[libvirt] [PATCH] blockcopy: check dst = identical device

[Eric Blake]

On 07/29/2014 07:37 AM, Eric Blake wrote:
> On 07/29/2014 03:59 AM, Chunyan Liu wrote:
>> Check whether dst is the same device as source, if yes, report
>> error and exit.
>>
>> Currently if dst is the same device as source, blockcopy is still
>> going and qemu 'drive-mirror' is executed. Considering that:
>> a). blockcopy to the same device is meaningless. b.) result is
>> unexpected. (tested with block device whose source path is /dev/sdaX,
>> after blockcopy, shutdown VM and then create VM from xml again, the
>> VM cannot be started.) This case should not be allowed.
>>
>> Signed-off-by: Chunyan Liu <cyliu at suse.com>
>> ---
>>  src/qemu/qemu_driver.c | 7 +++++++
>>  1 file changed, 7 insertions(+)
>>
>> diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
>> index 704ba39..87a3790 100644
>> --- a/src/qemu/qemu_driver.c
>> +++ b/src/qemu/qemu_driver.c
>> @@ -15309,6 +15309,13 @@ qemuDomainBlockCopy(virDomainObjPtr vm,
>>      }
>>  
>>      /* Prepare the destination file.  */
>> +    if (STREQ(disk->src->path, dest)) {
> 
> STREQ is too weak (consider symlinks, or even "a/b" vs. "a//b").  It
> will catch some cases, but not all.
> 
> I don't know that we can reliably detect all possible ways the user can
> shoot themselves in the foot, so I'm not sure this patch is a good idea.

A better idea would be to rely on the volume lease manager - obtaining a
lease should be impossible for an image already in use (and should even
cover the case of copying 'base <- active' onto 'base', which your
equality test wouldn't catch). I'm not sure why the lease manager is not
already flagging this issue - are we still using the nop lease manager
by default, and would the fcntl or sanlock lease manager do a better job?

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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