[Zucchini]: Stricter patch read

This enforces a stricter patch read in a few ways:
- Raw Deltas of 0 are forbidden. These are unused values which have no
  meaning and should not appear.
- Extra Data length must be equal to the size of the patch element
  minus the total length of equivalences.
- Element match headers must have regions of nonzero length.

This change also annotates a number of "unsafe" values read from the
patch that could be sources of error later if unchecked. It also adds
caveats about under what conditions emitted values are considered to
be safe/unsafe.

Bug: 837096
Change-Id: I1b7614d92b85c0a1546d8dccb7d371d28b2a4cd3
Reviewed-on: https://chromium-review.googlesource.com/1037186
Commit-Queue: Calder Kitagawa <ckitagawa@google.com>
Reviewed-by: Samuel Huang <huangs@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555396}
NOKEYCHECK=True
GitOrigin-RevId: 63f65c0a8b295b925c1dc92c7bed7625e7d99d46

1 files changed, 7 insertions, 6 deletions

diff --git a/patch_reader.h b/patch_reader.h
index 5d3c6dd..515da50 100644
--- a/patch_reader.h
+++ b/patch_reader.h
@@ -168,13 +168,13 @@ class ReferenceDeltaSource {
   // Core functions.
   bool Initialize(BufferSource* source);
   base::Optional<int32_t> GetNext();
-  bool Done() const { return reference_delta_.empty(); }
+  bool Done() const { return source_.empty(); }
 
   // Accessors for unittest.
-  BufferSource reference_delta() const { return reference_delta_; }
+  BufferSource reference_delta() const { return source_; }
 
  private:
-  BufferSource reference_delta_;
+  BufferSource source_;
 };
 
 // Source for additional targets.
@@ -236,9 +236,10 @@ class PatchElementReader {
 
  private:
   // Checks that "old" and "new" blocks of each item in |equivalences_| satisfy
-  // basic order and image bound constraints (using |element_match_| data).
-  // Returns true if successful.
-  bool ValidateEquivalences();
+  // basic order and image bound constraints (using |element_match_| data). Also
+  // validates that the amount of extra data is correct. Returns true if
+  // successful.
+  bool ValidateEquivalencesAndExtraData();
 
   ElementMatch element_match_;