From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:59657) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TM12T-0007U2-0U for qemu-devel@nongnu.org; Wed, 10 Oct 2012 14:29:50 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1TM12R-00039l-3d for qemu-devel@nongnu.org; Wed, 10 Oct 2012 14:29:48 -0400 Received: from mx1.redhat.com ([209.132.183.28]:5697) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TM12Q-00039U-Qk for qemu-devel@nongnu.org; Wed, 10 Oct 2012 14:29:47 -0400 Received: from int-mx09.intmail.prod.int.phx2.redhat.com (int-mx09.intmail.prod.int.phx2.redhat.com [10.5.11.22]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id q9AITjOB018750 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Wed, 10 Oct 2012 14:29:46 -0400 Message-ID: <5075BE97.1070407@redhat.com> Date: Wed, 10 Oct 2012 14:29:43 -0400 From: Jeff Cody MIME-Version: 1.0 References: <37ca1a150224c144f9329fc8408f78c388f50a02.1349848348.git.jcody@redhat.com> <5075BADF.2070402@redhat.com> In-Reply-To: <5075BADF.2070402@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH 1/2] block: make bdrv_find_backing_image compare canonical filenames Reply-To: jcody@redhat.com List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Eric Blake Cc: kwolf@redhat.com, pbonzini@redhat.com, qemu-devel@nongnu.org On 10/10/2012 02:13 PM, Eric Blake wrote: > On 10/09/2012 11:56 PM, Jeff Cody wrote: >> Currently, bdrv_find_backing_image compares bs->backing_file with >> what is passed in as a backing_file name. Mismatches may occur, >> however, when bs->backing_file and backing_file are both not >> absolute or relative. >> >> Use path_combine() to make sure any relative backing filenames are >> relative to the current image filename being searched, and then use >> realpath() to make all comparisons based on absolute filenames. >> >> This also changes bdrv_find_backing_image to no longer be recursive, >> but iterative. >> >> Signed-off-by: Jeff Cody >> --- >> block.c | 38 ++++++++++++++++++++++++++++++++------ >> 1 file changed, 32 insertions(+), 6 deletions(-) >> >> diff --git a/block.c b/block.c >> index e95f613..641b8fa 100644 >> --- a/block.c >> +++ b/block.c >> @@ -3123,18 +3123,44 @@ int bdrv_snapshot_load_tmp(BlockDriverState *bs, >> return -ENOTSUP; >> } >> >> +/* backing_file can either be relative, or absolute. If it is >> + * relative, it must be relative to the chain. So, passing in >> + * bs->filename from a BDS as backing_file should not be done, >> + * as that may be relative to the CWD rather than the chain. */ >> BlockDriverState *bdrv_find_backing_image(BlockDriverState *bs, >> const char *backing_file) >> { >> - if (!bs->drv) { >> + char filename_full[PATH_MAX]; >> + char backing_file_full[PATH_MAX]; >> + char filename_tmp[PATH_MAX]; > > That's a LOT of stack space, which risks stack overflow, will mostly be > unused, and still doesn't work if you have super-deep hierarchies larger > than PATH_MAX. Would you be better off using realpath(,NULL) for its > allocating semantics, and then free()ing the results? > That is the main reason I changed it from being a recursive function, to an iterative one. Do we know that realpath(,NULL) behaves the same on all platforms? We had a thread back in April that touched on the use of realpath, and concerns were raised then that realpath(,NULL) was not necessarily safe across all OSes: https://lists.gnu.org/archive/html/qemu-devel/2012-04/msg01417.html That said, if there is concern over the stack usage, to be safe I can manually g_malloc() each array.