diff --git a/payload_generator/delta_diff_generator.cc b/payload_generator/delta_diff_generator.cc
index 3355c41..ff759f6 100644
--- a/payload_generator/delta_diff_generator.cc
+++ b/payload_generator/delta_diff_generator.cc
@@ -729,6 +729,7 @@
     int blobs_fd,
     off_t* blobs_length,
     const string& old_image_path,
+    const uint64_t old_image_size,
     const string& new_image_path,
     Vertex* vertex,
     uint32_t minor_version) {
@@ -798,9 +799,13 @@
       TEST_AND_RETURN_FALSE_ERRNO(rc >= 0);
       TEST_AND_RETURN_FALSE(static_cast<size_t>(rc) == count);
 
-      rc = pread(old_image_fd, old_buf.data(), count, offset);
+      const off_t old_offset = min(offset, static_cast<off_t>(old_image_size));
+      const size_t old_count =
+          min(static_cast<size_t>(offset + count),
+              static_cast<size_t>(old_image_size)) - old_offset;
+      rc = pread(old_image_fd, old_buf.data(), old_count, old_offset);
       TEST_AND_RETURN_FALSE_ERRNO(rc >= 0);
-      TEST_AND_RETURN_FALSE(static_cast<size_t>(rc) == count);
+      TEST_AND_RETURN_FALSE(static_cast<size_t>(rc) == old_count);
 
       // Compare each block in the buffer to its counterpart in the old image
       // and only compress it if its content has changed.
@@ -808,6 +813,7 @@
       for (int i = 0; i < copy_block_cnt; ++i) {
         int buf_end_offset = buf_offset + kBlockSize;
         if (minor_version == kSourceMinorPayloadVersion ||
+            static_cast<size_t>(buf_end_offset) > old_count ||
             !std::equal(new_buf.begin() + buf_offset,
                         new_buf.begin() + buf_end_offset,
                         old_buf.begin() + buf_offset)) {
@@ -1046,6 +1052,7 @@
                                             data_file_fd,
                                             data_file_size,
                                             config.source.rootfs_part,
+                                            config.source.rootfs_size,
                                             config.target.rootfs_part,
                                             &unwritten_vertex,
                                             config.minor_version));
diff --git a/payload_generator/delta_diff_generator.h b/payload_generator/delta_diff_generator.h
index d34b9da..5aa506c 100644
--- a/payload_generator/delta_diff_generator.h
+++ b/payload_generator/delta_diff_generator.h
@@ -152,6 +152,7 @@
       int blobs_fd,
       off_t* blobs_length,
       const std::string& old_image_path,
+      const uint64_t old_image_size,
       const std::string& new_image_path,
       Vertex* vertex,
       uint32_t minor_version);
diff --git a/payload_generator/inplace_generator.cc b/payload_generator/inplace_generator.cc
index 869535c..febb5e6 100644
--- a/payload_generator/inplace_generator.cc
+++ b/payload_generator/inplace_generator.cc
@@ -722,6 +722,7 @@
                                               data_file_fd,
                                               data_file_size,
                                               config.source.rootfs_part,
+                                              config.source.rootfs_size,
                                               config.target.rootfs_part,
                                               &graph.back(),
                                               config.minor_version));