Fix Bytes slicing buffer overrun; fix bug in the IBM decoder which was

provoking it.

lib/bytes.cc

@@ -124,12 +124,16 @@ uint8_t& Bytes::operator [] (unsigned pos)
 Bytes Bytes::slice(unsigned start, unsigned len) const
 {
     start += _low;
-    boundsCheck(start);
     unsigned end = start + len;
-    if (end > _high)
+    if (start >= _high)
+    {
+        /* Asking for a completely out-of-range slice --- just return zeroes. */
+        return Bytes(len);
+    }
+    else if (end > _high)
     {
         /* Can't share the buffer, as we need to zero-pad the end. */
-        Bytes b(end - start);
+        Bytes b(len);
         std::uninitialized_copy(cbegin()+start, cend(), b.begin());
         return b;
     }

lib/ibm/decoder.cc

@@ -20,6 +20,8 @@ SectorVector AbstractIbmDecoder::decodeToSectors(const RawRecordVector& rawRecor
     {
         const Bytes bytes = decodeFmMfm(rawrecord->data);
         int headerSize = skipHeaderBytes();
+        if (bytes.size() < (headerSize + 1))
+            continue;
         Bytes data = bytes.slice(headerSize, bytes.size() - headerSize);
 
         switch (data[0])

tests/bytes.cc

@@ -98,6 +98,23 @@ static void test_writes()
     assert((b == Bytes{ 1, 2, 3, 4, 5 }));
 }
 
+static void test_slice()
+{
+    Bytes b = {1, 2, 3};
+
+    Bytes bs = b.slice(1, 1);
+    assert((bs == Bytes{ 2 }));
+
+    bs = b.slice(1, 2);
+    assert((bs == Bytes{ 2, 3 }));
+
+    bs = b.slice(1, 3);
+    assert((bs == Bytes{ 2, 3, 0 }));
+    
+    bs = b.slice(4, 2);
+    assert((bs == Bytes{ 0, 0 }));
+}
+
 int main(int argc, const char* argv[])
 {
     test_bounds();
@@ -105,5 +122,6 @@ int main(int argc, const char* argv[])
     test_equality();
     test_reads();
     test_writes();
+    test_slice();
     return 0;
 }