On Thu, May 30, 2013 at 06:12:00AM -0600, Eric Blake wrote: > On 05/30/2013 04:14 AM, Dongxu Wang wrote: > > Hi, > > > > Now block layer does not check whether backing_file chain can be a > > circle, do you think it is necessary to do a check? > > > > For example, 1.qcow2's backing_file is 2.qcow2 and 2.qcow2's > > backing_file is 1.qcow2, then any IO operation will be a fault. > > > > If necessary, I will post a patch to fix. > > Yes, it's worth patching. > > > > > [$] qemu-img create -f qcow2 1.qcow2 > > qemu-img: Image creation needs a size parameter > > [$] qemu-img create -f qcow2 1.qcow2 8M > > Formatting '1.qcow2', fmt=qcow2 size=8388608 encryption=off > > cluster_size=65536 lazy_refcounts=off > > [$] qemu-img create -f qcow2 2.qcow2 -o backing_file=1.qcow2 > > Formatting '2.qcow2', fmt=qcow2 size=8388608 backing_file='1.qcow2' > > encryption=off cluster_size=65536 lazy_refcounts=off > > [$] qemu-img create -f qcow2 1.qcow2 -o backing_file=2.qcow2 > > Formatting '1.qcow2', fmt=qcow2 size=8388608 backing_file='2.qcow2' > > encryption=off cluster_size=65536 lazy_refcounts=off > > [$] qemu-io -c "read -v 0 512" 1.qcow2 > > ^CSegmentation fault (core dumped) > > Also make sure that 'qemu-img info 1.qcow2' and 'qemu-img info > --backing-chain 1.qcow2' don't crash.
qemu-img info --backing-chain detects cycles and returns an error, see the hash table in collect_image_info_list(). We should protect bdrv_open() too. Stefan