On Tue, 08/14 10:22, Kevin Wolf wrote: > 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 <[email protected]> > > > > > > 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?
I can write a unit test around open/close/reopen in C, where it is convenient to check the lock status with F_OFD_GETLK/F_OFD_GETLK before/after the operations. Fam
