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

[Max Reitz]

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

> This leads to the following effect:
> 
> ./qemu-img create -f qcow2 base.qcow2 2M
> ./qemu-io -c "write -P 0x1 0 2M" base.qcow2
> 
> ./qemu-img create -f qcow2 -b base.qcow2 mid.qcow2 1M
> ./qemu-img create -f qcow2 -b mid.qcow2 top.qcow2 2M
> 
> Region 1M..2M is shadowed by short middle image, so guest sees zeroes:
> ./qemu-io -c "read -P 0 1M 1M" top.qcow2
> read 1048576/1048576 bytes at offset 1048576
> 1 MiB, 1 ops; 00.00 sec (22.795 GiB/sec and 23341.5807 ops/sec)
> 
> But after commit guest visible state is changed, which seems wrong for me:
> ./qemu-img commit top.qcow2 -b mid.qcow2
> 
> ./qemu-io -c "read -P 0 1M 1M" mid.qcow2
> Pattern verification failed at offset 1048576, 1048576 bytes
> read 1048576/1048576 bytes at offset 1048576
> 1 MiB, 1 ops; 00.00 sec (4.981 GiB/sec and 5100.4794 ops/sec)
> 
> ./qemu-io -c "read -P 1 1M 1M" mid.qcow2
> read 1048576/1048576 bytes at offset 1048576
> 1 MiB, 1 ops; 00.00 sec (3.365 GiB/sec and 3446.1606 ops/sec)
> 
> 
> I don't know, is it a real bug, as I don't know, do we support backing file 
> larger than
> its parent. Still, I'm not sure that this behavior of bdrv_is_allocated_above 
> don't lead
> to other problems.
> 
> =====
> 
> Hmm, bdrv_block_allocated_above behaves strange too:
> 
> with want_zero=true, it may report unallocated zeroes because of short 
> backing files, which
> are actually "allocated" in POV of backing chains. But I see this may 
> influence only
> qemu-img compare, and I don't see can it trigger some bug..
> 
> with want_zero=false, it may do no progress because of short backing file. 
> Moreover it may
> report EOF in the middle!! But want_zero=false used only in 
> bdrv_is_allocated, which considers
> onlyt top layer, so it seems OK. 
> 
> =====
> 
> So, I propose these series, still I'm not sure is there a real bug.
> 
> Vladimir Sementsov-Ogievskiy (4):
>   block/io: fix bdrv_co_block_status_above
>   block/io: bdrv_common_block_status_above: support include_base
>   block/io: bdrv_common_block_status_above: support bs == base
>   block/io: fix bdrv_is_allocated_above
> 
>  block/io.c                 | 104 ++++++++++++++++++-------------------
>  tests/qemu-iotests/154.out |   4 +-
>  2 files changed, 53 insertions(+), 55 deletions(-)
>

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