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Re: [Qemu-devel] [PATCH 0/4] mirror: Do not dereference invalid pointers

From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH 0/4] mirror: Do not dereference invalid pointers
Date: Thu, 19 Sep 2019 18:45:13 +0200
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On 18.09.19 17:38, Vladimir Sementsov-Ogievskiy wrote:
> 12.09.2019 16:56, Max Reitz wrote:
>> Hi,
>> The fix (patch 1) is pretty straightforward; patch 2 (which I need for
>> the test) may not be.
>> The biggest problem with patch 2 is that you can use it to uncover where
>> our permission handling is broken.  For example, devising the test case
>> (patch 4) was very difficult because I kept running into the
>> &error_abort that mirror_exit_common() passes when dropping the
>> mirror_top_bs.
>> The problem is that mirror_top_bs does not take the same permissions
>> that its parent takes.  Ergo using &error_abort when dropping it is
>> wrong: The parent may require more permissions that mirror_top_bs did,
>> and so dropping mirror_top_bs may fail.
>> Now what’s really bad is that this cannot be fixed with our current
>> permission system.  mirror_top_bs was introduced precisely so it does
>> not take CONSISTENT_READ, but can still allow parents to take it (for
>> active commits).  But what if there is actually something besides the
>> mirror job that unshares CONSISTENT_READ?
>> Imagine this:
>>        mirror target BB       mirror source BB
>>                    |             |
>>                    v             v
>> mirror_top_bs -> top -> mid -> base
>>                    ^
>>                    |
>>               other_parent
>> The source BB unshares CONSISTENT_READ on the base.  mirror_top_bs
>> ensures that its parents can read from top even though top itself cannot
>> allow CONSISTENT_READ to be taken.  So far so good.
>> But what if other_parent also unshares CONSISTENT_READ?  Then,
>> mirror_top_bs has no business allowing its parents to take it.
>> No idea how to fix that.  (I suppose mirror_top_bs would need some way
>> to verify that there is no other party that has unshared CONSISTENT_READ
>> but its associated source BB.
> May be we need grouped permissions?
> Some way to define group of children, which may unshare read permission
> for other children (out of the group), but still children in group may
> have read permission?

Hm, is that different from my idea below where one of mirror_top's
children unshares the read permission, and another is allowed to take it

(The problem is always that if some BDS has a parent that unshares this
permission, this condition propagates upwards through its other parents,
and we need to keep track of who unshared it in the first place.)

> But it don't work here as we are saying about children on different
> nodes.. And propagated through backing chain permissions..


>>  In the future, we want the source BB to
>> go away and instead have the source be an immediate BdrvChild of
>> mirror_top_bs.  Maybe we can then build something into the block layer
>> so that a node can only restore CONSISTENT_READ when it was that node
>> that broke it?)
>> Anyway.  You can see something arising from this problem simply by
>> unsharing CONSISTENT_READ on the target node.  (Just drop the src-perm
>> node from the test I add in patch 4.)  Replacing the source with the
>> target will then work fine (because mirror_top_bs doesn’t care about
>> CONSISTENT_READ being removed), but then you cannot drop mirror_top_bs –
>> because its parent does want CONSISTENT_READ.  Thus, the &error_abort
>> aborts.
>> While this is a more special case, I have no idea how to fix this one
>> either.
>> Soo...  This series just fixes one thing, and leaves another unfixed
>> because I have no idea how to fix it.  Worse, it adds parameters to
>> blkdebug to actually see the problem.  Do we want to let blkdebug be
>> able to crash qemu (because of a bug in qemu)?
> blkdebug is for debugging and not used by end users like libvirt, yes?



>> Max Reitz (4):
>>    mirror: Do not dereference invalid pointers
>>    blkdebug: Allow taking/unsharing permissions
>>    iotests: Add @error to wait_until_completed
>>    iotests: Add test for failing mirror complete
>>   qapi/block-core.json          |  29 +++++++++-
>>   block/blkdebug.c              | 106 +++++++++++++++++++++++++++++++++-
>>   block/mirror.c                |  13 +++--
>>   tests/qemu-iotests/041        |  44 ++++++++++++++
>>   tests/qemu-iotests/041.out    |   4 +-
>>   tests/qemu-iotests/iotests.py |  18 ++++--
>>   6 files changed, 200 insertions(+), 14 deletions(-)

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