Re: [Xen-users] Is using w! safe to share data between domains?

[Anthony Liguori <aliguori@xxxxxxxxxx>]

Your scheme has a race condition in it.  Suppose that a partition has 
opened sda3 read-only and right between after seeing the tag-file 
doesn't exist and re-mounting the partition, someone else comes in and 
creates the tag file.  You know have to partitions that have followed 
your rules and have simultaneously opened the partition read-write.
Even if you eliminated the race condition, in general, it's probably not 
a good idea to open a file system (even if it's read-only) while someone 
else has it open read-write.  There's no guarantee the meta-data is 
going to be in a consistent state.
Your best bet is probably to use networking to share data between 
partitions.
Regards,

Anthony Liguori
John A. Sullivan III wrote:

> I have a slightly unusual situation where I need to pass data from one
> domain to another but, for security reasons, one of the domains will not
> be on the network.  I would like to pass the data via a shared disk
> partition.  I would like to know if what I have done is safe.
>
> I created a disk partition named /dev/VG1/pkipass.
> Each domU disk definition contains: 'phy:VG1/pkipass,sda3,w!'
> None of the domUs automatically mounts this device.
>
> When one domU needs to deposit data for another domU to pick up (this
> exchange is always a manual effort to first deposit the shared data and
> then retrieve the shared data), we do the following:
>
> The depositing domU mounts sda3 read-only.
> It looks for a tag file -- this tag file is created when a domU has
> mounted the partition as read-write
> If the tag-file exists {
>         it unmounts the partition
>         it sleeps briefly
>         it retries
> }
> else {
>         it remounts the partition read-write
>         it creates the tag file
>         it deposits the data
>         it deletes the tag fie
>         it unmounts the partition
> }
>
> A similar process is used to retrieve the data.
>
> Is this safe?
>
> Thanks - John
>  

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