[lldb] Prevent passing a nullptr to std::string in ObjectFileMachO (llvm#100421)

Prevent passing a nullptr to std::string::insert in
ObjectFileMachO::GetDependentModules. Calling GetCString on an empty
ConstString will return a nullptr, which is undefined behavior. Instead,
use the GetString helper which will return an empty string in that case.

rdar://132388027
(cherry picked from commit e846fb4)

Author: JDevlieghere

lldb/source/Plugins/ObjectFile/Mach-O/ObjectFileMachO.cpp

@@ -140,6 +140,9 @@ using namespace lldb;
 using namespace lldb_private;
 using namespace llvm::MachO;
 
+static constexpr llvm::StringLiteral g_loader_path = "@loader_path";
+static constexpr llvm::StringLiteral g_executable_path = "@executable_path";
+
 LLDB_PLUGIN_DEFINE(ObjectFileMachO)
 
 static void PrintRegisterValue(RegisterContext *reg_ctx, const char *name,
@@ -5145,123 +5148,121 @@ UUID ObjectFileMachO::GetUUID() {
 }
 
 uint32_t ObjectFileMachO::GetDependentModules(FileSpecList &files) {
+  ModuleSP module_sp = GetModule();
+  if (!module_sp)
+    return 0;
+
   uint32_t count = 0;
-  ModuleSP module_sp(GetModule());
-  if (module_sp) {
-    std::lock_guard<std::recursive_mutex> guard(module_sp->GetMutex());
-    llvm::MachO::load_command load_cmd;
-    lldb::offset_t offset = MachHeaderSizeFromMagic(m_header.magic);
-    std::vector<std::string> rpath_paths;
-    std::vector<std::string> rpath_relative_paths;
-    std::vector<std::string> at_exec_relative_paths;
-    uint32_t i;
-    for (i = 0; i < m_header.ncmds; ++i) {
-      const uint32_t cmd_offset = offset;
-      if (m_data.GetU32(&offset, &load_cmd, 2) == nullptr)
-        break;
+  std::lock_guard<std::recursive_mutex> guard(module_sp->GetMutex());
+  llvm::MachO::load_command load_cmd;
+  lldb::offset_t offset = MachHeaderSizeFromMagic(m_header.magic);
+  std::vector<std::string> rpath_paths;
+  std::vector<std::string> rpath_relative_paths;
+  std::vector<std::string> at_exec_relative_paths;
+  uint32_t i;
+  for (i = 0; i < m_header.ncmds; ++i) {
+    const uint32_t cmd_offset = offset;
+    if (m_data.GetU32(&offset, &load_cmd, 2) == nullptr)
+      break;
 
-      switch (load_cmd.cmd) {
-      case LC_RPATH:
-      case LC_LOAD_DYLIB:
-      case LC_LOAD_WEAK_DYLIB:
-      case LC_REEXPORT_DYLIB:
-      case LC_LOAD_DYLINKER:
-      case LC_LOADFVMLIB:
-      case LC_LOAD_UPWARD_DYLIB: {
-        uint32_t name_offset = cmd_offset + m_data.GetU32(&offset);
-        // For LC_LOAD_DYLIB there is an alternate encoding
-        // which adds a uint32_t `flags` field for `DYLD_USE_*`
-        // flags.  This can be detected by a timestamp field with
-        // the `DYLIB_USE_MARKER` constant value.
-        bool is_delayed_init = false;
-        uint32_t use_command_marker = m_data.GetU32(&offset);
-        if (use_command_marker == 0x1a741800 /* DYLIB_USE_MARKER */) {
-          offset += 4; /* uint32_t current_version */
-          offset += 4; /* uint32_t compat_version */
-          uint32_t flags = m_data.GetU32(&offset);
-          // If this LC_LOAD_DYLIB is marked delay-init,
-          // don't report it as a dependent library -- it
-          // may be loaded in the process at some point,
-          // but will most likely not be load at launch.
-          if (flags & 0x08 /* DYLIB_USE_DELAYED_INIT */)
-            is_delayed_init = true;
-        }
-        const char *path = m_data.PeekCStr(name_offset);
-        if (path && !is_delayed_init) {
-          if (load_cmd.cmd == LC_RPATH)
-            rpath_paths.push_back(path);
-          else {
-            if (path[0] == '@') {
-              if (strncmp(path, "@rpath", strlen("@rpath")) == 0)
-                rpath_relative_paths.push_back(path + strlen("@rpath"));
-              else if (strncmp(path, "@executable_path",
-                               strlen("@executable_path")) == 0)
-                at_exec_relative_paths.push_back(path +
-                                                 strlen("@executable_path"));
-            } else {
-              FileSpec file_spec(path);
-              if (files.AppendIfUnique(file_spec))
-                count++;
-            }
+    switch (load_cmd.cmd) {
+    case LC_RPATH:
+    case LC_LOAD_DYLIB:
+    case LC_LOAD_WEAK_DYLIB:
+    case LC_REEXPORT_DYLIB:
+    case LC_LOAD_DYLINKER:
+    case LC_LOADFVMLIB:
+    case LC_LOAD_UPWARD_DYLIB: {
+      uint32_t name_offset = cmd_offset + m_data.GetU32(&offset);
+      // For LC_LOAD_DYLIB there is an alternate encoding
+      // which adds a uint32_t `flags` field for `DYLD_USE_*`
+      // flags.  This can be detected by a timestamp field with
+      // the `DYLIB_USE_MARKER` constant value.
+      bool is_delayed_init = false;
+      uint32_t use_command_marker = m_data.GetU32(&offset);
+      if (use_command_marker == 0x1a741800 /* DYLIB_USE_MARKER */) {
+        offset += 4; /* uint32_t current_version */
+        offset += 4; /* uint32_t compat_version */
+        uint32_t flags = m_data.GetU32(&offset);
+        // If this LC_LOAD_DYLIB is marked delay-init,
+        // don't report it as a dependent library -- it
+        // may be loaded in the process at some point,
+        // but will most likely not be load at launch.
+        if (flags & 0x08 /* DYLIB_USE_DELAYED_INIT */)
+          is_delayed_init = true;
+      }
+      const char *path = m_data.PeekCStr(name_offset);
+      if (path && !is_delayed_init) {
+        if (load_cmd.cmd == LC_RPATH)
+          rpath_paths.push_back(path);
+        else {
+          if (path[0] == '@') {
+            if (strncmp(path, "@rpath", strlen("@rpath")) == 0)
+              rpath_relative_paths.push_back(path + strlen("@rpath"));
+            else if (strncmp(path, "@executable_path",
+                             strlen("@executable_path")) == 0)
+              at_exec_relative_paths.push_back(path +
+                                               strlen("@executable_path"));
+          } else {
+            FileSpec file_spec(path);
+            if (files.AppendIfUnique(file_spec))
+              count++;
           }
         }
-      } break;
-
-      default:
-        break;
       }
-      offset = cmd_offset + load_cmd.cmdsize;
-    }
+    } break;
 
-    FileSpec this_file_spec(m_file);
-    FileSystem::Instance().Resolve(this_file_spec);
-
-    if (!rpath_paths.empty()) {
-      // Fixup all LC_RPATH values to be absolute paths
-      std::string loader_path("@loader_path");
-      std::string executable_path("@executable_path");
-      for (auto &rpath : rpath_paths) {
-        if (llvm::StringRef(rpath).starts_with(loader_path)) {
-          rpath.erase(0, loader_path.size());
-          rpath.insert(0, this_file_spec.GetDirectory().GetCString());
-        } else if (llvm::StringRef(rpath).starts_with(executable_path)) {
-          rpath.erase(0, executable_path.size());
-          rpath.insert(0, this_file_spec.GetDirectory().GetCString());
-        }
-      }
+    default:
+      break;
+    }
+    offset = cmd_offset + load_cmd.cmdsize;
+  }
 
-      for (const auto &rpath_relative_path : rpath_relative_paths) {
-        for (const auto &rpath : rpath_paths) {
-          std::string path = rpath;
-          path += rpath_relative_path;
-          // It is OK to resolve this path because we must find a file on disk
-          // for us to accept it anyway if it is rpath relative.
-          FileSpec file_spec(path);
-          FileSystem::Instance().Resolve(file_spec);
-          if (FileSystem::Instance().Exists(file_spec) &&
-              files.AppendIfUnique(file_spec)) {
-            count++;
-            break;
-          }
-        }
-      }
+  FileSpec this_file_spec(m_file);
+  FileSystem::Instance().Resolve(this_file_spec);
+
+  if (!rpath_paths.empty()) {
+    // Fixup all LC_RPATH values to be absolute paths.
+    const std::string this_directory =
+        this_file_spec.GetDirectory().GetString();
+    for (auto &rpath : rpath_paths) {
+      if (llvm::StringRef(rpath).starts_with(g_loader_path))
+        rpath = this_directory + rpath.substr(g_loader_path.size());
+      else if (llvm::StringRef(rpath).starts_with(g_executable_path))
+        rpath = this_directory + rpath.substr(g_executable_path.size());
     }
 
-    // We may have @executable_paths but no RPATHS.  Figure those out here.
-    // Only do this if this object file is the executable.  We have no way to
-    // get back to the actual executable otherwise, so we won't get the right
-    // path.
-    if (!at_exec_relative_paths.empty() && CalculateType() == eTypeExecutable) {
-      FileSpec exec_dir = this_file_spec.CopyByRemovingLastPathComponent();
-      for (const auto &at_exec_relative_path : at_exec_relative_paths) {
-        FileSpec file_spec =
-            exec_dir.CopyByAppendingPathComponent(at_exec_relative_path);
+    for (const auto &rpath_relative_path : rpath_relative_paths) {
+      for (const auto &rpath : rpath_paths) {
+        std::string path = rpath;
+        path += rpath_relative_path;
+        // It is OK to resolve this path because we must find a file on disk
+        // for us to accept it anyway if it is rpath relative.
+        FileSpec file_spec(path);
+        FileSystem::Instance().Resolve(file_spec);
         if (FileSystem::Instance().Exists(file_spec) &&
-            files.AppendIfUnique(file_spec))
+            files.AppendIfUnique(file_spec)) {
           count++;
+          break;
+        }
       }
     }
   }
+
+  // We may have @executable_paths but no RPATHS.  Figure those out here.
+  // Only do this if this object file is the executable.  We have no way to
+  // get back to the actual executable otherwise, so we won't get the right
+  // path.
+  if (!at_exec_relative_paths.empty() && CalculateType() == eTypeExecutable) {
+    FileSpec exec_dir = this_file_spec.CopyByRemovingLastPathComponent();
+    for (const auto &at_exec_relative_path : at_exec_relative_paths) {
+      FileSpec file_spec =
+          exec_dir.CopyByAppendingPathComponent(at_exec_relative_path);
+      if (FileSystem::Instance().Exists(file_spec) &&
+          files.AppendIfUnique(file_spec))
+        count++;
+    }
+  }
   return count;
 }