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 >>>> --- >>>> 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? Max