Commit 893a8f62 authored by MORITA Kazutaka's avatar MORITA Kazutaka Committed by Stefan Hajnoczi
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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: default avatarMORITA Kazutaka <>
Signed-off-by: default avatarAsias He <>
Signed-off-by: default avatarStefan Hajnoczi <>
parent 0d51b4de
...@@ -2571,7 +2571,35 @@ static int coroutine_fn bdrv_co_do_readv(BlockDriverState *bs, ...@@ -2571,7 +2571,35 @@ static int coroutine_fn bdrv_co_do_readv(BlockDriverState *bs,
} }
} }
if (!(bs->zero_beyond_eof && bs->growable)) {
ret = drv->bdrv_co_readv(bs, sector_num, nb_sectors, qiov); ret = drv->bdrv_co_readv(bs, sector_num, nb_sectors, qiov);
} else {
/* Read zeros after EOF of growable BDSes */
int64_t len, total_sectors, max_nb_sectors;
len = bdrv_getlength(bs);
if (len < 0) {
ret = len;
goto out;
total_sectors = len >> BDRV_SECTOR_BITS;
max_nb_sectors = MAX(0, total_sectors - sector_num);
if (max_nb_sectors > 0) {
ret = drv->bdrv_co_readv(bs, sector_num,
MIN(nb_sectors, max_nb_sectors), qiov);
} else {
ret = 0;
/* Reading beyond end of file is supposed to produce zeroes */
if (ret == 0 && total_sectors < sector_num + nb_sectors) {
uint64_t offset = MAX(0, total_sectors - sector_num);
uint64_t bytes = (sector_num + nb_sectors - offset) *
qemu_iovec_memset(qiov, offset * BDRV_SECTOR_SIZE, 0, bytes);
out: out:
tracked_request_end(&req); tracked_request_end(&req);
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