Index: sys/boot/efi/libefi/efipart.c
===================================================================
--- sys/boot/efi/libefi/efipart.c
+++ sys/boot/efi/libefi/efipart.c
@@ -925,7 +925,15 @@
 
 	/* make sure we don't read past disk end */
 	if ((off + size) / blkio->Media->BlockSize > d_offset + disk_blocks) {
-		size = d_offset + disk_blocks - off / blkio->Media->BlockSize;
+                size_t diskend = d_offset + disk_blocks;
+                size_t readstart = off / blkio->Media->BlockSize;
+
+                if (diskend <= readstart) {
+                        *rsize = 0;
+
+                        return (0);
+                }
+		size = diskend - readstart;
 		size = size * blkio->Media->BlockSize;
 	}