Fix for loop mounting of readonly images

Fix for loop mounting of readonly images

[Daniel P. Berrange]

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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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diff -r 2e80cd715047 tools/examples/block
--- a/tools/examples/block	Thu Feb 01 11:42:50 2007 +0000
+++ b/tools/examples/block	Thu Feb 01 11:58:38 2007 -0500
@@ -367,7 +367,13 @@ mount it read-write in a guest domain."
           fatal 'Failed to find an unused loop device'
         fi
 
-        do_or_die losetup "$loopdev" "$file"
+        cmode=`canonicalise_mode $mode`
+        if [ "$cmode" == 'r' ]
+        then
+          do_or_die losetup -r "$loopdev" "$file"
+        else
+          do_or_die losetup "$loopdev" "$file"
+        fi
         xenstore_write "$XENBUS_PATH/node" "$loopdev"
         write_dev "$loopdev"
         release_lock "block"

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Re: Fix for loop mounting of readonly images

[Ewan Mellor]

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.