[dump_syms/Mac] New -x option to prefer extern names when there's a mismatch

When built with -gmlt, .dSYMs are (by design) missing the
`DW_AT_linkage_name` which Breakpad uses to fill out the
(name-mangled) function names.

Thankfully, the .dSYM contains both the old-school LC_SYMTAB command
containing the STABS-format symbols (which include the fully-qualified
C++ symbol names we want, but no actual compilation unit data), as
well as the LC_SEGMENT_64 containing the __DWARF segment with the
minimal -gmlt debug information (which excludes the name-mangled C++
symbols).

Unfortunately, since the .dSYM's STABS does not define compilation
units, the usual path in `StabsReader` ignores all the fully-qualified
C++ symbol names for the functions:

bd9d94c708/src/common/stabs_reader.cc (100)

Fortunately, when built for macOS platforms (`HAVE_MACH_O_NLIST_H`),
`StabsReader` supports storing all the STABS-format symbols as
`Extern`s, regardless of whether or not they're in a compilation unit:

bd9d94c708/src/common/stabs_reader.cc (119)

Currently, when there's both a `Function` and an `Extern` with the same address, `Module` discards the `Extern`:

bd9d94c708/src/common/module.cc (161)

This CL adds a new `-x` option to the Mac `dump_syms` which prefers
the Extern function name if there's a mismatch.

Bug: https://bugs.chromium.org/p/google-breakpad/issues/detail?id=883
Change-Id: I0d32adc64fbf567600b0a5ca63c71c422b7f0f8c
Reviewed-on: https://chromium-review.googlesource.com/c/breakpad/breakpad/+/4453650
Reviewed-by: Joshua Peraza <jperaza@chromium.org>

src/common/dwarf_cu_to_module.cc

@@ -330,7 +330,10 @@ class DwarfCUToModule::GenericDIEHandler: public DIEHandler {
   // Use this from EndAttributes member functions, not ProcessAttribute*
   // functions; only the former can be sure that all the DIE's attributes
   // have been seen.
-  StringView ComputeQualifiedName();
+  //
+  // On return, if has_qualified_name is non-NULL, *has_qualified_name is set to
+  // true if the DIE includes a fully-qualified name, false otherwise.
+  StringView ComputeQualifiedName(bool* has_qualified_name);
 
   CUContext* cu_context_;
   DIEContext* parent_context_;
@@ -466,7 +469,8 @@ void DwarfCUToModule::GenericDIEHandler::ProcessAttributeString(
   }
 }
 
-StringView DwarfCUToModule::GenericDIEHandler::ComputeQualifiedName() {
+StringView DwarfCUToModule::GenericDIEHandler::ComputeQualifiedName(
+    bool* has_qualified_name) {
   // Use the demangled name, if one is available. Demangled names are
   // preferable to those inferred from the DWARF structure because they
   // include argument types.
@@ -482,6 +486,15 @@ StringView DwarfCUToModule::GenericDIEHandler::ComputeQualifiedName() {
   StringView* unqualified_name = nullptr;
   StringView* enclosing_name = nullptr;
   if (!qualified_name) {
+    if (has_qualified_name) {
+      // dSYMs built with -gmlt do not include the DW_AT_linkage_name
+      // with the unmangled symbol, but rather include it in the
+      // LC_SYMTAB STABS, which end up in the externs of the module.
+      //
+      // Remember this so the Module can copy over the extern name later.
+      *has_qualified_name = false;
+    }
+
     // Find the unqualified name. If the DIE has its own DW_AT_name
     // attribute, then use that; otherwise, check the specification.
     if (!name_attribute_.empty()) {
@@ -500,6 +513,10 @@ StringView DwarfCUToModule::GenericDIEHandler::ComputeQualifiedName() {
     } else if (parent_context_) {
       enclosing_name = &parent_context_->name;
     }
+  } else {
+    if (has_qualified_name) {
+      *has_qualified_name = true;
+    }
   }
 
   // Prepare the return value before upcoming mutations possibly invalidate the
@@ -722,7 +739,8 @@ class DwarfCUToModule::FuncHandler: public GenericDIEHandler {
         ranges_form_(DW_FORM_sec_offset),
         ranges_data_(0),
         inline_(false),
-        handle_inline_(handle_inline) {}
+        handle_inline_(handle_inline),
+        has_qualified_name_(false) {}
 
   void ProcessAttributeUnsigned(enum DwarfAttribute attr,
                                 enum DwarfForm form,
@@ -745,6 +763,7 @@ class DwarfCUToModule::FuncHandler: public GenericDIEHandler {
   bool inline_;
   vector<unique_ptr<Module::Inline>> child_inlines_;
   bool handle_inline_;
+  bool has_qualified_name_;
   DIEContext child_context_; // A context for our children.
 };
 
@@ -808,7 +827,7 @@ DIEHandler* DwarfCUToModule::FuncHandler::FindChildHandler(
 
 bool DwarfCUToModule::FuncHandler::EndAttributes() {
   // Compute our name, and record a specification, if appropriate.
-  name_ = ComputeQualifiedName();
+  name_ = ComputeQualifiedName(&has_qualified_name_);
   if (name_.empty() && abstract_origin_) {
     name_ = abstract_origin_->name;
   }
@@ -881,6 +900,9 @@ void DwarfCUToModule::FuncHandler::Finish() {
     scoped_ptr<Module::Function> func(new Module::Function(name, low_pc_));
     func->ranges = ranges;
     func->parameter_size = 0;
+    // If the name was unqualified, prefer the Extern name if there's a mismatch
+    // (the Extern name will be fully-qualified in that case).
+    func->prefer_extern_name = !has_qualified_name_;
     if (func->address) {
       // If the function address is zero this is a sign that this function
       // description is just empty debug data and should just be discarded.
@@ -915,7 +937,7 @@ void DwarfCUToModule::FuncHandler::Finish() {
 }
 
 bool DwarfCUToModule::NamedScopeHandler::EndAttributes() {
-  child_context_.name = ComputeQualifiedName();
+  child_context_.name = ComputeQualifiedName(NULL);
   if (child_context_.name.empty() && no_specification) {
     cu_context_->reporter->UnknownSpecification(offset_, specification_offset_);
   }

src/common/dwarf_cu_to_module_unittest.cc

@@ -267,6 +267,10 @@ class CUFixtureBase {
   void TestFunction(int i, const string& name,
                     Module::Address address, Module::Address size);
 
+  // Test that the I'th function (ordered by address) in the module
+  // this.module_ has the given prefer_extern_name.
+  void TestFunctionPreferExternName(int i, bool prefer_extern_name);
+
   // Test that the number of source lines owned by the I'th function
   // in the module this.module_ is equal to EXPECTED.
   void TestLineCount(int i, size_t expected);
@@ -615,6 +619,15 @@ void CUFixtureBase::TestFunction(int i, const string& name,
   EXPECT_EQ(0U,      function->parameter_size);
 }
 
+void CUFixtureBase::TestFunctionPreferExternName(int i,
+                                                 bool prefer_extern_name) {
+  FillFunctions();
+  ASSERT_LT((size_t)i, functions_.size());
+
+  Module::Function* function = functions_[i];
+  EXPECT_EQ(prefer_extern_name, function->prefer_extern_name);
+}
+
 void CUFixtureBase::TestLineCount(int i, size_t expected) {
   FillFunctions();
   ASSERT_LT((size_t) i, functions_.size());

src/common/mac/dump_syms.cc

@@ -420,7 +420,7 @@ bool DumpSymbols::CreateEmptyModule(scoped_ptr<Module>& module) {
 
   // Create a module to hold the debugging information.
   module.reset(new Module(module_name, "mac", selected_arch_name, identifier,
-                          "", enable_multiple_));
+                          "", enable_multiple_, prefer_extern_name_));
   return true;
 }
 

src/common/mac/dump_syms.h

@@ -57,7 +57,8 @@ class DumpSymbols {
   DumpSymbols(SymbolData symbol_data,
               bool handle_inter_cu_refs,
               bool enable_multiple = false,
-              const std::string& module_name = "")
+              const std::string& module_name = "",
+              bool prefer_extern_name = false)
       : symbol_data_(symbol_data),
         handle_inter_cu_refs_(handle_inter_cu_refs),
         object_filename_(),
@@ -68,7 +69,8 @@ class DumpSymbols {
         selected_object_file_(),
         selected_object_name_(),
         enable_multiple_(enable_multiple),
-        module_name_(module_name) {}
+        module_name_(module_name),
+        prefer_extern_name_(prefer_extern_name) {}
   ~DumpSymbols() = default;
 
   // Prepare to read debugging information from |filename|. |filename| may be
@@ -205,6 +207,15 @@ class DumpSymbols {
   // If non-empty, used as the module name. Otherwise, the basename of
   // |object_filename_| is used as the module name.
   const std::string module_name_;
+
+  // If a Function and an Extern share the same address but have a different
+  // name, prefer the name of the Extern.
+  //
+  // Use this when dumping Mach-O .dSYMs built with -gmlt (Minimum Line Tables),
+  // as the Function's fully-qualified name will only be present in the STABS
+  // (which are placed in the Extern), not in the DWARF symbols (which are
+  // placed in the Function).
+  bool prefer_extern_name_;
 };
 
 }  // namespace google_breakpad

src/common/module.cc

@@ -105,14 +105,16 @@ Module::Module(const string& name,
                const string& architecture,
                const string& id,
                const string& code_id /* = "" */,
-               bool enable_multiple_field /* = false*/)
+               bool enable_multiple_field /* = false*/,
+               bool prefer_extern_name /* = false*/)
     : name_(name),
       os_(os),
       architecture_(architecture),
       id_(id),
       code_id_(code_id),
       load_address_(0),
-      enable_multiple_field_(enable_multiple_field) {}
+      enable_multiple_field_(enable_multiple_field),
+      prefer_extern_name_(prefer_extern_name) {}
 
 Module::~Module() {
   for (FileByNameMap::iterator it = files_.begin(); it != files_.end(); ++it)
@@ -152,11 +154,14 @@ bool Module::AddFunction(Function* function) {
     it_ext = externs_.find(&arm_thumb_ext);
   }
   if (it_ext != externs_.end()) {
-    if (enable_multiple_field_) {
     Extern* found_ext = it_ext->get();
+    bool name_mismatch = found_ext->name != function->name;
+    if (enable_multiple_field_) {
       // If the PUBLIC is for the same symbol as the FUNC, don't mark multiple.
-      function->is_multiple |=
+      function->is_multiple |= name_mismatch || found_ext->is_multiple;
-          found_ext->name != function->name || found_ext->is_multiple;
+    }
+    if (name_mismatch && prefer_extern_name_) {
+      function->name = AddStringToPool(it_ext->get()->name);
     }
     externs_.erase(it_ext);
   }

src/common/module.h

@@ -131,6 +131,10 @@ class Module {
 
     // If this symbol has been folded with other symbols in the linked binary.
     bool is_multiple = false;
+
+    // If the function's name should be filled out from a matching Extern,
+    // should they not match.
+    bool prefer_extern_name = false;
   };
 
   struct InlineOrigin {
@@ -317,7 +321,8 @@ class Module {
          const string& architecture,
          const string& id,
          const string& code_id = "",
-         bool enable_multiple_field = false);
+         bool enable_multiple_field = false,
+         bool prefer_extern_name = false);
   ~Module();
 
   // Set the module's load address to LOAD_ADDRESS; addresses given
@@ -502,6 +507,15 @@ class Module {
   // at
   // https://chromium.googlesource.com/breakpad/breakpad/+/master/docs/symbol_files.md#records-3
   bool enable_multiple_field_;
+
+  // If a Function and an Extern share the same address but have a different
+  // name, prefer the name of the Extern.
+  //
+  // Use this when dumping Mach-O .dSYMs built with -gmlt (Minimum Line Tables),
+  // as the Function's fully-qualified name will only be present in the STABS
+  // (which are placed in the Extern), not in the DWARF symbols (which are
+  // placed in the Function).
+  bool prefer_extern_name_;
 };
 
 }  // namespace google_breakpad

src/common/module_unittest.cc

@@ -640,6 +640,37 @@ TEST(Module, ConstructFunctionsAndExternsWithSameAddress) {
                contents.c_str());
 }
 
+// If there exists an extern and a function at the same address, only write
+// out the FUNC entry.
+TEST(Module, ConstructFunctionsAndExternsWithSameAddressPreferExternName) {
+  stringstream s;
+  Module m(MODULE_NAME, MODULE_OS, MODULE_ARCH, MODULE_ID, "", false, true);
+
+  // Two externs.
+  auto extern1 = std::make_unique<Module::Extern>(0xabc0);
+  extern1->name = "extern1";
+  auto extern2 = std::make_unique<Module::Extern>(0xfff0);
+  extern2->name = "extern2";
+
+  m.AddExtern(std::move(extern1));
+  m.AddExtern(std::move(extern2));
+
+  Module::Function* function = new Module::Function("function2", 0xfff0);
+  Module::Range range(0xfff0, 0x10);
+  function->ranges.push_back(range);
+  function->parameter_size = 0;
+  m.AddFunction(function);
+
+  m.Write(s, ALL_SYMBOL_DATA);
+  string contents = s.str();
+
+  EXPECT_STREQ("MODULE " MODULE_OS " " MODULE_ARCH " " MODULE_ID " " MODULE_NAME
+               "\n"
+               "FUNC fff0 10 0 extern2\n"
+               "PUBLIC abc0 0 extern1\n",
+               contents.c_str());
+}
+
 // If there exists an extern and a function at the same address, only write
 // out the FUNC entry, and mark it with `m` if the multiple field is enabled.
 TEST(Module, ConstructFunctionsAndExternsWithSameAddressMultiple) {

src/tools/mac/dump_syms/dump_syms_tool.cc

@@ -64,7 +64,8 @@ struct Options {
         handle_inter_cu_refs(true),
         handle_inlines(false),
         enable_multiple(false),
-        module_name() {}
+        module_name(),
+        prefer_extern_name(false) {}
 
   string srcPath;
   string dsymPath;
@@ -75,6 +76,7 @@ struct Options {
   bool handle_inlines;
   bool enable_multiple;
   string module_name;
+  bool prefer_extern_name;
 };
 
 static bool StackFrameEntryComparator(const Module::StackFrameEntry* a,
@@ -151,7 +153,8 @@ static bool Start(const Options& options) {
       (options.handle_inlines ? INLINES : NO_DATA) |
       (options.cfi ? CFI : NO_DATA) | SYMBOLS_AND_FILES;
   DumpSymbols dump_symbols(symbol_data, options.handle_inter_cu_refs,
-                           options.enable_multiple, options.module_name);
+                           options.enable_multiple, options.module_name,
+                           options.prefer_extern_name);
 
   // For x86_64 binaries, the CFI data is in the __TEXT,__eh_frame of the
   // Mach-O file, which is not copied into the dSYM. Whereas in i386, the CFI
@@ -244,7 +247,7 @@ static void Usage(int argc, const char *argv[]) {
   fprintf(stderr, "Output a Breakpad symbol file from a Mach-o file.\n");
   fprintf(stderr,
           "Usage: %s [-a ARCHITECTURE] [-c] [-g dSYM path] "
-          "[-n MODULE] <Mach-o file>\n",
+          "[-n MODULE] [-x] <Mach-o file>\n",
           argv[0]);
   fprintf(stderr, "\t-i: Output module header information only.\n");
   fprintf(stderr, "\t-a: Architecture type [default: native, or whatever is\n");
@@ -260,6 +263,9 @@ static void Usage(int argc, const char *argv[]) {
   fprintf(stderr,
           "\t-n: Use MODULE as the name of the module rather than \n"
           "the basename of the Mach-O file/dSYM.\n");
+  fprintf(stderr,
+          "\t-x: Prefer the PUBLIC (extern) name over the FUNC if\n"
+          "they do not match.\n");
   fprintf(stderr, "\t-h: Usage\n");
   fprintf(stderr, "\t-?: Usage\n");
 }
@@ -269,7 +275,7 @@ static void SetupOptions(int argc, const char *argv[], Options *options) {
   extern int optind;
   signed char ch;
 
-  while ((ch = getopt(argc, (char* const*)argv, "ia:g:crdm?hn:")) != -1) {
+  while ((ch = getopt(argc, (char* const*)argv, "ia:g:crdm?hn:x")) != -1) {
     switch (ch) {
       case 'i':
         options->header_only = true;
@@ -302,6 +308,9 @@ static void SetupOptions(int argc, const char *argv[], Options *options) {
       case 'n':
         options->module_name = optarg;
         break;
+      case 'x':
+        options->prefer_extern_name = true;
+        break;
       case '?':
       case 'h':
         Usage(argc, argv);