path: root/lld/lib/ReaderWriter/PECOFF/LinkerGeneratedSymbolFile.h

//===- lib/ReaderWriter/PECOFF/LinkerGeneratedSymbolFile.cpp --------------===//
//
//                             The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//

#include "Atoms.h"

#include "lld/Core/ArchiveLibraryFile.h"
#include "lld/Core/Simple.h"
#include "lld/ReaderWriter/PECOFFLinkingContext.h"
#include "llvm/Support/Allocator.h"

#include <mutex>

namespace lld {
namespace pecoff {

namespace impl {

/// The defined atom for dllexported symbols with __imp_ prefix.
class ImpPointerAtom : public COFFLinkerInternalAtom {
public:
  ImpPointerAtom(const File &file, StringRef symbolName, uint64_t ordinal)
      : COFFLinkerInternalAtom(file, /*oridnal*/ 0, std::vector<uint8_t>(4),
                               symbolName),
        _ordinal(ordinal) {}

  uint64_t ordinal() const override { return _ordinal; }
  Scope scope() const override { return scopeGlobal; }
  ContentType contentType() const override { return typeData; }
  Alignment alignment() const override { return Alignment(4); }
  ContentPermissions permissions() const override { return permR__; }

private:
  uint64_t _ordinal;
};

class ImpSymbolFile : public SimpleFile {
public:
  ImpSymbolFile(StringRef defsym, StringRef undefsym, uint64_t ordinal)
      : SimpleFile(defsym), _undefined(*this, undefsym),
        _defined(*this, defsym, ordinal) {
    _defined.addReference(std::unique_ptr<COFFReference>(
        new COFFReference(&_undefined, 0, llvm::COFF::IMAGE_REL_I386_DIR32)));
    addAtom(_defined);
    addAtom(_undefined);
  };

private:
  SimpleUndefinedAtom _undefined;
  ImpPointerAtom _defined;
};

class VirtualArchiveLibraryFile : public ArchiveLibraryFile {
public:
  VirtualArchiveLibraryFile(StringRef filename)
      : ArchiveLibraryFile(filename) {}

  const atom_collection<DefinedAtom> &defined() const override {
    return _definedAtoms;
  }

  const atom_collection<UndefinedAtom> &undefined() const override {
    return _undefinedAtoms;
  }

  const atom_collection<SharedLibraryAtom> &sharedLibrary() const override {
    return _sharedLibraryAtoms;
  }

  const atom_collection<AbsoluteAtom> &absolute() const override {
    return _absoluteAtoms;
  }

  std::error_code
  parseAllMembers(std::vector<std::unique_ptr<File>> &result) const override {
    return std::error_code();
  }

private:
  atom_collection_vector<DefinedAtom> _definedAtoms;
  atom_collection_vector<UndefinedAtom> _undefinedAtoms;
  atom_collection_vector<SharedLibraryAtom> _sharedLibraryAtoms;
  atom_collection_vector<AbsoluteAtom> _absoluteAtoms;
};

// A file to make Resolver to resolve a symbol TO instead of a symbol FROM,
// using fallback mechanism for an undefined symbol. One can virtually rename an
// undefined symbol using this file.
class SymbolRenameFile : public SimpleFile {
public:
  SymbolRenameFile(StringRef from, StringRef to)
      : SimpleFile("<symbol-rename>"), _fromSym(from), _toSym(to),
        _from(*this, _fromSym, &_to), _to(*this, _toSym) {
    addAtom(_from);
  };

private:
  std::string _fromSym;
  std::string _toSym;
  COFFUndefinedAtom _from;
  COFFUndefinedAtom _to;
};

} // namespace impl

// A virtual file containing absolute symbol __ImageBase. __ImageBase (or
// ___ImageBase on x86) is a linker-generated symbol whose address is the same
// as the image base address.
class LinkerGeneratedSymbolFile : public SimpleFile {
public:
  LinkerGeneratedSymbolFile(const PECOFFLinkingContext &ctx)
      : SimpleFile("<linker-internal-file>"),
        _imageBaseAtom(*this, ctx.decorateSymbol("__ImageBase"),
                       Atom::scopeGlobal, ctx.getBaseAddress()) {
    addAtom(_imageBaseAtom);
  };

private:
  COFFAbsoluteAtom _imageBaseAtom;
};

// A LocallyImporteSymbolFile is an archive file containing _imp_
// symbols for local use.
//
// For each defined symbol, linker creates an implicit defined symbol
// by appending "_imp_" prefix to the original name. The content of
// the implicit symbol is a pointer to the original symbol
// content. This feature allows one to compile and link the following
// code without error, although _imp__hello is not defined in the
// code.
//
//   void hello() { printf("Hello\n"); }
//   extern void (*_imp__hello)();
//   int main() {
//      _imp__hello();
//      return 0;
//   }
//
// This odd feature is for the compatibility with MSVC link.exe.
class LocallyImportedSymbolFile : public impl::VirtualArchiveLibraryFile {
public:
  LocallyImportedSymbolFile(const PECOFFLinkingContext &ctx)
      : VirtualArchiveLibraryFile("__imp_"),
        _prefix(ctx.decorateSymbol("_imp_")), _ordinal(0) {}

  const File *find(StringRef sym, bool dataSymbolOnly) const override {
    if (!sym.startswith(_prefix))
      return nullptr;
    StringRef undef = sym.substr(_prefix.size());
    return new (_alloc) impl::ImpSymbolFile(sym, undef, _ordinal++);
  }

private:
  std::string _prefix;
  mutable uint64_t _ordinal;
  mutable llvm::BumpPtrAllocator _alloc;
};

class ResolvableSymbols {
public:
  void add(File *file) {
    std::lock_guard<std::mutex> lock(_mutex);
    if (_seen.count(file) > 0)
      return;
    _seen.insert(file);
    _queue.insert(file);
  }

  const std::set<std::string> &defined() {
    readAllSymbols();
    return _defined;
  }

private:
  // Files are read lazily, so that it has no runtime overhead if
  // no one accesses this class.
  void readAllSymbols() {
    std::lock_guard<std::mutex> lock(_mutex);
    for (File *file : _queue) {
      if (auto *archive = dyn_cast<ArchiveLibraryFile>(file)) {
        for (const std::string &sym : archive->getDefinedSymbols())
          _defined.insert(sym);
        continue;
      }
      for (const DefinedAtom *atom : file->defined())
        if (!atom->name().empty())
          _defined.insert(atom->name());
    }
    _queue.clear();
  }

  std::set<std::string> _defined;
  std::set<File *> _seen;
  std::set<File *> _queue;
  std::mutex _mutex;
};

// A ExportedSymbolRenameFile is a virtual archive file for dllexported symbols.
//
// One usually has to specify the exact symbol name to resolve it. That's true
// in most cases for PE/COFF, except the one described below.
//
// DLLExported symbols can be specified using a module definition file. In a
// file, one can write an EXPORT directive followed by symbol names. Such
// symbols may not be fully decorated -- one can omit "@" and the following
// number suffix for the stdcall function.
//
// If a symbol FOO is specified to be dllexported by a module definition file,
// linker has to search not only for FOO but also for FOO@[0-9]+. This ambiguous
// matching semantics does not fit well with Resolver.
//
// We could probably modify Resolver to resolve ambiguous symbols, but I think
// we don't want to do that because it'd be rarely used, and only this Windows
// specific feature would use it. It's probably not a good idea to make the core
// linker to be able to deal with it.
//
// So, instead of tweaking Resolver, we chose to do some hack here. An
// ExportedSymbolRenameFile maintains a set containing all possibly defined
// symbol names. That set would be a union of (1) all the defined symbols that
// are already parsed and read and (2) all the defined symbols in archive files
// that are not yet be parsed.
//
// If Resolver asks this file to return an atom for a dllexported symbol, find()
// looks up the set, doing ambiguous matching. If there's a symbol with @
// prefix, it returns an atom to rename the dllexported symbol, hoping that
// Resolver will find the new symbol with atsign from an archive file at the
// next visit.
class ExportedSymbolRenameFile : public impl::VirtualArchiveLibraryFile {
public:
  ExportedSymbolRenameFile(const PECOFFLinkingContext &ctx,
                           std::shared_ptr<ResolvableSymbols> syms)
      : VirtualArchiveLibraryFile("<export>"), _syms(syms) {
    for (const PECOFFLinkingContext::ExportDesc &desc : ctx.getDllExports())
      _exportedSyms.insert(desc.name);
  }

  const File *find(StringRef sym, bool dataSymbolOnly) const override {
    if (_exportedSyms.count(sym) == 0)
      return nullptr;
    std::string replace;
    if (!findSymbolWithAtsignSuffix(sym.str(), replace))
      return nullptr;
    return new (_alloc) impl::SymbolRenameFile(sym, replace);
  }

private:
  // Find a symbol that starts with a given symbol name followed
  // by @number suffix.
  bool findSymbolWithAtsignSuffix(std::string sym, std::string &res) const {
    sym.append("@");
    const std::set<std::string> &defined = _syms->defined();
    auto it = defined.lower_bound(sym);
    for (auto e = defined.end(); it != e; ++it) {
      if (!StringRef(*it).startswith(sym))
        return false;
      if (it->size() == sym.size())
        continue;
      StringRef suffix = StringRef(*it).substr(sym.size());
      if (suffix.find_first_not_of("0123456789") != StringRef::npos)
        continue;
      res = *it;
      return true;
    }
    return false;
  }

  std::set<std::string> _exportedSyms;
  mutable std::shared_ptr<ResolvableSymbols> _syms;
  mutable llvm::BumpPtrAllocator _alloc;
};

} // end namespace pecoff
} // end namespace lld