Re: [PATCH for-5.0 v2 15/23] mirror: Prevent loops

[Max Reitz <mreitz@redhat.com>]

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On 20.12.19 12:55, Vladimir Sementsov-Ogievskiy wrote:
> 20.12.2019 14:39, Max Reitz wrote:
>> On 13.12.19 12:18, Vladimir Sementsov-Ogievskiy wrote:
>>> 09.12.2019 17:43, Max Reitz wrote:
>>>> On 02.12.19 13:12, Vladimir Sementsov-Ogievskiy wrote:
>>>>> 11.11.2019 19:02, Max Reitz wrote:
>>>>>> While bdrv_replace_node() will not follow through with it, a specific
>>>>>> @replaces asks the mirror job to create a loop.
>>>>>>
>>>>>> For example, say both the source and the target share a child where the
>>>>>> source is a filter; by letting @replaces point to the common child, you
>>>>>> ask for a loop.
>>>>>>
>>>>>> Or if you use @replaces in drive-mirror with sync=none and
>>>>>> mode=absolute-paths, you generally ask for a loop (@replaces must point
>>>>>> to a child of the source, and sync=none makes the source the backing
>>>>>> file of the target after the job).
>>>>>>
>>>>>> bdrv_replace_node() will not create those loops, but by doing so, it
>>>>>> ignores the user-requested configuration, which is not ideally either.
>>>>>> (In the first example above, the target's child will remain what it was,
>>>>>> which may still be reasonable.  But in the second example, the target
>>>>>> will just not become a child of the source, which is precisely what was
>>>>>> requested with @replaces.)
>>>>>>
>>>>>> So prevent such configurations, both before the job, and before it
>>>>>> actually completes.
>>>>>>
>>>>>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>>>>>> ---
>>>>>>     block.c                   | 30 ++++++++++++++++++++++++
>>>>>>     block/mirror.c            | 19 +++++++++++++++-
>>>>>>     blockdev.c                | 48 ++++++++++++++++++++++++++++++++++++++-
>>>>>>     include/block/block_int.h |  3 +++
>>>>>>     4 files changed, 98 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/block.c b/block.c
>>>>>> index 0159f8e510..e3922a0474 100644
>>>>>> --- a/block.c
>>>>>> +++ b/block.c
>>>>>> @@ -6259,6 +6259,36 @@ out:
>>>>>>         return to_replace_bs;
>>>>>>     }
>>>>>>     
>>>>>> +/*
>>>>>> + * Return true iff @child is a (recursive) child of @parent, with at
>>>>>> + * least @min_level edges between them.
>>>>>> + *
>>>>>> + * (If @min_level == 0, return true if @child == @parent.  For
>>>>>> + * @min_level == 1, @child needs to be at least a real child; for
>>>>>> + * @min_level == 2, it needs to be at least a grand-child; and so on.)
>>>>>> + */
>>>>>> +bool bdrv_is_child_of(BlockDriverState *child, BlockDriverState *parent,
>>>>>> +                      int min_level)
>>>>>> +{
>>>>>> +    BdrvChild *c;
>>>>>> +
>>>>>> +    if (child == parent && min_level <= 0) {
>>>>>> +        return true;
>>>>>> +    }
>>>>>> +
>>>>>> +    if (!parent) {
>>>>>> +        return false;
>>>>>> +    }
>>>>>> +
>>>>>> +    QLIST_FOREACH(c, &parent->children, next) {
>>>>>> +        if (bdrv_is_child_of(child, c->bs, min_level - 1)) {
>>>>>> +            return true;
>>>>>> +        }
>>>>>> +    }
>>>>>> +
>>>>>> +    return false;
>>>>>> +}
>>>>>> +
>>>>>>     /**
>>>>>>      * Iterates through the list of runtime option keys that are said to
>>>>>>      * be "strong" for a BDS.  An option is called "strong" if it changes
>>>>>> diff --git a/block/mirror.c b/block/mirror.c
>>>>>> index 68a4404666..b258c7e98b 100644
>>>>>> --- a/block/mirror.c
>>>>>> +++ b/block/mirror.c
>>>>>> @@ -701,7 +701,24 @@ static int mirror_exit_common(Job *job)
>>>>>>              * there.
>>>>>>              */
>>>>>>             if (bdrv_recurse_can_replace(src, to_replace)) {
>>>>>> -            bdrv_replace_node(to_replace, target_bs, &local_err);
>>>>>> +            /*
>>>>>> +             * It is OK for @to_replace to be an immediate child of
>>>>>> +             * @target_bs, because that is what happens with
>>>>>> +             * drive-mirror sync=none mode=absolute-paths: target_bs's
>>>>>> +             * backing file will be the source node, which is also
>>>>>> +             * to_replace (by default).
>>>>>> +             * bdrv_replace_node() handles this case by not letting
>>>>>> +             * target_bs->backing point to itself, but to the source
>>>>>> +             * still.
>>>>>> +             */
>>>>>> +            if (!bdrv_is_child_of(to_replace, target_bs, 2)) {
>>>>>> +                bdrv_replace_node(to_replace, target_bs, &local_err);
>>>>>> +            } else {
>>>>>> +                error_setg(&local_err, "Can no longer replace '%s' by '%s', "
>>>>>> +                           "because the former is now a child of the latter, "
>>>>>> +                           "and doing so would thus create a loop",
>>>>>> +                           to_replace->node_name, target_bs->node_name);
>>>>>> +            }
>>>>>
>>>>> you may swap if and else branch, dropping "!" mark..
>>>>
>>>> Yes, but I just personally prefer to have the error case in the else branch.
>>>>
>>>>>>             } else {
>>>>>>                 error_setg(&local_err, "Can no longer replace '%s' by '%s', "
>>>>>>                            "because it can no longer be guaranteed that doing so "
>>>>>> diff --git a/blockdev.c b/blockdev.c
>>>>>> index 9dc2238bf3..d29f147f72 100644
>>>>>> --- a/blockdev.c
>>>>>> +++ b/blockdev.c
>>>>>> @@ -3824,7 +3824,7 @@ static void blockdev_mirror_common(const char *job_id, BlockDriverState *bs,
>>>>>>         }
>>>>>>     
>>>>>>         if (has_replaces) {
>>>>>> -        BlockDriverState *to_replace_bs;
>>>>>> +        BlockDriverState *to_replace_bs, *target_backing_bs;
>>>>>>             AioContext *replace_aio_context;
>>>>>>             int64_t bs_size, replace_size;
>>>>>>     
>>>>>> @@ -3839,6 +3839,52 @@ static void blockdev_mirror_common(const char *job_id, BlockDriverState *bs,
>>>>>>                 return;
>>>>>>             }
>>>>>>     
>>>>>> +        if (bdrv_is_child_of(to_replace_bs, target, 1)) {
>>>>>> +            error_setg(errp, "Replacing %s by %s would result in a loop, "
>>>>>> +                       "because the former is a child of the latter",
>>>>>> +                       to_replace_bs->node_name, target->node_name);
>>>>>> +            return;
>>>>>> +        }
>>>>>
>>>>> here min_level=1, so we don't handle the case, described in mirror_exit_common..
>>>>> I don't see why.. blockdev_mirror_common is called from qmp_drive_mirror,
>>>>> including the case with MIRROR_SYNC_MODE_NONE and NEW_IMAGE_MODE_ABSOLUTE_PATHS..
>>>>>
>>>>> What I'm missing?
>>>>
>>>> Hmm.  Well.
>>>>
>>>> If it broke drive-mirror sync=none, I suppose I would have noticed by
>>>> running the iotests.  But I didn’t, and that’s because this code here is
>>>> reached only if the user actually specified @replaces.  (As opposed to
>>>> the mirror_exit_common code, where @to_replace may simply be @src if not
>>>> overridden by the user.)
>>>>
>>>> The only reason why I allow it in mirror_exit_common is because we have
>>>> to.  But if the user manually specifies this configuration, we can’t
>>>> guarantee it’s safe.
>>>>
>>>> OTOH, well, if we allow it for drive-mirror sync=none, why not allow it
>>>> when manually specified with blockdev-mirror?
>>>>
>>>> What’s your opinion?
>>>
>>> Hmm, I think, that allowing to_replaces to be direct backing child of target
>>> (like in mirror_exit_common) is safe enough. User doesn't know that
>>> such replacing includes also replacing own child of the target,
>>> which leads to the loop.. It's not obvious. And behavior of
>>> bdrv_replace_node() which just doesn't create this loop, doesn't
>>> seem something too tricky. Hmm..
>>>
>>> We could mention in qapi spec, that replacing doesn't break backing
>>> link of the target, for it to be absolutely defined.
>>>
>>> But should we allow replaces to be some other (not backing and not filtered)
>>> child of target?..
>>
>> Well, my opinion is that this is a bit of weird thing to do and that it
>> basically does ask for a loop.
>>
>> I’m OK with excluding the sync=none case, because (1) that’s so
>> obviously a loop that it can’t be what the user honestly wants; (2) how
>> it’s resolved is rather obvious, too: There is exactly one edge that
>> causes the loop, so you simply don’t change that one; (3) drive-mirror
>> sync=none does this case automatically, so we should probably allow
>> users to do it manually with blockdev-mirror, too.
>>
>>>>>> +
>>>>>> +        if (backing_mode == MIRROR_SOURCE_BACKING_CHAIN ||
>>>>>> +            backing_mode == MIRROR_OPEN_BACKING_CHAIN)
>>>>>> +        {
>>>>>> +            /*
>>>>>> +             * While we do not quite know what OPEN_BACKING_CHAIN
>>>>>> +             * (used for mode=existing) will yield, it is probably
>>>>>> +             * best to restrict it exactly like SOURCE_BACKING_CHAIN,
>>>>>> +             * because that is our best guess.
>>>>>> +             */
>>>>>> +            switch (sync) {
>>>>>> +            case MIRROR_SYNC_MODE_FULL:
>>>>>> +                target_backing_bs = NULL;
>>>>>> +                break;
>>>>>> +
>>>>>> +            case MIRROR_SYNC_MODE_TOP:
>>>>>> +                target_backing_bs = backing_bs(bs);
>>>>>> +                break;
>>>>>> +
>>>>>> +            case MIRROR_SYNC_MODE_NONE:
>>>>>> +                target_backing_bs = bs;
>>>>>> +                break;
>>>>>> +
>>>>>> +            default:
>>>>>> +                abort();
>>>>>> +            }
>>>>>> +        } else {
>>>>>> +            assert(backing_mode == MIRROR_LEAVE_BACKING_CHAIN);
>>>>>> +            target_backing_bs = backing_bs(target);
>>>>>> +        }
>>>>>> +
>>>>>> +        if (bdrv_is_child_of(to_replace_bs, target_backing_bs, 0)) {
>>>>>> +            error_setg(errp, "Replacing '%s' by '%s' with this sync mode would "
>>>>>> +                       "result in a loop, because the former would be a child "
>>>>>> +                       "of the latter's backing file ('%s') after the mirror "
>>>>>> +                       "job", to_replace_bs->node_name, target->node_name,
>>>>>> +                       target_backing_bs->node_name);
>>>>>> +            return;
>>>>>> +        }
>>>>>
>>>>> hmm.. so for MODE_NONE we disallow to_replace == src?
>>>>
>>>> I suppose that’s basically the same as above.  Should we allow this case
>>>> when specified explicitly by the user?
>>>>
>>>
>>> I'm a bit more closer to allowing it, for consistency with automatic path, with
>>> unspecified replaces. Are we sure that nobody uses it?
>>
>> Well, there are multiple cases, as shown in the commit message.  I think
>> that for drive-mirror sync=none, nobody uses @replaces, because it just
>> doesn’t work.
>>
>> But, well, that’s just because drive-mirror does graph manipulation that
>> blockdev-mirror doesn’t (i.e., changing the target’s backing file on
>> completion).  So maybe we should just prevent loops for drive-mirror,
>> but let the user do what they want when they use blockdev-mirror?
>>
> 
> Well, the question finally is, how much to restrict from things for which we
> don't know are they useful or not. I don't know) I think, finally, I'm OK with
> either way we discussed, or with this patch as is. If it breaks some existing
> scenario it will be easy to fix.

OK.  I hope next-year-me has a good and consistent idea on what to do.

Max


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