qemu /
qemu /
893a8f6220368a9ebff9a74bd48359928545cf6a block: Produce zeros when protocols reading beyond end of file
While Asias is debugging an issue creating qcow2 images on top of
non-file protocols. It boils down to this example using NBD:
$ qemu-io -c 'open -g nbd+unix:///?socket=/tmp/nbd.sock' -c 'read -v 0 512'
Notice the open -g option to set bs->growable. This means you can
read/write beyond end of file. Reading beyond end of file is supposed
to produce zeroes.
We rely on this behavior in qcow2_create2() during qcow2 image
creation. We create a new file and then write the qcow2 header
structure using bdrv_pwrite(). Since QCowHeader is not a multiple of
sector size, block.c first uses bdrv_read() on the empty file to fetch
the first sector (should be all zeroes).
Here is the output from the qemu-io NBD example above:
$ qemu-io -c 'open -g nbd+unix:///?socket=/tmp/nbd.sock' -c 'read -v 0 512'
00000000: ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ................
00000010: ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ................
00000020: ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ................
...
We are not zeroing the buffer! As a result qcow2 image creation on top
of protocols is not guaranteed to work even when file creation is
supported by the protocol.
[Adapted this patch to use bs->zero_beyond_eof.
-- Stefan]
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Asias He <asias@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
1 file changed
