Re: [Xen-devel] Fix for loop mounting of readonly images

[Ewan Mellor <ewan@xxxxxxxxxxxxx>]

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@xxxxxxxxxx>

Applied, thank you.

Ewan.

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