[ELF] - do not create special symbols when creating relocatable output

[grimar]

Summary

start_/_end_​<section-name> symbols and such ones like :
preinit_array_start/end
init_array_start/end
fini_array_start/end

should not be created by linker when creating relocatable files.
Patch fixes that, testcase is provided.

[ruiu]

LGTM

ELF/Writer.cpp

1004

Instead of here, please add "if (Config->Relocatable) return;" at beginning of addStartEndSymbols.

[grimar]

ELF/Writer.cpp

1004

But what about addStartStopSymbols() ? Should I add the same there ? I not sure that it is good because will iterate over all RegularSections, thats not wasting but somewhat confusing to do that without reasons.

[ruiu]

ELF/Writer.cpp

1004

That makes sense. Let's go with your code then.

ELF/Writer.cpp

template <class ELFT> bool Writer<ELFT>::createSections() {
   sortInitFini(Out<ELFT>::Dynamic->InitArraySec);
   sortInitFini(Out<ELFT>::Dynamic->FiniArraySec);
   sortCtorsDtors(Factory.lookup(".ctors", SHT_PROGBITS, SHF_WRITE | SHF_ALLOC));
   sortCtorsDtors(Factory.lookup(".dtors", SHT_PROGBITS, SHF_WRITE | SHF_ALLOC));
 
   // The linker needs to define SECNAME_start, SECNAME_end and SECNAME_stop
   // symbols for sections, so that the runtime can get the start and end
   // addresses of each section by section name. Add such symbols.
+  if (!Config->Relocatable) {
     addStartEndSymbols();
     for (OutputSectionBase<ELFT> *Sec : RegularSections)
       addStartStopSymbols(Sec);
+  }
   if (isOutputDynamic())
     Symtab.addSynthetic("_DYNAMIC", *Out<ELFT>::Dynamic, 0, STV_HIDDEN);
 
   // Define __rel[a]_iplt_{start,end} symbols if needed.
   addRelIpltSymbols();
 
   // Scan relocations. This must be done after every symbol is declared so that
   // we can correctly decide if a dynamic relocation is needed.

test/ELF/relocatable-symbols.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: ld.lld -r %t -o %tout
+# RUN: llvm-objdump -d %tout | FileCheck -check-prefix=DISASM %s
+# RUN: llvm-readobj -symbols -r %tout | FileCheck -check-prefix=SYMBOL %s
+
+# DISASM:      _start:
+# DISASM-NEXT:   0: {{.*}} callq 0
+# DISASM-NEXT:   5: {{.*}} callq 0
+# DISASM-NEXT:   a: {{.*}} callq 0
+# DISASM-NEXT:   f: {{.*}} callq 0
+# DISASM-NEXT:  14: {{.*}} callq 0
+# DISASM-NEXT:  19: {{.*}} callq 0
+# DISASM-NEXT:  1e: {{.*}} callq 0
+# DISASM-NEXT:  23: {{.*}} callq 0
+# DISASM-NEXT:  28: {{.*}} callq 0
+# DISASM-NEXT:  2d: {{.*}} callq 0
+# DISASM-NEXT:  32: {{.*}} callq 0
+# DISASM-NEXT:  37: {{.*}} callq 0
+# DISASM-NEXT: Disassembly of section foo:
+# DISASM-NEXT: foo:
+# DISASM-NEXT:  0: 90 nop
+# DISASM-NEXT:  1: 90 nop
+# DISASM-NEXT:  2: 90 nop
+# DISASM-NEXT: Disassembly of section bar:
+# DISASM-NEXT: bar:
+# DISASM-NEXT:  0: 90 nop
+# DISASM-NEXT:  1: 90 nop
+# DISASM-NEXT:  2: 90 nop
+
+# SYMBOL:      Relocations [
+# SYMBOL-NEXT:  Section ({{.*}}) .rela.text {
+# SYMBOL-NEXT:     0x1 R_X86_64_PC32 __start_foo 0x0
+# SYMBOL-NEXT:     0x6 R_X86_64_PC32 __stop_foo 0x0
+# SYMBOL-NEXT:     0xB R_X86_64_PC32 __start_bar 0x0
+# SYMBOL-NEXT:     0x10 R_X86_64_PC32 __stop_bar 0x0
+# SYMBOL-NEXT:     0x15 R_X86_64_PC32 __start_doo 0x0
+# SYMBOL-NEXT:     0x1A R_X86_64_PC32 __stop_doo 0x0
+# SYMBOL-NEXT:     0x1F R_X86_64_PC32 __preinit_array_start 0x0
+# SYMBOL-NEXT:     0x24 R_X86_64_PC32 __preinit_array_end 0x0
+# SYMBOL-NEXT:     0x29 R_X86_64_PC32 __init_array_start 0x0
+# SYMBOL-NEXT:     0x2E R_X86_64_PC32 __init_array_end 0x0
+# SYMBOL-NEXT:     0x33 R_X86_64_PC32 __fini_array_start 0x0
+# SYMBOL-NEXT:     0x38 R_X86_64_PC32 __fini_array_end 0x0
+# SYMBOL-NEXT:   }
+# SYMBOL-NEXT: ]
+# SYMBOL:      Symbol {
+# SYMBOL:        Name: __fini_array_end
+# SYMBOL-NEXT:   Value: 0x0
+# SYMBOL-NEXT:   Size: 0
+# SYMBOL-NEXT:   Binding: Global
+# SYMBOL-NEXT:   Type: None
+# SYMBOL-NEXT:   Other: 0
+# SYMBOL-NEXT:   Section: Undefined
+# SYMBOL-NEXT: }
+# SYMBOL-NEXT: Symbol {
+# SYMBOL-NEXT:   Name: __fini_array_start
+# SYMBOL-NEXT:   Value: 0x0
+# SYMBOL-NEXT:   Size: 0
+# SYMBOL-NEXT:   Binding: Global
+# SYMBOL-NEXT:   Type: None
+# SYMBOL-NEXT:   Other: 0
+# SYMBOL-NEXT:   Section: Undefined
+# SYMBOL-NEXT: }
+# SYMBOL-NEXT: Symbol {
+# SYMBOL-NEXT:   Name: __init_array_end
+# SYMBOL-NEXT:   Value: 0x0
+# SYMBOL-NEXT:   Size: 0
+# SYMBOL-NEXT:   Binding: Global
+# SYMBOL-NEXT:   Type: None
+# SYMBOL-NEXT:   Other: 0
+# SYMBOL-NEXT:   Section: Undefined
+# SYMBOL-NEXT: }
+# SYMBOL-NEXT: Symbol {
+# SYMBOL-NEXT:   Name: __init_array_start
+# SYMBOL-NEXT:   Value: 0x0
+# SYMBOL-NEXT:   Size: 0
+# SYMBOL-NEXT:   Binding: Global
+# SYMBOL-NEXT:   Type: None
+# SYMBOL-NEXT:   Other: 0
+# SYMBOL-NEXT:   Section: Undefined
+# SYMBOL-NEXT: }
+# SYMBOL-NEXT: Symbol {
+# SYMBOL-NEXT:   Name: __preinit_array_end
+# SYMBOL-NEXT:   Value: 0x0
+# SYMBOL-NEXT:   Size: 0
+# SYMBOL-NEXT:   Binding: Global
+# SYMBOL-NEXT:   Type: None
+# SYMBOL-NEXT:   Other: 0
+# SYMBOL-NEXT:   Section: Undefined
+# SYMBOL-NEXT: }
+# SYMBOL-NEXT: Symbol {
+# SYMBOL-NEXT:   Name: __preinit_array_start
+# SYMBOL-NEXT:   Value: 0x0
+# SYMBOL-NEXT:   Size: 0
+# SYMBOL-NEXT:   Binding: Global
+# SYMBOL-NEXT:   Type: None
+# SYMBOL-NEXT:   Other: 0
+# SYMBOL-NEXT:   Section: Undefined
+# SYMBOL-NEXT: }
+# SYMBOL-NEXT: Symbol {
+# SYMBOL-NEXT:   Name: __start_bar
+# SYMBOL-NEXT:   Value: 0x0
+# SYMBOL-NEXT:   Size: 0
+# SYMBOL-NEXT:   Binding: Global
+# SYMBOL-NEXT:   Type: None
+# SYMBOL-NEXT:   Other: 0
+# SYMBOL-NEXT:   Section: Undefined
+# SYMBOL-NEXT: }
+# SYMBOL-NEXT: Symbol {
+# SYMBOL-NEXT:   Name: __start_doo
+# SYMBOL-NEXT:   Value: 0x0
+# SYMBOL-NEXT:   Size: 0
+# SYMBOL-NEXT:   Binding: Global
+# SYMBOL-NEXT:   Type: None
+# SYMBOL-NEXT:   Other: 0
+# SYMBOL-NEXT:   Section: Undefined
+# SYMBOL-NEXT: }
+# SYMBOL-NEXT: Symbol {
+# SYMBOL-NEXT:   Name: __start_foo
+# SYMBOL-NEXT:   Value: 0x0
+# SYMBOL-NEXT:   Size: 0
+# SYMBOL-NEXT:   Binding: Global
+# SYMBOL-NEXT:   Type: None
+# SYMBOL-NEXT:   Other: 0
+# SYMBOL-NEXT:   Section: Undefined
+# SYMBOL-NEXT: }
+# SYMBOL-NEXT: Symbol {
+# SYMBOL-NEXT:   Name: __stop_bar
+# SYMBOL-NEXT:   Value: 0x0
+# SYMBOL-NEXT:   Size: 0
+# SYMBOL-NEXT:   Binding: Global
+# SYMBOL-NEXT:   Type: None
+# SYMBOL-NEXT:   Other: 0
+# SYMBOL-NEXT:   Section: Undefined
+# SYMBOL-NEXT: }
+# SYMBOL-NEXT: Symbol {
+# SYMBOL-NEXT:   Name: __stop_doo
+# SYMBOL-NEXT:   Value: 0x0
+# SYMBOL-NEXT:   Size: 0
+# SYMBOL-NEXT:   Binding: Global
+# SYMBOL-NEXT:   Type: None
+# SYMBOL-NEXT:   Other: 0
+# SYMBOL-NEXT:   Section: Undefined
+# SYMBOL-NEXT: }
+# SYMBOL-NEXT: Symbol {
+# SYMBOL-NEXT:   Name: __stop_foo
+# SYMBOL-NEXT:   Value: 0x0
+# SYMBOL-NEXT:   Size: 0
+# SYMBOL-NEXT:   Binding: Global
+# SYMBOL-NEXT:   Type: None
+# SYMBOL-NEXT:   Other: 0
+# SYMBOL-NEXT:   Section: Undefined
+# SYMBOL-NEXT: }
+
+.global _start
+.text
+_start:
+ call __start_foo
+ call __stop_foo
+
+ call __start_bar
+ call __stop_bar
+
+ call __start_doo
+ call __stop_doo
+
+ call __preinit_array_start
+ call __preinit_array_end
+ call __init_array_start
+ call __init_array_end
+ call __fini_array_start
+ call __fini_array_end
+
+.section foo,"ax"
+ nop
+ nop
+ nop
+
+.section bar,"ax"
+ nop
+ nop
+ nop