Re: virtio-fs: Fix file ID collisions (was: 9p: Fix file ID collisions)

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Christian Schoenebeck (qemu_oss@crudebyte.com) wrote:
> On Dienstag, 15. Oktober 2019 11:20:39 CEST Greg Kurz wrote:
> > On Tue, 08 Oct 2019 14:05:28 +0200
> > 
> > Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:
> > > I wonder though whether virtio-fs suffers from the same file ID collisions
> > > problem when sharing multiple file systems.
> > 
> > I gave a try and it seems that virtio-fs might expose the inode numbers from
> > different devices in the host, unvirtualized AND with the same device in
> > the guest:
> > 
> > # mkdir -p /var/tmp/virtio-fs/proc
> > # mount --bind /proc /var/tmp/virtio-fs/proc
> > # virtiofsd -o vhost_user_socket=/tmp/vhostqemu -o source=/var/tmp/virtio-fs
> > -o cache=always
> > 
> > and then started QEMU with:
> > 
> > -chardev socket,id=char0,path=/tmp/vhostqemu \
> > -device vhost-user-fs-pci,queue-size=1024,chardev=char0,tag=myfs \
> > -m 4G -object memory-backend-file,id=mem,size=4G,mem-path=/dev/shm,share=on
> > \ -numa node,memdev=mem
> > 
> > In the host:
> > 
> > $ stat /var/tmp/virtio-fs
> >   File: /var/tmp/virtio-fs
> >   Size: 4096            Blocks: 8          IO Block: 4096   directory
> > Device: fd00h/64768d    Inode: 787796      Links: 4
> > Access: (0775/drwxrwxr-x)  Uid: ( 1000/    greg)   Gid: ( 1000/    greg)
> > Context: unconfined_u:object_r:user_tmp_t:s0
> > Access: 2019-10-15 11:08:52.070080922 +0200
> > Modify: 2019-10-15 11:02:09.887404446 +0200
> > Change: 2019-10-15 11:02:09.887404446 +0200
> >  Birth: 2019-10-13 19:13:04.009699354 +0200
> > [greg@bahia ~]$ stat /var/tmp/virtio-fs/FOO
> >   File: /var/tmp/virtio-fs/FOO
> >   Size: 0               Blocks: 0          IO Block: 4096   regular empty
> > file Device: fd00h/64768d    Inode: 790740      Links: 1
> > Access: (0664/-rw-rw-r--)  Uid: ( 1000/    greg)   Gid: ( 1000/    greg)
> > Context: unconfined_u:object_r:user_tmp_t:s0
> > Access: 2019-10-15 11:02:09.888404448 +0200
> > Modify: 2019-10-15 11:02:09.888404448 +0200
> > Change: 2019-10-15 11:02:09.888404448 +0200
> >  Birth: 2019-10-15 11:02:09.887404446 +0200
> > [greg@bahia ~]$ stat /var/tmp/virtio-fs/proc/fs
> >   File: /var/tmp/virtio-fs/proc/fs
> >   Size: 0               Blocks: 0          IO Block: 1024   directory
> > Device: 4h/4d   Inode: 4026531845  Links: 5
> > Access: (0555/dr-xr-xr-x)  Uid: (    0/    root)   Gid: (    0/    root)
> > Context: system_u:object_r:proc_t:s0
> > Access: 2019-10-01 14:50:09.223233901 +0200
> > Modify: 2019-10-01 14:50:09.223233901 +0200
> > Change: 2019-10-01 14:50:09.223233901 +0200
> >  Birth: -
> > 
> > In the guest:
> > 
> > [greg@localhost ~]$ stat /mnt
> >   File: /mnt
> >   Size: 4096            Blocks: 8          IO Block: 4096   directory
> > Device: 2dh/45d Inode: 787796      Links: 4
> > Access: (0775/drwxrwxr-x)  Uid: ( 1000/    greg)   Gid: ( 1000/    greg)
> > Context: system_u:object_r:unlabeled_t:s0
> > Access: 2019-10-15 11:08:52.070080922 +0200
> > Modify: 2019-10-15 11:02:09.887404446 +0200
> > Change: 2019-10-15 11:02:09.887404446 +0200
> >  Birth: -
> > [greg@localhost ~]$ stat /mnt/FOO
> >   File: /mnt/FOO
> >   Size: 0               Blocks: 0          IO Block: 4096   regular empty
> > file Device: 2dh/45d Inode: 790740      Links: 1
> > Access: (0664/-rw-rw-r--)  Uid: ( 1000/    greg)   Gid: ( 1000/    greg)
> > Context: system_u:object_r:unlabeled_t:s0
> > Access: 2019-10-15 11:02:09.888404448 +0200
> > Modify: 2019-10-15 11:02:09.888404448 +0200
> > Change: 2019-10-15 11:02:09.888404448 +0200
> >  Birth: -
> > [greg@localhost ~]$ stat /mnt/proc/fs
> >   File: /mnt/proc/fs
> >   Size: 0               Blocks: 0          IO Block: 1024   directory
> > Device: 2dh/45d Inode: 4026531845  Links: 5
> > Access: (0555/dr-xr-xr-x)  Uid: (    0/    root)   Gid: (    0/    root)
> > Context: system_u:object_r:unlabeled_t:s0
> > Access: 2019-10-01 14:50:09.223233901 +0200
> > Modify: 2019-10-01 14:50:09.223233901 +0200
> > Change: 2019-10-01 14:50:09.223233901 +0200
> >  Birth: -
> > 
> > Unless I'm missing something, it seems that "virtio-fs" has the same
> > issue we had on 9pfs before Christian's patches... :-\
> 
> Is a fix for this desired for virtio-fs?

Yes I think so;  we had originally thought we were hiding the host inode
numbers; but that's not true - since we pass both a device and inode
number in virtiofs, unlike 9p, it seems we can probably get away with
only remapping device IDs rather than inode numbers; but that requires
some understanding of how multiple block device IDs are supposed to look
like to the guest kernel.

Dave

> Greg, did you have to update kernel version on either host or guest side to 
> get virtio-fs running? Or were the discussed kernel changes just for optional 
> acceleration purposes (i.e. DAX)?
> 
> Best regards,
> Christian Schoenebeck
> 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK