Refactor Objects::recover_stream_length

Change how the maximum stream length is calculated. For streams not in the
xref table, instead of returning 0 length return the maximum length that
does not overlap with a known object or xref table.

libqpdf/QPDF_objects.cc

@@ -93,13 +93,13 @@ namespace
 void
 Xref_table::test()
 {
-    std::cout << "id, gen, offset, length, next\n";
+    std::cout << "id, gen, offset, length, next, upper_bound\n";
     int i = 0;
     for (auto const& entry: table) {
         if (entry.type() == 1) {
             std::cout << i << ", " << entry.gen() << ", " << entry.type() << ", " << entry.offset()
                       << ", " << entry.length() << ", " << (entry.offset() + toO(entry.length()))
-                      << '\n';
+                      << ", " << upper_bound(entry.offset() + 1) << '\n';
         }
         ++i;
     }
@@ -149,7 +149,7 @@ Xref_table::initialize()
     // PDF spec says %%EOF must be found within the last 1024 bytes of the file.  We add an extra
     // 30 characters to leave room for the startxref stuff.
     file->seek(0, SEEK_END);
-    qpdf_offset_t end_offset = file->tell();
+    end_offset = file->tell();
     // Sanity check on object ids. All objects must appear in xref table / stream. In all realistic
     // scenarios at least 3 bytes are required.
     if (max_id_ > end_offset / 3) {
@@ -1129,26 +1129,6 @@ Xref_table::insert_free(QPDFObjGen og)
     }
 }
 
-QPDFObjGen
-Xref_table::at_offset(qpdf_offset_t offset) const noexcept
-{
-    int id = 0;
-    int gen = 0;
-    qpdf_offset_t start = 0;
-
-    int i = 0;
-    for (auto const& item: table) {
-        auto o = item.offset();
-        if (start < o && o <= offset) {
-            start = o;
-            id = i;
-            gen = item.gen();
-        }
-        ++i;
-    }
-    return QPDFObjGen(id, gen);
-}
-
 std::map<QPDFObjGen, QPDFXRefEntry>
 Xref_table::as_map() const
 {
@@ -1409,6 +1389,30 @@ QPDF::findEndstream()
     return false;
 }
 
+// Return the smallest offset that is known to belong to a different item(object/xre table) from the
+// item at start.
+qpdf_offset_t
+Xref_table::upper_bound(qpdf_offset_t start) const noexcept
+{
+    auto upb = end_offset;
+    if (start >= end_offset) {
+        // Shouldn't be possible.
+        return start;
+    }
+    for (auto const& e: table) {
+        if (auto offset = e.offset(); offset > start) {
+            // Should never happen.
+            upb = std::min(upb, offset);
+        }
+    }
+    for (auto const& e: offsets) {
+        if (e.first > start) {
+            upb = std::min(upb, e.first);
+        }
+    }
+    return upb;
+}
+
 size_t
 Objects::recover_stream_length(QPDFObjGen og, qpdf_offset_t stream_offset)
 {
@@ -1416,30 +1420,19 @@ Objects::recover_stream_length(QPDFObjGen og, qpdf_offset_t stream_offset)
     qpdf.warn(qpdf.damagedPDF(stream_offset, "attempting to recover stream length"));
 
     PatternFinder ef(qpdf, &QPDF::findEndstream);
-    size_t length = 0;
-    if (m->file->findFirst("end", stream_offset, 0, ef)) {
+
+    auto length = xref.length(og);
+    length = length ? length - std::min(length, toS(stream_offset - xref.offset(og)))
+                    : toS(xref.upper_bound(stream_offset) - stream_offset);
+
+    if (m->file->findFirst("end", stream_offset, length, ef)) {
         length = toS(m->file->getLastOffset() - stream_offset);
-    }
-
-    if (length) {
-        // Make sure this is inside this object
-        auto found = xref.at_offset(stream_offset + toO(length));
-        if (found == QPDFObjGen() || found == og) {
-            // If we are trying to recover an XRef stream the xref table will not contain and
-            // won't contain any entries, therefore we cannot check the found length. Otherwise we
-            // found endstream\endobj within the space allowed for this object, so we're probably
-            // in good shape.
-        } else {
-            QTC::TC("qpdf", "QPDF found wrong endstream in recovery");
-            length = 0;
-        }
-    }
-
-    if (length == 0) {
-        qpdf.warn(qpdf.damagedPDF(stream_offset, "unable to recover stream data; treating stream as empty"));
     } else {
-        qpdf.warn(qpdf.damagedPDF(stream_offset, "recovered stream length: " + std::to_string(length)));
+        // NB findFirst ignores 'length' when reading data into the buffer and therefore leaves the
+        // file position beyond the end of the object if the target is not found.
+        m->file->seek(stream_offset + toO(length), SEEK_SET);
     }
+    qpdf.warn(qpdf.damagedPDF(stream_offset, "recovered stream length: " + std::to_string(length)));
 
     QTC::TC("qpdf", "QPDF recovered stream length");
     return length;

libqpdf/qpdf/QPDF_objects.hh

@@ -78,7 +78,7 @@ class QPDF::Objects
             return table[id].type();
         }
 
-        // Returns 0 if og is not in table.
+        // Returns 0 if og is not in table or is not an uncompressed object.
         qpdf_offset_t
         offset(QPDFObjGen og) const noexcept
         {
@@ -89,6 +89,18 @@ class QPDF::Objects
             return table[static_cast<size_t>(id)].offset();
         }
 
+        // (Maximum possible) size of object. Returns 0 if og is not in table or is not an
+        // uncompressed object.
+        size_t
+        length(QPDFObjGen og) const noexcept
+        {
+            int id = og.getObj();
+            if (id < 1 || static_cast<size_t>(id) >= table.size()) {
+                return 0;
+            }
+            return table[static_cast<size_t>(id)].length();
+        }
+
         // Returns 0 if id is not in table.
         int
         stream_number(int id) const noexcept
@@ -108,8 +120,6 @@ class QPDF::Objects
             return table[static_cast<size_t>(id)].stream_index();
         }
 
-        QPDFObjGen at_offset(qpdf_offset_t offset) const noexcept;
-
         std::map<QPDFObjGen, QPDFXRefEntry> as_map() const;
 
         bool
@@ -228,6 +238,8 @@ class QPDF::Objects
             return first_item_offset_;
         }
 
+        qpdf_offset_t upper_bound(qpdf_offset_t start) const noexcept;
+
         void test();
 
       private:
@@ -399,6 +411,7 @@ class QPDF::Objects
         // to the value of /Size. If the file is damaged, max_id_ becomes the maximum object id in
         // the xref table after reconstruction.
         int max_id_{std::numeric_limits<int>::max() - 1};
+        qpdf_offset_t end_offset{0}; // used for object length calc.
 
         // Linearization data
         bool uncompressed_after_compressed_{false};

libtests/qtest/xref/incremental-1-bad.out

@@ -1,12 +1,12 @@
 WARNING: incremental-1-bad.pdf: file is damaged
 WARNING: incremental-1-bad.pdf (offset 1241): xref not found
 WARNING: incremental-1-bad.pdf: Attempting to reconstruct cross-reference table
-id, gen, offset, length, next
-1, 0, 1, 9, 93, 102
-2, 0, 1, 102, 72, 174
-3, 0, 1, 1108, 172, 1280
-4, 1, 1, 987, 26, 1013
-5, 0, 1, 442, 35, 477
-6, 0, 1, 477, 118, 595
-7, 0, 1, 1013, 95, 1108
+id, gen, offset, length, next, upper_bound
+1, 0, 1, 9, 93, 102, 102
+2, 0, 1, 102, 72, 174, 442
+3, 0, 1, 1108, 172, 1280, 1462
+4, 1, 1, 987, 26, 1013, 1013
+5, 0, 1, 442, 35, 477, 477
+6, 0, 1, 477, 118, 595, 987
+7, 0, 1, 1013, 95, 1108, 1108
 xref done

libtests/qtest/xref/incremental-1.out

@@ -1,9 +1,9 @@
-id, gen, offset, length, next
-1, 0, 1, 9, 54, 63
-2, 0, 1, 63, 72, 135
-3, 0, 1, 1069, 172, 1241
-4, 1, 1, 948, 26, 974
-5, 0, 1, 403, 35, 438
-6, 0, 1, 438, 118, 556
-7, 0, 1, 974, 95, 1069
+id, gen, offset, length, next, upper_bound
+1, 0, 1, 9, 54, 63, 63
+2, 0, 1, 63, 72, 135, 403
+3, 0, 1, 1069, 172, 1241, 1423
+4, 1, 1, 948, 26, 974, 974
+5, 0, 1, 403, 35, 438, 438
+6, 0, 1, 438, 118, 556, 948
+7, 0, 1, 974, 95, 1069, 1069
 xref done

qpdf/qpdf.testcov

@@ -98,7 +98,6 @@ QPDF loop detected traversing objects 0
 QPDF reconstructed xref table 0
 QPDF recovered in readObjectAtOffset 0
 QPDF recovered stream length 0
-QPDF found wrong endstream in recovery 0
 QPDF_Stream pipeStreamData with null pipeline 0
 QPDFWriter not recompressing /FlateDecode 0
 QPDF_encryption xref stream from encrypted file 0

qpdf/qtest/qpdf/bad39-recover.out

@@ -1,11 +1,28 @@
 WARNING: bad39.pdf (object 4 0, offset 385): expected endstream
 WARNING: bad39.pdf (object 4 0, offset 341): attempting to recover stream length
-WARNING: bad39.pdf (object 4 0, offset 341): unable to recover stream data; treating stream as empty
+WARNING: bad39.pdf (object 4 0, offset 341): recovered stream length: 62
+WARNING: bad39.pdf (object 4 0, offset 403): expected endobj
 /QTest is indirect and has type stream (10)
 /QTest is a stream.  Dictionary: << /Length 44 >>
 Raw stream data:
+BT
+  /F1 24 Tf
+  72 720 Td
+  (Potato) Tj
+ET
+enxstream
+enxobj
+
 
 Uncompressed stream data:
+BT
+  /F1 24 Tf
+  72 720 Td
+  (Potato) Tj
+ET
+enxstream
+enxobj
+
 
 End of stream data
 unparse: 4 0 R

qpdf/qtest/qpdf/issue-141a.out

@@ -1,7 +1,7 @@
 WARNING: issue-141a.pdf: can't find PDF header
 WARNING: issue-141a.pdf (xref stream: object 9 0, offset 10): stream dictionary lacks /Length key
 WARNING: issue-141a.pdf (xref stream: object 9 0, offset 47): attempting to recover stream length
-WARNING: issue-141a.pdf (xref stream: object 9 0, offset 47): unable to recover stream data; treating stream as empty
+WARNING: issue-141a.pdf (xref stream: object 9 0, offset 47): recovered stream length: 0
 WARNING: issue-141a.pdf: file is damaged
 WARNING: issue-141a.pdf (xref stream, offset 3): Cross-reference stream's /W indicates entry size of 0
 WARNING: issue-141a.pdf: Attempting to reconstruct cross-reference table