From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:43706) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fpUbb-0002ll-K6 for qemu-devel@nongnu.org; Tue, 14 Aug 2018 04:23:08 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fpUba-00044q-Iw for qemu-devel@nongnu.org; Tue, 14 Aug 2018 04:23:07 -0400 Date: Tue, 14 Aug 2018 10:22:59 +0200 From: Kevin Wolf Message-ID: <20180814082259.GB5025@dhcp-200-186.str.redhat.com> References: <20180813023958.5352-1-famz@redhat.com> <20180813134206.GF4323@localhost.localdomain> <20180814081252.GJ7549@lemon.usersys.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180814081252.GJ7549@lemon.usersys.redhat.com> Subject: Re: [Qemu-devel] [PATCH v2] file-posix: Skip effectiveless OFD lock operations List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Fam Zheng Cc: qemu-devel@nongnu.org, Max Reitz , qemu-block@nongnu.org Am 14.08.2018 um 10:12 hat Fam Zheng geschrieben: > On Mon, 08/13 15:42, Kevin Wolf wrote: > > Am 13.08.2018 um 04:39 hat Fam Zheng geschrieben: > > > If we know we've already locked the bytes, don't do it again; similarly > > > don't unlock a byte if we haven't locked it. This doesn't change the > > > behavior, but fixes a corner case explained below. > > > > > > Libvirt had an error handling bug that an image can get its (ownership, > > > file mode, SELinux) permissions changed (RHBZ 1584982) by mistake behind > > > QEMU. Specifically, an image in use by Libvirt VM has: > > > > > > $ ls -lhZ b.img > > > -rw-r--r--. qemu qemu system_u:object_r:svirt_image_t:s0:c600,c690 b.img > > > > > > Trying to attach it a second time won't work because of image locking. > > > And after the error, it becomes: > > > > > > $ ls -lhZ b.img > > > -rw-r--r--. root root system_u:object_r:virt_image_t:s0 b.img > > > > > > Then, we won't be able to do OFD lock operations with the existing fd. > > > In other words, the code such as in blk_detach_dev: > > > > > > blk_set_perm(blk, 0, BLK_PERM_ALL, &error_abort); > > > > > > can abort() QEMU, out of environmental changes. > > > > > > This patch is an easy fix to this and the change is regardlessly > > > reasonable, so do it. > > > > > > Signed-off-by: Fam Zheng > > > > Thanks, applied to the block branch. > > Self-NACK. This breaks raw_abort_perm_update(). The extra bytes locked by > raw_check_perm() are not tracked by s->perm (it is only updated in > raw_set_perm()), thus will not get released. This patch is "misusing" s->perm > and s->shared_perm. > > I'll revise the implementation and send another version together with dropping > s->lock_fd. Oops! I'm dequeuing the patch for now. Also, getting rid of s->lock_fd sounds good! I wonder if we can give this some test coverage, too, so that we'll notice the breakage earlier next time. Maybe we can check from Python which bits are locked? Kevin