Support marking folded symbols on Posix

This is similar to the Windows change at
https://chromium-review.googlesource.com/c/breakpad/breakpad/+/773418/

When a `Module` is created with `enable_multiple_field_` = true,
all FUNCs and PUBLICs that share the same address will be collapsed
into a single entry, and that entry will be marked with `m` for
multiple in the final output.

`enable_multiple_field_` is temporary just in case people are
depending on the current behavior.

Support for `dump_syms` executables will be added in a follow-up.

Bug: google-breakpad:751
Change-Id: I631a148ed00138924c7bcb5ad6db8b9a6610dd03
Reviewed-on: https://chromium-review.googlesource.com/c/breakpad/breakpad/+/3905122
Reviewed-by: Mark Mentovai <mark@chromium.org>

src/common/module.cc

@@ -96,15 +96,19 @@ void Module::InlineOriginMap::SetReference(uint64_t offset,
   references_[offset] = specification_offset;
 }
 
-Module::Module(const string& name, const string& os,
-               const string& architecture, const string& id,
-               const string& code_id /* = "" */) :
-    name_(name),
+Module::Module(const string& name,
+               const string& os,
+               const string& architecture,
+               const string& id,
+               const string& code_id /* = "" */,
+               bool enable_multiple_field /* = false*/)
+    : name_(name),
       os_(os),
       architecture_(architecture),
       id_(id),
       code_id_(code_id),
-    load_address_(0) { }
+      load_address_(0),
+      enable_multiple_field_(enable_multiple_field) {}
 
 Module::~Module() {
   for (FileByNameMap::iterator it = files_.begin(); it != files_.end(); ++it)
@@ -150,6 +154,12 @@ 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;
+      // If the PUBLIC is for the same symbol as the FUNC, don't mark multiple.
+      function->is_multiple |=
+          found_ext->name != function->name || found_ext->is_multiple;
+    }
     delete *it_ext;
     externs_.erase(it_ext);
   }
@@ -164,13 +174,27 @@ bool Module::AddFunction(Function* function) {
     }
   }
 #endif
-
+  if (enable_multiple_field_) {
+    FunctionSet::iterator existing_function = std::find_if(
+        functions_.begin(), functions_.end(),
+        [&](Function* other) { return other->address == function->address; });
+    if (existing_function != functions_.end()) {
+      (*existing_function)->is_multiple = true;
+      // Free the duplicate that was not inserted because this Module
+      // now owns it.
+      return false;
+    }
+    std::pair<FunctionSet::iterator, bool> ret = functions_.insert(function);
+    // We just checked!
+    assert(ret.second);
+  } else {
     std::pair<FunctionSet::iterator, bool> ret = functions_.insert(function);
     if (!ret.second && (*ret.first != function)) {
       // Free the duplicate that was not inserted because this Module
       // now owns it.
       return false;
     }
+  }
   return true;
 }
 
@@ -188,7 +212,8 @@ void Module::AddExtern(Extern* ext) {
   }
 
   std::pair<ExternSet::iterator,bool> ret = externs_.insert(ext);
-  if (!ret.second) {
+  if (!ret.second && enable_multiple_field_) {
+    (*ret.first)->is_multiple = true;
     // Free the duplicate that was not inserted because this Module
     // now owns it.
     delete ext;
@@ -378,11 +403,10 @@ bool Module::Write(std::ostream& stream, SymbolData symbol_data) {
       vector<Line>::iterator line_it = func->lines.begin();
       for (auto range_it = func->ranges.cbegin();
            range_it != func->ranges.cend(); ++range_it) {
-        stream << "FUNC " << hex
-               << (range_it->address - load_address_) << " "
-               << range_it->size << " "
-               << func->parameter_size << " "
-               << func->name << dec << "\n";
+        stream << "FUNC " << (func->is_multiple ? "m " : "") << hex
+               << (range_it->address - load_address_) << " " << range_it->size
+               << " " << func->parameter_size << " " << func->name << dec
+               << "\n";
 
         if (!stream.good())
           return ReportError();
@@ -422,9 +446,9 @@ bool Module::Write(std::ostream& stream, SymbolData symbol_data) {
     for (ExternSet::const_iterator extern_it = externs_.begin();
          extern_it != externs_.end(); ++extern_it) {
       Extern* ext = *extern_it;
-      stream << "PUBLIC " << hex
-             << (ext->address - load_address_) << " 0 "
-             << ext->name << dec << "\n";
+      stream << "PUBLIC " << (ext->is_multiple ? "m " : "") << hex
+             << (ext->address - load_address_) << " 0 " << ext->name << dec
+             << "\n";
     }
   }
 

src/common/module.h

@@ -128,6 +128,9 @@ class Module {
 
     // Inlined call sites belonging to this functions.
     vector<std::unique_ptr<Inline>> inlines;
+
+    // If this symbol has been folded with other symbols in the linked binary.
+    bool is_multiple = false;
   };
 
   struct InlineOrigin {
@@ -241,6 +244,8 @@ class Module {
     explicit Extern(const Address& address_input) : address(address_input) {}
     const Address address;
     string name;
+    // If this symbol has been folded with other symbols in the linked binary.
+    bool is_multiple = false;
   };
 
   // A map from register names to postfix expressions that recover
@@ -294,8 +299,14 @@ class Module {
 
   // Create a new module with the given name, operating system,
   // architecture, and ID string.
-  Module(const string& name, const string& os, const string& architecture,
-         const string& id, const string& code_id = "");
+  // NB: `enable_multiple_field` is temporary while transitioning to enabling
+  // writing the multiple field permanently.
+  Module(const string& name,
+         const string& os,
+         const string& architecture,
+         const string& id,
+         const string& code_id = "",
+         bool enable_multiple_field = false);
   ~Module();
 
   // Set the module's load address to LOAD_ADDRESS; addresses given
@@ -471,6 +482,13 @@ class Module {
   ExternSet externs_;
 
   unordered_set<string> common_strings_;
+
+  // Whether symbols sharing an address should be collapsed into a single entry
+  // and marked with an `m` in the output. See
+  // https://bugs.chromium.org/p/google-breakpad/issues/detail?id=751 and docs
+  // at
+  // https://chromium.googlesource.com/breakpad/breakpad/+/master/docs/symbol_files.md#records-3
+  bool enable_multiple_field_;
 };
 
 }  // namespace google_breakpad

src/common/module_unittest.cc

@@ -506,6 +506,34 @@ TEST(Module, ConstructFunctionsWithSameAddress) {
                contents.c_str());
 }
 
+// If multiple fields are enabled, only one function is included per address.
+// The entry will be tagged with `m` to show that there are multiple symbols
+// at that address.
+// TODO(lgrey): Remove the non-multiple versions of these tests and remove the
+// suffixes from the suffxed ones when removing `enable_multiple_field_`.
+TEST(Module, ConstructFunctionsWithSameAddressMultiple) {
+  stringstream s;
+  Module m(MODULE_NAME, MODULE_OS, MODULE_ARCH, MODULE_ID, "", true);
+
+  // Two functions.
+  Module::Function* function1 = generate_duplicate_function("_without_form");
+  Module::Function* function2 = generate_duplicate_function("_and_void");
+
+  m.AddFunction(function1);
+  // If this succeeds, we'll have a double-free with the `delete` below. Avoid
+  // that.
+  ASSERT_FALSE(m.AddFunction(function2));
+  delete function2;
+
+  m.Write(s, ALL_SYMBOL_DATA);
+  string contents = s.str();
+  EXPECT_STREQ(
+      "MODULE os-name architecture id-string name with spaces\n"
+      "FUNC m d35402aac7a7ad5c 200b26e605f99071 f14ac4fed48c4a99"
+      " _without_form\n",
+      contents.c_str());
+}
+
 // Externs should be written out as PUBLIC records, sorted by
 // address.
 TEST(Module, ConstructExterns) {
@@ -554,6 +582,29 @@ TEST(Module, ConstructDuplicateExterns) {
                "PUBLIC ffff 0 _xyz\n",
                contents.c_str());
 }
+// Externs with the same address  have the `m` tag if the multiple field are
+// enabled.
+TEST(Module, ConstructDuplicateExternsMultiple) {
+  stringstream s;
+  Module m(MODULE_NAME, MODULE_OS, MODULE_ARCH, MODULE_ID, "", true);
+
+  // Two externs.
+  Module::Extern* extern1 = new Module::Extern(0xffff);
+  extern1->name = "_xyz";
+  Module::Extern* extern2 = new Module::Extern(0xffff);
+  extern2->name = "_abc";
+
+  m.AddExtern(extern1);
+  m.AddExtern(extern2);
+
+  m.Write(s, ALL_SYMBOL_DATA);
+  string contents = s.str();
+
+  EXPECT_STREQ("MODULE " MODULE_OS " " MODULE_ARCH " " MODULE_ID " " MODULE_NAME
+               "\n"
+               "PUBLIC m ffff 0 _xyz\n",
+               contents.c_str());
+}
 
 // If there exists an extern and a function at the same address, only write
 // out the FUNC entry.
@@ -586,6 +637,37 @@ TEST(Module, ConstructFunctionsAndExternsWithSameAddress) {
                contents.c_str());
 }
 
+// If there exists an extern and a function at the same address, only write
+// out the FUNC entry, and mark it with `m` if the multiple field is enabled.
+TEST(Module, ConstructFunctionsAndExternsWithSameAddressMultiple) {
+  stringstream s;
+  Module m(MODULE_NAME, MODULE_OS, MODULE_ARCH, MODULE_ID, "", true);
+
+  // Two externs.
+  Module::Extern* extern1 = new Module::Extern(0xabc0);
+  extern1->name = "abc";
+  Module::Extern* extern2 = new Module::Extern(0xfff0);
+  extern2->name = "xyz";
+
+  m.AddExtern(extern1);
+  m.AddExtern(extern2);
+
+  Module::Function* function = new Module::Function("_xyz", 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 m fff0 10 0 _xyz\n"
+               "PUBLIC abc0 0 abc\n",
+               contents.c_str());
+}
+
 // If there exists an extern and a function at the same address, only write
 // out the FUNC entry. For ARM THUMB, the extern that comes from the ELF
 // symbol section has bit 0 set.