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

[Vladimir Sementsov-Ogievskiy]

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?

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(-)
> 


-- 
Best regards,
Vladimir