diff --git a/filesystem_iterator.cc b/filesystem_iterator.cc
new file mode 100644
index 0000000..9d98dea
--- /dev/null
+++ b/filesystem_iterator.cc
@@ -0,0 +1,147 @@
+// Copyright (c) 2009 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "update_engine/filesystem_iterator.h"
+#include <sys/types.h>
+#include <dirent.h>
+#include <errno.h>
+#include <set>
+#include <string>
+#include <vector>
+#include "chromeos/obsolete_logging.h"
+#include "update_engine/utils.h"
+
+using std::set;
+using std::string;
+using std::vector;
+
+namespace chromeos_update_engine {
+
+// We use a macro here for two reasons:
+// 1. We want to be able to return from the caller's function.
+// 2. We can use the #macro_arg ism to get a string of the calling statement,
+//    which we can log.
+
+#define RETURN_ERROR_IF_FALSE(_statement)                               \
+  do {                                                                  \
+    bool _result = (_statement);                                        \
+    if (!_result) {                                                     \
+      string _message = utils::ErrnoNumberAsString(errno);              \
+      LOG(INFO) << #_statement << " failed: " << _message << ". Aborting"; \
+      is_end_ = true;                                                   \
+      is_err_ = true;                                                   \
+      return;                                                           \
+    }                                                                   \
+  } while (0)
+
+FilesystemIterator::FilesystemIterator(
+    const std::string& path,
+    const std::set<std::string>& excl_prefixes)
+    : excl_prefixes_(excl_prefixes),
+      is_end_(false),
+      is_err_(false) {
+  root_path_ = utils::NormalizePath(path, true);
+  RETURN_ERROR_IF_FALSE(lstat(root_path_.c_str(), &stbuf_) == 0);
+  root_dev_ = stbuf_.st_dev;
+}
+
+FilesystemIterator::~FilesystemIterator() {
+  for (vector<DIR*>::iterator it = dirs_.begin(); it != dirs_.end(); ++it) {
+    LOG_IF(ERROR, closedir(*it) != 0) << "closedir failed";
+  }
+}
+
+// Returns full path for current file
+std::string FilesystemIterator::GetFullPath() const {
+  return root_path_ + GetPartialPath();
+}
+
+std::string FilesystemIterator::GetPartialPath() const {
+  std::string ret;
+  for (vector<string>::const_iterator it = names_.begin();
+       it != names_.end(); ++it) {
+    ret += "/";
+    ret += *it;
+  }
+  return ret;
+}
+
+// Increments to the next file
+void FilesystemIterator::Increment() {
+  // If we're currently on a dir, descend into children, but only if
+  // we're on the same device as the root device
+
+  bool entering_dir = false;  // true if we're entering into a new dir
+  if (S_ISDIR(stbuf_.st_mode) && (stbuf_.st_dev == root_dev_)) {
+    DIR* dir = opendir(GetFullPath().c_str());
+    if ((!dir) && ((errno == ENOTDIR) || (errno == ENOENT))) {
+      // opendir failed b/c either it's not a dir or it doesn't exist.
+      // that's fine. let's just skip over this.
+      LOG(ERROR) << "Can't descend into " << GetFullPath();
+    } else {
+      RETURN_ERROR_IF_FALSE(dir);
+      entering_dir = true;
+      dirs_.push_back(dir);
+    }
+  }
+
+  if (!entering_dir && names_.empty()) {
+    // root disappeared while we tried to descend into it
+    is_end_ = true;
+    return;
+  }
+
+  if (!entering_dir)
+    names_.pop_back();
+
+  IncrementInternal();
+  for (set<string>::const_iterator it = excl_prefixes_.begin();
+       it != excl_prefixes_.end(); ++it) {
+    if (utils::StringHasPrefix(GetPartialPath(), *it)) {
+      Increment();
+      break;
+    }
+  }
+  return;
+}
+
+// Assumes that we need to find the next child of dirs_.back(), or if
+// there are none more, go up the chain
+void FilesystemIterator::IncrementInternal() {
+  CHECK_EQ(dirs_.size(), names_.size() + 1);
+  for (;;) {
+    struct dirent dir_entry;
+    struct dirent* dir_entry_pointer;
+    int r;
+    RETURN_ERROR_IF_FALSE(
+        (r = readdir_r(dirs_.back(), &dir_entry, &dir_entry_pointer)) == 0);
+    if (dir_entry_pointer) {
+      // Found an entry
+      names_.push_back(dir_entry_pointer->d_name);
+      // Validate
+      RETURN_ERROR_IF_FALSE(lstat(GetFullPath().c_str(), &stbuf_) == 0);
+      if (strcmp(dir_entry_pointer->d_name, ".") &&
+          strcmp(dir_entry_pointer->d_name, "..")) {
+        // Done
+        return;
+      }
+      // Child didn't work out. Try again
+      names_.pop_back();
+    } else {
+      // No more children in this dir. Pop it and try again
+      RETURN_ERROR_IF_FALSE(closedir(dirs_.back()) == 0);
+      dirs_.pop_back();
+      if (dirs_.empty()) {
+        CHECK(names_.empty());
+        // Done with the entire iteration
+        is_end_ = true;
+        return;
+      }
+      CHECK(!names_.empty());
+      names_.pop_back();
+    }
+  }
+}
+
+}  //   namespace chromeos_update_engine