Daniel P. Berrange
2007-Feb-02 20:43 UTC
[Xen-devel] Fix for loop mounting of readonly images
We hit an wierd edge case when booting a fullyvirt guest VM off an ISO
image stored on NFS, where the hotplug scripts would fail to map the
ISO to the backend. This only happened if the ISO image was marked readonly,
but on a read-write NFS mount. It worked fine on a readonly NFS mount!
After a little debugging appears that losetup was failing with:
# mount -orw nfs1:/some/path /mnt
# strace -e trace=open losetup /dev/loop0 /mnt/rhel4-i386-AS/images/boot.iso
2>&1 | grep boot.iso
open("/mnt/rhel4-i386-AS/images/boot.iso", O_RDWR) = -1 EACCES
(Permission denied)
/mnt/rhel4-i386-AS/images/boot.iso: Permission denied
When the NFS mount is mounted read-only it works, because it falls back
to setting up a readonly loop device:
# mount -oro nfs1:/some/path /mnt
# strace -e trace=open losetup /dev/loop0 /mnt/rhel4-i386-AS/images/boot.iso
2>&1 | grep boot.iso
open("/mnt/rhel4-i386-AS/images/boot.iso", O_RDWR) = -1 EROFS
(Read-only file system)
open("/mnt/rhel4-i386-AS/images/boot.iso", O_RDONLY) = 3
Which made me wonder - why don''t we request a readonly loopdevice in
the
first place, given that the ISO is being exported read-only to the guest.
So I''m ataching a patch to teh block hotplug scripts which will
automatically
pass the ''-r'' option to losetup if the device sharing mode is
''r''
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Regards,
Dan.
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Ewan Mellor
2007-Feb-06 15:35 UTC
Re: [Xen-devel] Fix for loop mounting of readonly images
On Fri, Feb 02, 2007 at 08:43:13PM +0000, Daniel P. Berrange wrote:> We hit an wierd edge case when booting a fullyvirt guest VM off an ISO > image stored on NFS, where the hotplug scripts would fail to map the > ISO to the backend. This only happened if the ISO image was marked readonly, > but on a read-write NFS mount. It worked fine on a readonly NFS mount! > > After a little debugging appears that losetup was failing with: > > # mount -orw nfs1:/some/path /mnt > # strace -e trace=open losetup /dev/loop0 /mnt/rhel4-i386-AS/images/boot.iso 2>&1 | grep boot.iso > open("/mnt/rhel4-i386-AS/images/boot.iso", O_RDWR) = -1 EACCES (Permission denied) > /mnt/rhel4-i386-AS/images/boot.iso: Permission denied > > When the NFS mount is mounted read-only it works, because it falls back > to setting up a readonly loop device: > > # mount -oro nfs1:/some/path /mnt > # strace -e trace=open losetup /dev/loop0 /mnt/rhel4-i386-AS/images/boot.iso 2>&1 | grep boot.iso > open("/mnt/rhel4-i386-AS/images/boot.iso", O_RDWR) = -1 EROFS (Read-only file system) > open("/mnt/rhel4-i386-AS/images/boot.iso", O_RDONLY) = 3 > > Which made me wonder - why don''t we request a readonly loopdevice in the > first place, given that the ISO is being exported read-only to the guest. > > So I''m ataching a patch to teh block hotplug scripts which will automatically > pass the ''-r'' option to losetup if the device sharing mode is ''r'' > > Signed-off-by: Daniel P. Berrange <berrange@redhat.com>Applied, thank you. Ewan. _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel