Re: [PATCH 0/4] fix & merge block_status_above and is_allocated_above

[Max Reitz]

On 19.11.19 13:02, Denis V. Lunev wrote:
> On 11/19/19 1:22 PM, Max Reitz wrote:
>> On 16.11.19 17:34, Vladimir Sementsov-Ogievskiy wrote:
>>> Hi all!
>>>
>>> I wanted to understand, what is the real difference between 
>>> bdrv_block_status_above
>>> and bdrv_is_allocated_above, IMHO bdrv_is_allocated_above should work 
>>> through
>>> bdrv_block_status_above..
>>>
>>> And I found the problem: bdrv_is_allocated_above considers space after EOF 
>>> as
>>> UNALLOCATED for intermediate nodes..
>>>
>>> UNALLOCATED is not about allocation at fs level, but about should we go to 
>>> backing or
>>> not.. And it seems incorrect for me, as in case of short backing file, 
>>> we'll read
>>> zeroes after EOF, instead of going further by backing chain.
>> Should we, though?  It absolutely makes sense to me to consider post-EOF
>> space as unallocated because, well, it is as unallocated as it gets.
>>
>> So from my POV it would make more sense to fall back to the backing file
>> for post-EOF reads.
>>
>> OTOH, I don’t know whether changing that behavior would qualify as a
>> possible security issue now, because maybe someone has sensitive
>> information in the tail of some disk and then truncated the overlay so
>> as to hide it?  But honestly, that seems ridiculous and I can’t imagine
>> people to do that.  (It would work only for the tail, and why not just
>> write zeroes there, which works everywhere?)  So in practice I don’t
>> believe that to be a problem.
>>
>> Max
> 
> That seems to be wrong from my POW. Once we get block device truncated,
> it exposed that tail to the guest with all zeroes.
> 
> Let us assume that we have virtual disk of length L. We create new top
> delta of
> length X (less then L) and new top delta after with length Y (more than L),
> like the following:
> 
> [.........................] Y
> [........] X
> [...................] L
> 
> Once the guest creates FS  on state Y it relies on the fact that data from X
> to Y is all zeroes.
> 
> Any operations with backing chain must keep guest content to be tha same,
> i.e. if we commit from Y to L, virtual disk content should be preserved,
> i.e.
> read as all zero even if there is some data in L from X to L.
> 
> If we commit from X to Y, the range from X to L should remain all zeroes.
> 
> This is especially valid for backups, which can not be changed and are
> validated by the software from time to time.
> 
> Does this makes sense?

All right then.  But then there’s the case of commit not shrinking the
backing file, so the guest content won’t be the same if you commit a
short overlay into a longer backing file.

Max

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