--- a/ffstools/tiffs-rd/basics.c	Mon Jan 27 03:05:56 2014 +0000
+++ b/ffstools/tiffs-rd/basics.c	Mon Jan 27 03:46:11 2014 +0000
@@ -32,13 +32,19 @@
 		fprintf(stderr, "error: %s is not a regular file\n", imgfile);
 		exit(1);
 	}
-	if (st.st_size < total_ffs_size) {
+	if (st.st_size < imgfile_offset) {
+		fprintf(stderr,
+		"error: offset given with -o exceeds the size of the file\n");
+		exit(1);
+	}
+	if (st.st_size - imgfile_offset < total_ffs_size) {
 		fprintf(stderr,
 			"error: %s is shorter than FFS size of 0x%lx bytes\n",
 			imgfile, (u_long)total_ffs_size);
 		exit(1);
 	}
-	image = mmap(NULL, total_ffs_size, PROT_READ, MAP_PRIVATE, fd, 0);
+	image = mmap(NULL, total_ffs_size, PROT_READ, MAP_PRIVATE, fd,
+			imgfile_offset);
 	if (image == MAP_FAILED) {
 		perror("mmap");
 		exit(1);