diff --git a/common/fake_boot_control.h b/common/fake_boot_control.h
index 5c6c160..3eccc80 100644
--- a/common/fake_boot_control.h
+++ b/common/fake_boot_control.h
@@ -85,9 +85,9 @@
     current_slot_ = slot;
   }
 
-  void SetPartitionDevice(const std::string partition_name,
+  void SetPartitionDevice(const std::string& partition_name,
                           BootControlInterface::Slot slot,
-                          const std::string device) {
+                          const std::string& device) {
     DCHECK(slot < num_slots_);
     devices_[slot][partition_name] = device;
   }
diff --git a/common/fake_hardware.h b/common/fake_hardware.h
index f7c0286..5d0fca3 100644
--- a/common/fake_hardware.h
+++ b/common/fake_hardware.h
@@ -109,15 +109,15 @@
     is_oobe_complete_ = false;
   }
 
-  void SetHardwareClass(std::string hardware_class) {
+  void SetHardwareClass(const std::string& hardware_class) {
     hardware_class_ = hardware_class;
   }
 
-  void SetFirmwareVersion(std::string firmware_version) {
+  void SetFirmwareVersion(const std::string& firmware_version) {
     firmware_version_ = firmware_version;
   }
 
-  void SetECVersion(std::string ec_version) {
+  void SetECVersion(const std::string& ec_version) {
     ec_version_ = ec_version;
   }
 
diff --git a/common/subprocess_unittest.cc b/common/subprocess_unittest.cc
index 58d4191..7dbdf98 100644
--- a/common/subprocess_unittest.cc
+++ b/common/subprocess_unittest.cc
@@ -88,7 +88,7 @@
 void ExpectedEnvVars(int return_code, const string& output) {
   EXPECT_EQ(0, return_code);
   const std::set<string> allowed_envs = {"LD_LIBRARY_PATH", "PATH"};
-  for (string key_value : brillo::string_utils::Split(output, "\n")) {
+  for (const string& key_value : brillo::string_utils::Split(output, "\n")) {
     auto key_value_pair = brillo::string_utils::SplitAtFirst(
         key_value, "=", true);
     EXPECT_NE(allowed_envs.end(), allowed_envs.find(key_value_pair.first));
diff --git a/common/utils.cc b/common/utils.cc
index a352961..101e0b8 100644
--- a/common/utils.cc
+++ b/common/utils.cc
@@ -246,7 +246,7 @@
   return true;
 }
 
-bool WriteAll(FileDescriptorPtr fd, const void* buf, size_t count) {
+bool WriteAll(const FileDescriptorPtr& fd, const void* buf, size_t count) {
   const char* c_buf = static_cast<const char*>(buf);
   ssize_t bytes_written = 0;
   while (bytes_written < static_cast<ssize_t>(count)) {
@@ -257,7 +257,7 @@
   return true;
 }
 
-bool PWriteAll(FileDescriptorPtr fd,
+bool PWriteAll(const FileDescriptorPtr& fd,
                const void* buf,
                size_t count,
                off_t offset) {
@@ -283,7 +283,7 @@
   return true;
 }
 
-bool PReadAll(FileDescriptorPtr fd, void* buf, size_t count, off_t offset,
+bool PReadAll(const FileDescriptorPtr& fd, void* buf, size_t count, off_t offset,
               ssize_t* out_bytes_read) {
   TEST_AND_RETURN_FALSE_ERRNO(fd->Seek(offset, SEEK_SET) !=
                               static_cast<off_t>(-1));
diff --git a/common/utils.h b/common/utils.h
index 3987484..6bc030b 100644
--- a/common/utils.h
+++ b/common/utils.h
@@ -69,8 +69,8 @@
 bool WriteAll(int fd, const void* buf, size_t count);
 bool PWriteAll(int fd, const void* buf, size_t count, off_t offset);
 
-bool WriteAll(FileDescriptorPtr fd, const void* buf, size_t count);
-bool PWriteAll(FileDescriptorPtr fd,
+bool WriteAll(const FileDescriptorPtr& fd, const void* buf, size_t count);
+bool PWriteAll(const FileDescriptorPtr& fd,
                const void* buf,
                size_t count,
                off_t offset);
@@ -88,7 +88,7 @@
 bool PReadAll(int fd, void* buf, size_t count, off_t offset,
               ssize_t* out_bytes_read);
 
-bool PReadAll(FileDescriptorPtr fd, void* buf, size_t count, off_t offset,
+bool PReadAll(const FileDescriptorPtr& fd, void* buf, size_t count, off_t offset,
               ssize_t* out_bytes_read);
 
 // Opens |path| for reading and appends its entire content to the container
diff --git a/payload_consumer/delta_performer.cc b/payload_consumer/delta_performer.cc
index 072cd57..a1cf2ab 100644
--- a/payload_consumer/delta_performer.cc
+++ b/payload_consumer/delta_performer.cc
@@ -135,7 +135,7 @@
 // Discard the tail of the block device referenced by |fd|, from the offset
 // |data_size| until the end of the block device. Returns whether the data was
 // discarded.
-bool DiscardPartitionTail(FileDescriptorPtr fd, uint64_t data_size) {
+bool DiscardPartitionTail(const FileDescriptorPtr& fd, uint64_t data_size) {
   uint64_t part_size = fd->BlockDevSize();
   if (!part_size || part_size <= data_size)
     return false;
@@ -951,8 +951,7 @@
 #endif  // !defined(BLKZEROOUT)
 
   brillo::Blob zeros;
-  for (int i = 0; i < operation.dst_extents_size(); i++) {
-    Extent extent = operation.dst_extents(i);
+  for (const Extent& extent : operation.dst_extents()) {
     const uint64_t start = extent.start_block() * block_size_;
     const uint64_t length = extent.num_blocks() * block_size_;
     if (attempt_ioctl) {
@@ -1030,7 +1029,7 @@
 // each block in |extents|. For example, [(3, 2), (8, 1)] would give [3, 4, 8].
 void ExtentsToBlocks(const RepeatedPtrField<Extent>& extents,
                      vector<uint64_t>* blocks) {
-  for (Extent ext : extents) {
+  for (const Extent& ext : extents) {
     for (uint64_t j = 0; j < ext.num_blocks(); j++)
       blocks->push_back(ext.start_block() + j);
   }
@@ -1039,7 +1038,7 @@
 // Takes |extents| and returns the number of blocks in those extents.
 uint64_t GetBlockCount(const RepeatedPtrField<Extent>& extents) {
   uint64_t sum = 0;
-  for (Extent ext : extents) {
+  for (const Extent& ext : extents) {
     sum += ext.num_blocks();
   }
   return sum;
@@ -1151,8 +1150,7 @@
     string* positions_string) {
   string ret;
   uint64_t length = 0;
-  for (int i = 0; i < extents.size(); i++) {
-    Extent extent = extents.Get(i);
+  for (const Extent& extent : extents) {
     int64_t start = extent.start_block() * block_size;
     uint64_t this_length =
         min(full_length - length,
@@ -1709,7 +1707,7 @@
 }
 
 bool DeltaPerformer::CanResumeUpdate(PrefsInterface* prefs,
-                                     string update_check_response_hash) {
+                                     const string& update_check_response_hash) {
   int64_t next_operation = kUpdateStateOperationInvalid;
   if (!(prefs->GetInt64(kPrefsUpdateStateNextOperation, &next_operation) &&
         next_operation != kUpdateStateOperationInvalid &&
diff --git a/payload_consumer/delta_performer.h b/payload_consumer/delta_performer.h
index 7af59e6..74143e0 100644
--- a/payload_consumer/delta_performer.h
+++ b/payload_consumer/delta_performer.h
@@ -140,7 +140,7 @@
   // Returns true if a previous update attempt can be continued based on the
   // persistent preferences and the new update check response hash.
   static bool CanResumeUpdate(PrefsInterface* prefs,
-                              std::string update_check_response_hash);
+                              const std::string& update_check_response_hash);
 
   // Resets the persistent update progress state to indicate that an update
   // can't be resumed. Performs a quick update-in-progress reset if |quick| is
diff --git a/payload_generator/ab_generator.cc b/payload_generator/ab_generator.cc
index 8c736a5..efb8ccf 100644
--- a/payload_generator/ab_generator.cc
+++ b/payload_generator/ab_generator.cc
@@ -110,7 +110,7 @@
   int curr_src_ext_index = 0;
   Extent curr_src_ext = original_op.src_extents(curr_src_ext_index);
   for (int i = 0; i < original_op.dst_extents_size(); i++) {
-    Extent dst_ext = original_op.dst_extents(i);
+    const Extent& dst_ext = original_op.dst_extents(i);
     // The new operation which will have only one dst extent.
     InstallOperation new_op;
     uint64_t blocks_left = dst_ext.num_blocks();
@@ -165,7 +165,7 @@
 
   uint32_t data_offset = original_op.data_offset();
   for (int i = 0; i < original_op.dst_extents_size(); i++) {
-    Extent dst_ext = original_op.dst_extents(i);
+    const Extent& dst_ext = original_op.dst_extents(i);
     // Make a new operation with only one dst extent.
     InstallOperation new_op;
     *(new_op.add_dst_extents()) = dst_ext;
diff --git a/payload_generator/ab_generator_unittest.cc b/payload_generator/ab_generator_unittest.cc
index 224880d..3fd2323 100644
--- a/payload_generator/ab_generator_unittest.cc
+++ b/payload_generator/ab_generator_unittest.cc
@@ -42,7 +42,7 @@
 
 namespace {
 
-bool ExtentEquals(Extent ext, uint64_t start_block, uint64_t num_blocks) {
+bool ExtentEquals(const Extent& ext, uint64_t start_block, uint64_t num_blocks) {
   return ext.start_block() == start_block && ext.num_blocks() == num_blocks;
 }
 
diff --git a/payload_generator/delta_diff_utils.cc b/payload_generator/delta_diff_utils.cc
index 41bf0b3..50fbdf2 100644
--- a/payload_generator/delta_diff_utils.cc
+++ b/payload_generator/delta_diff_utils.cc
@@ -370,7 +370,7 @@
   // TODO(deymo): Produce ZERO operations instead of calling DeltaReadFile().
   size_t num_ops = aops->size();
   new_visited_blocks->AddExtents(new_zeros);
-  for (Extent extent : new_zeros) {
+  for (const Extent& extent : new_zeros) {
     TEST_AND_RETURN_FALSE(DeltaReadFile(aops,
                                         "",
                                         new_part,
@@ -391,7 +391,7 @@
   uint64_t used_blocks = 0;
   old_visited_blocks->AddExtents(old_identical_blocks);
   new_visited_blocks->AddExtents(new_identical_blocks);
-  for (Extent extent : new_identical_blocks) {
+  for (const Extent& extent : new_identical_blocks) {
     // We split the operation at the extent boundary or when bigger than
     // chunk_blocks.
     for (uint64_t op_block_offset = 0; op_block_offset < extent.num_blocks();
diff --git a/payload_generator/fake_filesystem.cc b/payload_generator/fake_filesystem.cc
index c765286..234e2f6 100644
--- a/payload_generator/fake_filesystem.cc
+++ b/payload_generator/fake_filesystem.cc
@@ -39,7 +39,7 @@
 }
 
 void FakeFilesystem::AddFile(const std::string& filename,
-                             const std::vector<Extent> extents) {
+                             const std::vector<Extent>& extents) {
   File file;
   file.name = filename;
   file.extents = extents;
diff --git a/payload_generator/fake_filesystem.h b/payload_generator/fake_filesystem.h
index a14b8d3..1b13920 100644
--- a/payload_generator/fake_filesystem.h
+++ b/payload_generator/fake_filesystem.h
@@ -43,7 +43,7 @@
   // Fake methods.
 
   // Add a file to the list of fake files.
-  void AddFile(const std::string& filename, const std::vector<Extent> extents);
+  void AddFile(const std::string& filename, const std::vector<Extent>& extents);
 
   // Sets the PAYLOAD_MINOR_VERSION key stored by LoadSettings(). Use a negative
   // value to produce an error in LoadSettings().
diff --git a/payload_generator/inplace_generator.cc b/payload_generator/inplace_generator.cc
index be8b487..bc140e8 100644
--- a/payload_generator/inplace_generator.cc
+++ b/payload_generator/inplace_generator.cc
@@ -704,7 +704,7 @@
 bool InplaceGenerator::AddInstallOpToGraph(Graph* graph,
                                            Vertex::Index existing_vertex,
                                            vector<Block>* blocks,
-                                           const InstallOperation operation,
+                                           const InstallOperation& operation,
                                            const string& op_name) {
   Vertex::Index vertex = existing_vertex;
   if (vertex == Vertex::kInvalidIndex) {
diff --git a/payload_generator/inplace_generator.h b/payload_generator/inplace_generator.h
index 48a1fac..f108639 100644
--- a/payload_generator/inplace_generator.h
+++ b/payload_generator/inplace_generator.h
@@ -193,7 +193,7 @@
   static bool AddInstallOpToGraph(Graph* graph,
                                   Vertex::Index existing_vertex,
                                   std::vector<Block>* blocks,
-                                  const InstallOperation operation,
+                                  const InstallOperation& operation,
                                   const std::string& op_name);
 
   // Apply the transformation stored in |the_map| to the |collection| vector
diff --git a/payload_generator/payload_signer_unittest.cc b/payload_generator/payload_signer_unittest.cc
index e159f08..62b6e7a 100644
--- a/payload_generator/payload_signer_unittest.cc
+++ b/payload_generator/payload_signer_unittest.cc
@@ -174,7 +174,7 @@
   EXPECT_EQ(1, signatures.signatures_size());
   const Signatures_Signature& signature = signatures.signatures(0);
   EXPECT_EQ(1U, signature.version());
-  const string sig_data = signature.data();
+  const string& sig_data = signature.data();
   ASSERT_EQ(arraysize(kDataSignature), sig_data.size());
   for (size_t i = 0; i < arraysize(kDataSignature); i++) {
     EXPECT_EQ(kDataSignature[i], static_cast<uint8_t>(sig_data[i]));