[dsymutil] Find relocations that correspond to debug map entries.

These 'valid relocations' in the debug_info section will be how
dsymutil identifies the DIEs it needs to keep in the linked debug
information.

llvm-svn: 229178

llvm/tools/dsymutil/DwarfLinker.cpp

@@ -12,6 +12,7 @@
 #include "dsymutil.h"
 #include "llvm/DebugInfo/DWARF/DWARFContext.h"
 #include "llvm/DebugInfo/DWARF/DWARFDebugInfoEntry.h"
+#include "llvm/Object/MachO.h"
 #include <string>
 
 namespace llvm {
@@ -44,6 +45,19 @@ private:
 };
 
 /// \brief The core of the Dwarf linking logic.
+///
+/// The link of the dwarf information from the object files will be
+/// driven by the selection of 'root DIEs', which are DIEs that
+/// describe variables or functions that are present in the linked
+/// binary (and thus have entries in the debug map). All the debug
+/// information that will be linked (the DIEs, but also the line
+/// tables, ranges, ...) is derived from that set of root DIEs.
+///
+/// The root DIEs are identified because they contain relocations that
+/// correspond to a debug map entry at specific places (the low_pc for
+/// a function, the location for a variable). These relocations are
+/// called ValidRelocs in the DwarfLinker and are gathered as a very
+/// first step when we start processing a DebugMapObject.
 class DwarfLinker {
 public:
   DwarfLinker(StringRef OutputFilename, bool Verbose)
@@ -59,6 +73,45 @@ private:
   /// \brief Called at the end of a debug object link.
   void endDebugObject();
 
+  /// \defgroup FindValidRelocations Translate debug map into a list
+  /// of relevant relocations
+  ///
+  /// @{
+  struct ValidReloc {
+    uint32_t Offset;
+    uint32_t Size;
+    uint64_t Addend;
+    const DebugMapObject::DebugMapEntry *Mapping;
+
+    ValidReloc(uint32_t Offset, uint32_t Size, uint64_t Addend,
+               const DebugMapObject::DebugMapEntry *Mapping)
+        : Offset(Offset), Size(Size), Addend(Addend), Mapping(Mapping) {}
+
+    bool operator<(const ValidReloc &RHS) const { return Offset < RHS.Offset; }
+  };
+
+  /// \brief The valid relocations for the current DebugMapObject.
+  /// This vector is sorted by relocation offset.
+  std::vector<ValidReloc> ValidRelocs;
+
+  /// \brief Index into ValidRelocs of the next relocation to
+  /// consider. As we walk the DIEs in acsending file offset and as
+  /// ValidRelocs is sorted by file offset, keeping this index
+  /// uptodate is all we have to do to have a cheap lookup during the
+  /// root DIE selection.
+  unsigned NextValidReloc;
+
+  bool findValidRelocsInDebugInfo(const object::ObjectFile &Obj,
+                                  const DebugMapObject &DMO);
+
+  bool findValidRelocs(const object::SectionRef &Section,
+                       const object::ObjectFile &Obj,
+                       const DebugMapObject &DMO);
+
+  void findValidRelocsMachO(const object::SectionRef &Section,
+                            const object::MachOObjectFile &Obj,
+                            const DebugMapObject &DMO);
+  /// @}
 private:
   std::string OutputFilename;
   bool Verbose;
@@ -83,9 +136,96 @@ void GatherDIEParents(const DWARFDebugInfoEntryMinimal *DIE, unsigned ParentIdx,
 
 void DwarfLinker::startDebugObject(DWARFContext &Dwarf) {
   Units.reserve(Dwarf.getNumCompileUnits());
+  NextValidReloc = 0;
 }
 
-void DwarfLinker::endDebugObject() { Units.clear(); }
+void DwarfLinker::endDebugObject() {
+  Units.clear();
+  ValidRelocs.clear();
+}
+
+/// \brief Iterate over the relocations of the given \p Section and
+/// store the ones that correspond to debug map entries into the
+/// ValidRelocs array.
+void DwarfLinker::findValidRelocsMachO(const object::SectionRef &Section,
+                                       const object::MachOObjectFile &Obj,
+                                       const DebugMapObject &DMO) {
+  StringRef Contents;
+  Section.getContents(Contents);
+  DataExtractor Data(Contents, Obj.isLittleEndian(), 0);
+
+  for (const object::RelocationRef &Reloc : Section.relocations()) {
+    object::DataRefImpl RelocDataRef = Reloc.getRawDataRefImpl();
+    MachO::any_relocation_info MachOReloc = Obj.getRelocation(RelocDataRef);
+    unsigned RelocSize = 1 << Obj.getAnyRelocationLength(MachOReloc);
+    uint64_t Offset64;
+    if ((RelocSize != 4 && RelocSize != 8) || Reloc.getOffset(Offset64)) {
+      errs() << "warning: unsupported relocation in debug_info section.\n";
+      continue;
+    }
+    uint32_t Offset = Offset64;
+    // Mach-o uses REL relocations, the addend is at the relocation offset.
+    uint64_t Addend = Data.getUnsigned(&Offset, RelocSize);
+
+    auto Sym = Reloc.getSymbol();
+    if (Sym != Obj.symbol_end()) {
+      StringRef SymbolName;
+      if (Sym->getName(SymbolName)) {
+        errs() << "warning: error getting relocation symbol name.\n";
+        continue;
+      }
+      if (const auto *Mapping = DMO.lookupSymbol(SymbolName))
+        ValidRelocs.emplace_back(Offset64, RelocSize, Addend, Mapping);
+    } else if (const auto *Mapping = DMO.lookupObjectAddress(Addend)) {
+      // Do not store the addend. The addend was the address of the
+      // symbol in the object file, the address in the binary that is
+      // stored in the debug map doesn't need to be offseted.
+      ValidRelocs.emplace_back(Offset64, RelocSize, 0, Mapping);
+    }
+  }
+}
+
+/// \brief Dispatch the valid relocation finding logic to the
+/// appropriate handler depending on the object file format.
+bool DwarfLinker::findValidRelocs(const object::SectionRef &Section,
+                                  const object::ObjectFile &Obj,
+                                  const DebugMapObject &DMO) {
+  // Dispatch to the right handler depending on the file type.
+  if (auto *MachOObj = dyn_cast<object::MachOObjectFile>(&Obj))
+    findValidRelocsMachO(Section, *MachOObj, DMO);
+  else
+    errs() << "warning: unsupported object file type: " << Obj.getFileName()
+           << '\n';
+
+  if (ValidRelocs.empty())
+    return false;
+
+  // Sort the relocations by offset. We will walk the DIEs linearly in
+  // the file, this allows us to just keep an index in the relocation
+  // array that we advance during our walk, rather than resorting to
+  // some associative container. See DwarfLinker::NextValidReloc.
+  std::sort(ValidRelocs.begin(), ValidRelocs.end());
+  return true;
+}
+
+/// \brief Look for relocations in the debug_info section that match
+/// entries in the debug map. These relocations will drive the Dwarf
+/// link by indicating which DIEs refer to symbols present in the
+/// linked binary.
+/// \returns wether there are any valid relocations in the debug info.
+bool DwarfLinker::findValidRelocsInDebugInfo(const object::ObjectFile &Obj,
+                                             const DebugMapObject &DMO) {
+  // Find the debug_info section.
+  for (const object::SectionRef &Section : Obj.sections()) {
+    StringRef SectionName;
+    Section.getName(SectionName);
+    SectionName = SectionName.substr(SectionName.find_first_not_of("._"));
+    if (SectionName != "debug_info")
+      continue;
+    return findValidRelocs(Section, Obj, DMO);
+  }
+  return false;
+}
 
 bool DwarfLinker::link(const DebugMap &Map) {
 
@@ -103,6 +243,13 @@ bool DwarfLinker::link(const DebugMap &Map) {
       continue;
     }
 
+    // Look for relocations that correspond to debug map entries.
+    if (!findValidRelocsInDebugInfo(*ErrOrObj, *Obj)) {
+      if (Verbose)
+        outs() << "No valid relocations found. Skipping.\n";
+      continue;
+    }
+
     // Setup access to the debug info.
     DWARFContextInMemory DwarfContext(*ErrOrObj);
     startDebugObject(DwarfContext);