I reproduced the problem by mounting everything by hand
Code:
sudo mkdir /tmp/testMountSd
sudo chmod 777 /tmp/testMountSd
sudo mount -t exfat -o defaults,umask=000 /dev/mmcblk5p1 /tmp/testMountSd
sudo mkdir /tmp/testMountImage
sudo chmod 555 /tmp/testMountImage
sudo mount --type="squashfs" --options="loop" --source="/tmp/testMountSd/dragonbox/packages/SlayTheDragons1.1.1.dbp" --target="/tmp/testMountImage"
sudo mkdir /tmp/testMountUnion
sudo chmod 777 /tmp/testMountUnion
sudo mount -t aufs -o noplink,br=/tmp/testMountSd/dragonbox/appdata/SlayTheDragons=rw+nolwh:/tmp/testMountImage=ro none /tmp/testMountUnion
=> results in the message "Segmentation fault"
I don't think the file permissions in exFAT are the problem (since I mounted with all the permissions and the problem still occurs).