[libvirt] [PATCH] qemu: Forcibly mknod() even if it exists

[Daniel P. Berrangé]

On Wed, Nov 20, 2019 at 11:34:24AM +0100, Michal Privoznik wrote:
> On 11/20/19 11:05 AM, Daniel P. Berrangé wrote:
> > On Wed, Nov 20, 2019 at 10:51:47AM +0100, Michal Privoznik wrote:
> > > Another weird bug appeared concerning qemu namespaces. Basically
> > > the problem is as follows:
> > > 
> > > 1) Issue an API that causes libvirt to create a node in domain's
> > >     namespace, say /dev/sda with 8:0 as major:minor (the API can
> > >     be attach-disk for instance). Or simply create the node from
> > >     a console by hand.
> > > 
> > > 2) Detach the disk from qemu.
> > > 
> > > 3) Do something that makes /dev/sda change it's minor number.
> > 
> > Wait, what ?
> > 
> > major/minor numbers for SCSI disks are a defined standard
> > IIUC
> > 
> > $ grep SCSI ./admin-guide/devices.txt | grep block
> >     8 block	SCSI disk devices (0-15)
> >    11 block	SCSI CD-ROM devices
> >    65 block	SCSI disk devices (16-31)
> >    66 block	SCSI disk devices (32-47)
> >    67 block	SCSI disk devices (48-63)
> >    68 block	SCSI disk devices (64-79)
> >    69 block	SCSI disk devices (80-95)
> >    70 block	SCSI disk devices (96-111)
> >    71 block	SCSI disk devices (112-127)
> >   128 block       SCSI disk devices (128-143)
> >   129 block       SCSI disk devices (144-159)
> >   130 block       SCSI disk devices (160-175)
> >   131 block       SCSI disk devices (176-191)
> >   132 block       SCSI disk devices (192-207)
> >   133 block       SCSI disk devices (208-223)
> >   134 block       SCSI disk devices (224-239)
> >   135 block       SCSI disk devices (240-255)
> > 
> > 
> > IOW, /dev/sda should always be 8,0
> > 
> > There is, however, the possibiity of dynamically assign
> > major/minor numbers so its possible we'll see a block
> > device with a changable number, but AFAIK, such a block
> > device should never be call /dev/sda !??!
> > 
> > IOW, the commit is fine, but is this commit message
> > really accurate ?
> 
> Ah, the bug talks about /dev/nvmeN that has changed MIN number. I haven't
> found corresponding docs on NVMe though. Is s/sda/nvmeN/ on the commit
> message enough?

Yeah, that makes more sense as nvme devices are allocated from the
dynamic range.

Regards,
Daniel
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