fix(manifest): flag backup files listed in the manifest but not stored

An incomplete iTunes backup still lists in its Manifest.db the files it
failed to acquire, so a module which found nothing for one of them looks
exactly like a module which found nothing on a device that never had the
artifact. The Manifest module now records a "missing" flag on those
records and reports the total, so a gap in the acquisition is visible in
manifest.json and in the command output.

The stored file IDs are collected by walking the backup folder once, which
keeps the check off the per-entry filesystem lookup path: on a 20k entry
manifest the module runs in the same time as before the change.

docs/ios/records.md

@@ -204,6 +204,8 @@ This JSON file is created by mvt-ios' `Manifest` module. The module extracts rec
 
 If indicators are provided through the command-line, they are checked against the original relative path in case. In some cases, there might be records of files created containing a domain name in their name, for example in the case of browser cache folders. Any matches are stored in *manifest_detected.json*.
 
+An incomplete backup still lists in its manifest the files it failed to acquire. Those records carry a `"missing": true` field, and the module reports how many of them it found. Use it to tell a module which returned nothing because the artifact was not acquired apart from one which found nothing on a device that never had it.
+
 ---
 
 ### `os_analytics_ad_daily.json`

src/mvt/ios/modules/backup/manifest.py

@@ -147,6 +147,12 @@ class Manifest(IOSExtraction):
             )
         names = [description[0] for description in cur.description]
 
+        # An incomplete backup still lists the files it failed to acquire in
+        # its manifest. Only regular files are stored in the backup folder, so
+        # directories and symlinks (flags 2 and 4) are not looked up.
+        stored_file_ids = self._get_stored_backup_file_ids()
+        missing_files = 0
+
         for file_entry in cur:
             file_data = {}
             for index, value in enumerate(file_entry):
@@ -160,6 +166,18 @@ class Manifest(IOSExtraction):
                 "created": "",
             }
 
+            if file_data["flags"] == 1 and file_data["fileID"] not in stored_file_ids:
+                # Without this, a module which found nothing for one of these
+                # files would look like a negative result rather than a gap in
+                # the acquisition.
+                cleaned_metadata["missing"] = True
+                missing_files += 1
+                self.log.debug(
+                    "File %s is listed in the manifest but was not found in the "
+                    "backup folder",
+                    cleaned_metadata["relative_path"],
+                )
+
             if file_data["file"]:
                 try:
                     file_plist = plistlib.load(io.BytesIO(file_data["file"]))
@@ -197,3 +215,10 @@ class Manifest(IOSExtraction):
         conn.close()
 
         self.log.info("Extracted a total of %d file metadata items", len(self.results))
+
+        if missing_files:
+            self.log.info(
+                "Found %d files listed in the manifest but missing from the backup "
+                "folder. The backup might be incomplete.",
+                missing_files,
+            )

src/mvt/ios/modules/base.py

@@ -216,6 +216,41 @@ class IOSExtraction(MVTModule):
 
         return None
 
+    def _get_stored_backup_file_ids(self) -> set[str]:
+        """List the IDs of the files actually stored in the backup folder.
+
+        A backup folder stores each file under a two character subfolder named
+        after the first two characters of its file ID. Walking the folders once
+        is cheaper than a filesystem lookup per file ID, and it keeps callers
+        which compare a whole manifest against the folder from paying a
+        `resolve()` for every entry.
+
+        :returns: The file IDs found in the backup folder, empty if there is
+                  no backup folder to walk.
+        """
+        if not self.target_path:
+            return set()
+
+        file_ids: set[str] = set()
+        try:
+            with os.scandir(self.target_path) as entries:
+                for entry in entries:
+                    if not entry.is_dir():
+                        continue
+                    with os.scandir(entry.path) as sub_entries:
+                        for sub_entry in sub_entries:
+                            if sub_entry.is_file():
+                                file_ids.add(sub_entry.name)
+        except OSError as exc:
+            self.log.debug(
+                "Unable to list the files stored in the backup folder %s: %s",
+                self.target_path,
+                exc,
+            )
+            return set()
+
+        return file_ids
+
     def _get_fs_files_from_patterns(self, root_paths: list) -> Iterator[str]:
         if not self.target_path:
             return

tests/ios_backup/test_manifest.py

@@ -5,8 +5,11 @@
 
 import gc
 import logging
+import shutil
 import warnings
 
+import pytest
+
 from mvt.common.indicators import Indicators
 from mvt.common.module import run_module
 from mvt.ios.modules.base import IOSExtraction
@@ -14,6 +17,30 @@ from mvt.ios.modules.backup.manifest import Manifest
 
 from ..utils import get_ios_backup_folder
 
+# fileID of HomeDomain::Library/SMS/sms.db in the test backup. It is one of the
+# few files the test backup actually stores.
+SMS_FILE_ID = "3d0d7e5fb2ce288813306e4d4636395e047a3d28"
+
+
+@pytest.fixture
+def backup_without_stored_files(tmp_path):
+    """A copy of the test backup with every stored file removed.
+
+    The test backup only ships a handful of the files its manifest lists, so
+    dropping what it does store leaves a backup where every regular file the
+    manifest mentions is missing from the folder.
+    """
+    backup_path = tmp_path / "backup"
+    shutil.copytree(get_ios_backup_folder(), backup_path)
+
+    for file_id_folder in backup_path.iterdir():
+        if not file_id_folder.is_dir():
+            continue
+        for backup_file in file_id_folder.iterdir():
+            backup_file.unlink()
+
+    return str(backup_path)
+
 
 class TestIOSExtraction:
     def test_get_backup_files_from_manifest_closes_connection(self):
@@ -41,6 +68,41 @@ class TestManifestModule:
         assert len(m.timeline) == 5881
         assert len(m.alertstore.alerts) == 0
 
+    def test_manifest_flags_the_files_missing_from_an_incomplete_backup(self):
+        m = Manifest(target_path=get_ios_backup_folder())
+        run_module(m)
+
+        missing = [result for result in m.results if result.get("missing")]
+        # The test backup only stores a handful of the files its manifest
+        # lists, and every one of the rest is a regular file (flags 1).
+        assert len(missing) == 1079
+        assert all(result["flags"] == 1 for result in missing)
+
+        stored = [result for result in m.results if result["file_id"] == SMS_FILE_ID]
+        assert len(stored) == 1
+        assert "missing" not in stored[0]
+
+    def test_manifest_flags_every_stored_file_removed_from_the_backup_folder(
+        self, backup_without_stored_files
+    ):
+        m = Manifest(target_path=backup_without_stored_files)
+        run_module(m)
+
+        missing = [result for result in m.results if result.get("missing")]
+        assert len(missing) == 1089
+
+    def test_manifest_flags_a_file_removed_from_the_backup_folder(self, tmp_path):
+        backup_path = tmp_path / "backup"
+        shutil.copytree(get_ios_backup_folder(), backup_path)
+        (backup_path / SMS_FILE_ID[:2] / SMS_FILE_ID).unlink()
+
+        m = Manifest(target_path=str(backup_path))
+        run_module(m)
+
+        removed = [result for result in m.results if result["file_id"] == SMS_FILE_ID]
+        assert len(removed) == 1
+        assert removed[0]["missing"] is True
+
     def test_detection(self, indicator_file):
         m = Manifest(target_path=get_ios_backup_folder())
         ind = Indicators(log=logging.getLogger())