From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:56706) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1V6WTh-0008LP-L5 for qemu-devel@nongnu.org; Mon, 05 Aug 2013 21:54:31 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1V6WTY-0003Lw-TJ for qemu-devel@nongnu.org; Mon, 05 Aug 2013 21:54:25 -0400 Received: from mx1.redhat.com ([209.132.183.28]:28189) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1V6WTY-0003Kb-KC for qemu-devel@nongnu.org; Mon, 05 Aug 2013 21:54:16 -0400 From: Asias He Date: Tue, 6 Aug 2013 09:53:40 +0800 Message-Id: <1375754020-20545-1-git-send-email-asias@redhat.com> In-Reply-To: <20130805124110.GI2654@dhcp-200-207.str.redhat.com> References: <20130805124110.GI2654@dhcp-200-207.str.redhat.com> Subject: [Qemu-devel] [PATCH v2] block: Produce zeros when protocols reading beyond end of file List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: Kevin Wolf , Asias He , MORITA Kazutaka , Stefan Hajnoczi From: MORITA Kazutaka 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. Signed-off-by: MORITA Kazutaka Signed-off-by: Asias He --- block.c | 30 +++++++++++++++++++++++++++++- 1 file changed, 29 insertions(+), 1 deletion(-) diff --git a/block.c b/block.c index 01b66d8..f3cd9fb 100644 --- a/block.c +++ b/block.c @@ -2544,7 +2544,35 @@ static int coroutine_fn bdrv_co_do_readv(BlockDriverState *bs, } } - ret = drv->bdrv_co_readv(bs, sector_num, nb_sectors, qiov); + if (!bs->growable) { + 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) * + BDRV_SECTOR_SIZE; + qemu_iovec_memset(qiov, offset * BDRV_SECTOR_SIZE, 0, bytes); + } + } out: tracked_request_end(&req); -- 1.8.3.1