Re: [Qemu-block] [PATCH v2] file-posix: Skip effectiveless OFD lock operations

[Fam Zheng]

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 <address@hidden>
> > > 
> > > 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