feat(reporting): report meta-data information about chunks.

Allow handlers to provide a dict value as part of a ValidChunk metadata
attribute. That dictionnary can contain any relevant metadata
information from the perspective of the handler, but we advise handler
writers to report parsed information such as header values.

This metadata dict is later reported as part of our ChunkReports and
available in the JSON report file if the user requested one.

The idea is to expose metadata to further analysis steps through the
unblob report. For example, a binary analysis toolkit would read the load
address and architecture from a uImage chunk to analyze the file
extracted from that chunk with the right settings.

A note on the 'as_dict' implementation.

The initial idea was to implement it in dissect.cstruct (see
fox-it/dissect.cstruct#29), but due to expected
changes in the project's API I chose to implement it in unblob so we're
not dependent on another project.

unblob/file_utils.py

@@ -8,7 +8,7 @@
 from pathlib import Path
 from typing import Iterator, Tuple
 
-from dissect.cstruct import cstruct
+from dissect.cstruct import Instance, cstruct
 from pyperscan import Scan
 
 from .logging import format_hex
@@ -311,3 +311,26 @@ def read_until_past(file: File, pattern: bytes):
             return file.tell()
         if next_byte not in pattern:
             return file.tell() - 1
+
+
+def as_dict(obj):
+    """Convert a Python class instance to a dictionary."""
+    if isinstance(obj, dict):
+        return obj
+    if isinstance(obj, list):
+        return [as_dict(item) for item in obj]
+    if isinstance(obj, Instance):
+        result = {}
+        for k, v in obj._values.items():  # noqa: SLF001
+            result[k] = v
+        return result
+
+    result = {}
+    for key, value in obj.__dict__.items():
+        if key.startswith("_"):
+            continue
+        if isinstance(value, (list, tuple)):
+            result[key] = [as_dict(item) for item in value]
+        else:
+            result[key] = as_dict(value)
+    return result

unblob/handlers/archive/sevenzip.py

@@ -23,6 +23,7 @@
 from structlog import get_logger
 
 from unblob.extractors import Command
+from unblob.file_utils import as_dict
 
 from ...models import File, HexString, StructHandler, ValidChunk
 
@@ -70,4 +71,6 @@ def calculate_chunk(self, file: File, start_offset: int) -> Optional[ValidChunk]
         # We read the signature header here to get the offset to the header database
         first_db_header = start_offset + len(header) + header.next_header_offset
         end_offset = first_db_header + header.next_header_size
-        return ValidChunk(start_offset=start_offset, end_offset=end_offset)
+        return ValidChunk(
+            start_offset=start_offset, end_offset=end_offset, metadata=as_dict(header)
+        )

unblob/models.py

@@ -88,6 +88,7 @@ class ValidChunk(Chunk):
 
     handler: "Handler" = attr.ib(init=False, eq=False)
     is_encrypted: bool = attr.ib(default=False)
+    metadata: dict = attr.ib(default={})
 
     def extract(self, inpath: Path, outdir: Path):
         if self.is_encrypted:
@@ -108,6 +109,7 @@ def as_report(self, extraction_reports: List[Report]) -> ChunkReport:
             size=self.size,
             handler_name=self.handler.NAME,
             is_encrypted=self.is_encrypted,
+            metadata=self.metadata,
             extraction_reports=extraction_reports,
         )
 
@@ -188,7 +190,7 @@ def default(self, obj):
 
         if isinstance(obj, bytes):
             try:
-                return obj.decode()
+                return obj.decode("utf-8", errors="surrogateescape")
             except UnicodeDecodeError:
                 return str(obj)
 

unblob/report.py

@@ -4,7 +4,7 @@
 import traceback
 from enum import Enum
 from pathlib import Path
-from typing import List, Optional, Union, final
+from typing import Dict, List, Optional, Union, final
 
 import attr
 
@@ -181,6 +181,7 @@ class ChunkReport(Report):
     end_offset: int
     size: int
     is_encrypted: bool
+    metadata: Dict
     extraction_reports: List[Report]