ptr<InputFile>(nullptr);
   return createObjectFile(MBRef, File->getName());
 }
 
 template <class ELFT> static void doInitSymbols() {
+  ElfSym<ELFT>::Etext.setBinding(STB_GLOBAL);
+  ElfSym<ELFT>::Edata.setBinding(STB_GLOBAL);
   ElfSym<ELFT>::End.setBinding(STB_GLOBAL);
   ElfSym<ELFT>::Ignored.setBinding(STB_WEAK);
   ElfSym<ELFT>::Ignored.setVisibility(STV_HIDDEN);
 }
 
 void elf2::initSymbols() {
   doInitSymbols<ELF32LE>();
   doInitSymbols<ELF32BE>();

lld/trunk/ELF/Writer.cpp

 template <class ELFT> void Writer<ELFT>::addReservedSymbols() {
   // __tls_get_addr is defined by the dynamic linker for dynamic ELFs. For
   // static linking the linker is required to optimize away any references to
   // __tls_get_addr, so it's not defined anywhere. Create a hidden definition
   // to avoid the undefined symbol error.
   if (!isOutputDynamic())
     Symtab.addIgnored("__tls_get_addr");
 
+  auto Define = [this](StringRef Name, StringRef Alias, Elf_Sym &Sym) {
+    if (Symtab.find(Name))
+      Symtab.addAbsolute(Name, Sym);
+    if (SymbolBody *B = Symtab.find(Alias))
+      if (B->isUndefined())
+        Symtab.addAbsolute(Alias, Sym);
+  };
+
   // If the "_end" symbol is referenced, it is expected to point to the address
   // right after the data segment. Usually, this symbol points to the end
   // of .bss section or to the end of .data section if .bss section is absent.
   // We don't know the final address of _end yet, so just add a symbol here,
   // and fix ElfSym<ELFT>::End.st_value later.
-  if (Symtab.find("_end"))
-    Symtab.addAbsolute("_end", ElfSym<ELFT>::End);
-
   // Define "end" as an alias to "_end" if it is used but not defined.
   // We don't want to define that unconditionally because we don't want to
   // break programs that uses "end" as a regular symbol.
-  if (SymbolBody *B = Symtab.find("end"))
-    if (B->isUndefined())
-      Symtab.addAbsolute("end", ElfSym<ELFT>::End);
+  // The similar history with _etext/etext and _edata/edata:
+  // Address of _etext is the first location after the last read-only loadable
+  // segment. Address of _edata points to the end of the last non SHT_NOBITS
+  // section. That is how gold/bfd do. We update the values for these symbols
+  // later, after assigning sections to segments.
+  Define("_end", "end", ElfSym<ELFT>::End);
+  Define("_etext", "etext", ElfSym<ELFT>::Etext);
+  Define("_edata", "edata", ElfSym<ELFT>::Edata);
 }
 
 // Sort input sections by section name suffixes for
 // __attribute__((init_priority(N))).
 template <class ELFT> static void sortInitFini(OutputSectionBase<ELFT> *S) {
   if (S)
     reinterpret_cast<OutputSection<ELFT> *>(S)->sortInitFini();
 }
if (Out<ELFT>::RelaPlt) {
     uintX_t Start = Out<ELFT>::RelaPlt->getVA();
     ElfSym<ELFT>::RelaIpltStart.st_value = Start;
     ElfSym<ELFT>::RelaIpltEnd.st_value = Start + Out<ELFT>::RelaPlt->getSize();
   }
 
   // Update MIPS _gp absolute symbol so that it points to the static data.
   if (Config->EMachine == EM_MIPS)
     ElfSym<ELFT>::MipsGp.st_value = getMipsGpAddr<ELFT>();
+
+  // _etext points to location after the last read-only loadable segment.
+  // _edata points to the end of the last non SHT_NOBITS section.
+  for (OutputSectionBase<ELFT> *Sec : OutputSections) {
+    if (!(Sec->getFlags() & SHF_ALLOC))
+      continue;
+    if (!(Sec->getFlags() & SHF_WRITE))
+      ElfSym<ELFT>::Etext.st_value = Sec->getVA() + Sec->getSize();
+    if (Sec->getType() != SHT_NOBITS)
+      ElfSym<ELFT>::Edata.st_value = Sec->getVA() + Sec->getSize();
+  }
 }
 
 template <class ELFT> void Writer<ELFT>::writeHeader() {
   uint8_t *Buf = Buffer->getBufferStart();
   memcpy(Buf, "\177ELF", 4);
 
   auto &FirstObj = cast<ELFFileBase<ELFT>>(*Config->FirstElf);
 

lld/trunk/test/ELF/edata-etext.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: ld.lld %t.o -o %t
+# RUN: llvm-readobj -sections -symbols %t | FileCheck %s
+
+## This checks that:
+## 1) Address of _etext is the first location after the last read-only loadable segment.
+## 2) Address of _edata points to the end of the last non SHT_NOBITS section.
+##    That is how gold/bfd do. At the same time specs says: "If the address of _edata is
+##    greater than the address of _etext, the address of _end is same as the address
+##    of _edata." (https://docs.oracle.com/cd/E53394_01/html/E54766/u-etext-3c.html).
+## 3) Address of _end is different from _edata because of 2.
+# CHECK:      Section {
+# CHECK:         Index: 1
+# CHECK:         Name: .text
+# CHECK-NEXT:    Type: SHT_PROGBITS
+# CHECK-NEXT:    Flags [
+# CHECK-NEXT:      SHF_ALLOC
+# CHECK-NEXT:      SHF_EXECINSTR
+# CHECK-NEXT:    ]
+# CHECK-NEXT:    Address: 0x11000
+# CHECK-NEXT:    Offset: 0x1000
+# CHECK-NEXT:    Size: 1
+# CHECK-NEXT:    Link:
+# CHECK-NEXT:    Info:
+# CHECK-NEXT:    AddressAlignment:
+# CHECK-NEXT:    EntrySize: 0
+# CHECK-NEXT:  }
+# CHECK-NEXT:  Section {
+# CHECK-NEXT:    Index: 2
+# CHECK-NEXT:    Name: .data
+# CHECK-NEXT:    Type: SHT_PROGBITS
+# CHECK-NEXT:    Flags [
+# CHECK-NEXT:      SHF_ALLOC
+# CHECK-NEXT:      SHF_WRITE
+# CHECK-NEXT:    ]
+# CHECK-NEXT:    Address: 0x12000
+# CHECK-NEXT:    Offset: 0x2000
+# CHECK-NEXT:    Size: 2
+# CHECK-NEXT:    Link:
+# CHECK-NEXT:    Info:
+# CHECK-NEXT:    AddressAlignment:
+# CHECK-NEXT:    EntrySize:
+# CHECK-NEXT:  }
+# CHECK-NEXT:  Section {
+# CHECK-NEXT:    Index: 3
+# CHECK-NEXT:    Name: .bss
+# CHECK-NEXT:    Type: SHT_NOBITS
+# CHECK-NEXT:    Flags [
+# CHECK-NEXT:      SHF_ALLOC
+# CHECK-NEXT:      SHF_WRITE
+# CHECK-NEXT:    ]
+# CHECK-NEXT:    Address: 0x12004
+# CHECK-NEXT:    Offset: 0x2002
+# CHECK-NEXT:    Size: 6
+# CHECK-NEXT:    Link:
+# CHECK-NEXT:    Info:
+# CHECK-NEXT:    AddressAlignment:
+# CHECK-NEXT:    EntrySize:
+# CHECK-NEXT:  }
+# CHECK:      Symbols [
+# CHECK-NEXT:  Symbol {
+# CHECK-NEXT:    Name:
+# CHECK-NEXT:    Value: 0x0
+# CHECK-NEXT:    Size: 0
+# CHECK-NEXT:    Binding: Local
+# CHECK-NEXT:    Type: None
+# CHECK-NEXT:    Other: 0
+# CHECK-NEXT:    Section: Undefined
+# CHECK-NEXT:  }
+# CHECK-NEXT:  Symbol {
+# CHECK-NEXT:    Name: _start
+# CHECK-NEXT:    Value: 0x11000
+# CHECK-NEXT:    Size: 0
+# CHECK-NEXT:    Binding: Global
+# CHECK-NEXT:    Type: None
+# CHECK-NEXT:    Other: 0
+# CHECK-NEXT:    Section: .text
+# CHECK-NEXT:  }
+# CHECK-NEXT:  Symbol {
+# CHECK-NEXT:    Name: _edata
+# CHECK-NEXT:    Value: 0x12002
+# CHECK-NEXT:    Size: 0
+# CHECK-NEXT:    Binding: Global
+# CHECK-NEXT:    Type: None
+# CHECK-NEXT:    Other: 0
+# CHECK-NEXT:    Section: Absolute
+# CHECK-NEXT:  }
+# CHECK-NEXT:  Symbol {
+# CHECK-NEXT:    Name: _end
+# CHECK-NEXT:    Value: 0x1200A
+# CHECK-NEXT:    Size: 0
+# CHECK-NEXT:    Binding: Global
+# CHECK-NEXT:    Type: None
+# CHECK-NEXT:    Other: 0
+# CHECK-NEXT:    Section: Absolute
+# CHECK-NEXT:  }
+# CHECK-NEXT:  Symbol {
+# CHECK-NEXT:    Name: _etext
+# CHECK-NEXT:    Value: 0x11001
+# CHECK-NEXT:    Size: 0
+# CHECK-NEXT:    Binding: Global
+# CHECK-NEXT:    Type: None
+# CHECK-NEXT:    Other: 0
+# CHECK-NEXT:    Section: Absolute
+# CHECK-NEXT:  }
+# CHECK-NEXT: ]
+
+.global _start,_end,_etext,_edata
+.text
+_start:
+  nop
+.data
+  .word 1
+.bss
+  .align 4
+  .space 6