Commit 1981b1b6 authored by Fangrui Song's avatar Fangrui Song
Browse files

[ELF] Demote symbols in /DISCARD/ discarded sections to Undefined (#69295) 



When an input section is matched by /DISCARD/ in a linker script, GNU ld
reports errors for relocations referencing symbols defined in the section:

    `.aaa' referenced in section `.bbb' of a.o: defined in discarded section `.aaa' of a.o

Implement the error by demoting eligible symbols to `Undefined` and changing
STB_WEAK to STB_GLOBAL. As a side benefit, in relocatable links, relocations
referencing symbols defined relative to /DISCARD/ discarded sections no longer
set symbol/type to zeros.

It's arguable whether a weak reference to a discarded symbol should lead to
errors. GNU ld reports an error and our demoting approach reports an error as
well.

Close #58891

Co-authored-by: default avatarBevin Hansson <bevin.hansson@ericsson.com>
parent ef0e0adc

lld/ELF/LinkerScript.cpp

+1 −0
Original line number Diff line number Diff line
@@ -613,6 +613,7 @@ void LinkerScript::processSectionCommands() { 
        discard(*s);

      discardSynthetic(*osec);

      osec->commands.clear();

      seenDiscard = true;

      return false;

    }

lld/ELF/LinkerScript.h

+1 −0
Original line number Diff line number Diff line
@@ -356,6 +356,7 @@ public: 


  bool hasSectionsCommand = false;

  bool seenDataAlign = false;

  bool seenDiscard = false;

  bool seenRelroEnd = false;

  bool errorOnMissingSection = false;

  std::string backwardDotErr;

lld/ELF/MapFile.cpp

+1 −1
Original line number Diff line number Diff line
@@ -229,7 +229,7 @@ static void writeCref(raw_fd_ostream &os) { 
      if (isa<SharedSymbol>(sym))

        map[sym].insert(file);

      if (auto *d = dyn_cast<Defined>(sym))

        if (!d->isLocal() && (!d->section || d->section->isLive()))

        if (!d->isLocal())

          map[d].insert(file);

    }

  }

lld/ELF/Relocations.cpp

+3 −3
Original line number Diff line number Diff line
@@ -507,8 +507,7 @@ int64_t RelocationScanner::computeMipsAddend(const RelTy &rel, RelExpr expr, 
template <class ELFT>

static std::string maybeReportDiscarded(Undefined &sym) {

  auto *file = dyn_cast_or_null<ObjFile<ELFT>>(sym.file);

  if (!file || !sym.discardedSecIdx ||

      file->getSections()[sym.discardedSecIdx] != &InputSection::discarded)

  if (!file || !sym.discardedSecIdx)

    return "";

  ArrayRef<typename ELFT::Shdr> objSections =

      file->template getELFShdrs<ELFT>();
@@ -1575,6 +1574,7 @@ template <class ELFT> void elf::scanRelocations() { 
        scanner.template scanSection<ELFT>(*sec);

      if (part.armExidx && part.armExidx->isLive())

        for (InputSection *sec : part.armExidx->exidxSections)

          if (sec->isLive())

            scanner.template scanSection<ELFT>(*sec);

    }

  });

lld/ELF/Symbols.cpp

+10 −6
Original line number Diff line number Diff line
@@ -316,12 +316,13 @@ void elf::maybeWarnUnorderableSymbol(const Symbol *sym) { 
  if (!config->warnSymbolOrdering)

    return;



  // If UnresolvedPolicy::Ignore is used, no "undefined symbol" error/warning

  // is emitted. It makes sense to not warn on undefined symbols.

  // If UnresolvedPolicy::Ignore is used, no "undefined symbol" error/warning is

  // emitted. It makes sense to not warn on undefined symbols (excluding those

  // demoted by demoteSymbols).

  //

  // Note, ld.bfd --symbol-ordering-file= does not warn on undefined symbols,

  // but we don't have to be compatible here.

  if (sym->isUndefined() &&

  if (sym->isUndefined() && !cast<Undefined>(sym)->discardedSecIdx &&

      config->unresolvedSymbols == UnresolvedPolicy::Ignore)

    return;
@@ -330,9 +331,12 @@ void elf::maybeWarnUnorderableSymbol(const Symbol *sym) { 


  auto report = [&](StringRef s) { warn(toString(file) + s + sym->getName()); };



  if (sym->isUndefined())

  if (sym->isUndefined()) {

    if (cast<Undefined>(sym)->discardedSecIdx)

      report(": unable to order discarded symbol: ");

    else

      report(": unable to order undefined symbol: ");

  else if (sym->isShared())

  } else if (sym->isShared())

    report(": unable to order shared symbol: ");

  else if (d && !d->section)

    report(": unable to order absolute symbol: ");