QEMU-Devel Archive on lore.kernel.org
 help / color / Atom feed
From: Max Reitz <mreitz@redhat.com>
To: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>,
	"qemu-block@nongnu.org" <qemu-block@nongnu.org>
Cc: Kevin Wolf <kwolf@redhat.com>,
	"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>
Subject: Re: [Qemu-devel] [PATCH v6 35/42] block: Fix check_to_replace_node()
Date: Thu, 15 Aug 2019 19:01:48 +0200
Message-ID: <39c41687-e192-e63a-984f-6d60b075df1a@redhat.com> (raw)
In-Reply-To: <92ded75c-caed-c674-f2ef-72b15b4405b4@virtuozzo.com>

[-- Attachment #1.1: Type: text/plain, Size: 10509 bytes --]

On 15.08.19 17:21, Vladimir Sementsov-Ogievskiy wrote:
> 09.08.2019 19:14, Max Reitz wrote:
>> Currently, check_to_replace_node() only allows mirror to replace a node
>> in the chain of the source node, and only if it is the first non-filter
>> node below the source.  Well, technically, the idea is that you can
>> exactly replace a quorum child by mirroring from quorum.
>>
>> This has (probably) two reasons:
>> (1) We do not want to create loops.
>> (2) @replaces and @device should have exactly the same content so
>>      replacing them does not cause visible data to change.
>>
>> This has two issues:
>> (1) It is overly restrictive.  It is completely fine for @replaces to be
>>      a filter.
>> (2) It is not restrictive enough.  You can create loops with this as
>>      follows:
>>
>> $ qemu-img create -f qcow2 /tmp/source.qcow2 64M
>> $ qemu-system-x86_64 -qmp stdio
>> {"execute": "qmp_capabilities"}
>> {"execute": "object-add",
>>   "arguments": {"qom-type": "throttle-group", "id": "tg0"}}
>> {"execute": "blockdev-add",
>>   "arguments": {
>>       "node-name": "source",
>>       "driver": "throttle",
>>       "throttle-group": "tg0",
>>       "file": {
>>           "node-name": "filtered",
>>           "driver": "qcow2",
>>           "file": {
>>               "driver": "file",
>>               "filename": "/tmp/source.qcow2"
>>           } } } }
>> {"execute": "drive-mirror",
>>   "arguments": {
>>       "job-id": "mirror",
>>       "device": "source",
>>       "target": "/tmp/target.qcow2",
>>       "format": "qcow2",
>>       "node-name": "target",
>>       "sync" :"none",
>>       "replaces": "filtered"
>>   } }
>> {"execute": "block-job-complete", "arguments": {"device": "mirror"}}
>>
>> And qemu crashes because of a stack overflow due to the loop being
>> created (target's backing file is source, so when it replaces filtered,
>> it points to itself through source).
>>
>> (blockdev-mirror can be broken similarly.)
>>
>> So let us make the checks for the two conditions above explicit, which
>> makes the whole function exactly as restrictive as it needs to be.
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>> ---
>>   include/block/block.h |  1 +
>>   block.c               | 83 +++++++++++++++++++++++++++++++++++++++----
>>   blockdev.c            | 34 ++++++++++++++++--
>>   3 files changed, 110 insertions(+), 8 deletions(-)
>>
>> diff --git a/include/block/block.h b/include/block/block.h
>> index 6ba853fb90..8da706cd89 100644
>> --- a/include/block/block.h
>> +++ b/include/block/block.h
>> @@ -404,6 +404,7 @@ bool bdrv_is_first_non_filter(BlockDriverState *candidate);
>>   
>>   /* check if a named node can be replaced when doing drive-mirror */
>>   BlockDriverState *check_to_replace_node(BlockDriverState *parent_bs,
>> +                                        BlockDriverState *backing_bs,
>>                                           const char *node_name, Error **errp);
>>   
>>   /* async block I/O */
>> diff --git a/block.c b/block.c
>> index 915b80153c..4858d3e718 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -6290,7 +6290,59 @@ bool bdrv_is_first_non_filter(BlockDriverState *candidate)
>>       return false;
>>   }
>>   
>> +static bool is_child_of(BlockDriverState *child, BlockDriverState *parent)
>> +{
>> +    BdrvChild *c;
>> +
>> +    if (!parent) {
>> +        return false;
>> +    }
>> +
>> +    QLIST_FOREACH(c, &parent->children, next) {
>> +        if (c->bs == child || is_child_of(child, c->bs)) {
>> +            return true;
>> +        }
>> +    }
>> +
>> +    return false;
>> +}
>> +
>> +/*
>> + * Return true if there are only filters in [@top, @base).  Note that
>> + * this may include quorum (which bdrv_chain_contains() cannot
>> + * handle).
> 
> More presizely: return true if exists chain of filters from top to base or if
> top == base.
> 
> I keep in mind backup-top filter:
> 
> [backup-top]
> |          \target

backup-top can’t be a filter if it has two children with different
contents, though.

(commit-top and mirror-top aren’t filters either.)

That’s why there must be a unique chain [@top, @base).

I should probably not that it will return true if top == base, though, yes.

> |backing    -------->[target]
> V                    /
> [source]  <---------/backing
> 
>> + */
>> +static bool is_filtered_child(BlockDriverState *top, BlockDriverState *base)
>> +{
>> +    BdrvChild *c;
>> +
>> +    if (!top) {
>> +        return false;
>> +    }
>> +
>> +    if (top == base) {
>> +        return true;
>> +    }
>> +
>> +    if (!top->drv->is_filter) {
>> +        return false;
>> +    }
>> +
>> +    QLIST_FOREACH(c, &top->children, next) {
>> +        if (is_filtered_child(c->bs, base)) {
>> +            return true;
>> +        }
>> +    }
> 
> interesting, how much is it better to somehow reuse DFS search written in should_update_child()..
> [just note, don't do it in these series please]
> 
>> +
>> +    return false;
>> +}
>> +
>> +/*
>> + * @parent_bs is mirror's source BDS, @backing_bs is the BDS which
>> + * will be attached to the target when mirror completes.
>> + */
>>   BlockDriverState *check_to_replace_node(BlockDriverState *parent_bs,
>> +                                        BlockDriverState *backing_bs,
>>                                           const char *node_name, Error **errp)
>>   {
>>       BlockDriverState *to_replace_bs = bdrv_find_node(node_name);
>> @@ -6309,13 +6361,32 @@ BlockDriverState *check_to_replace_node(BlockDriverState *parent_bs,
>>           goto out;
>>       }
>>   
>> -    /* We don't want arbitrary node of the BDS chain to be replaced only the top
>> -     * most non filter in order to prevent data corruption.
>> -     * Another benefit is that this tests exclude backing files which are
>> -     * blocked by the backing blockers.
>> +    /*
>> +     * If to_replace_bs is (recursively) a child of backing_bs,
>> +     * replacing it may create a loop.  We cannot allow that.
>>        */
>> -    if (!bdrv_recurse_is_first_non_filter(parent_bs, to_replace_bs)) {
>> -        error_setg(errp, "Only top most non filter can be replaced");
>> +    if (to_replace_bs == backing_bs || is_child_of(to_replace_bs, backing_bs)) {
> 
> first condition is covered by second, so first may be omitted.

It is not.  is_child_of() does not return true if child == parent.

>> +        error_setg(errp, "Replacing this node would result in a loop");
>> +        to_replace_bs = NULL;
>> +        goto out;
>> +    }
>> +
>> +    /*
>> +     * Mirror is designed in such a way that when it completes, the
>> +     * source BDS is seamlessly replaced.  
> 
> Not source but to_replace_bs is replaced?

It has originally been designed to replace the source.  If it could
replace any arbitrary BDS, all of this would be moot.

>> It is therefore not allowed
>> +     * to replace a BDS where this condition would be violated, as that
>> +     * would defeat the purpose of mirror and could lead to data
>> +     * corruption.
>> +     * Therefore, between parent_bs and to_replace_bs there may be
>> +     * only filters (and the one on top must be a filter, too), so
>> +     * their data always stays in sync and mirror can complete and
>> +     * replace to_replace_bs without any possible corruptions.
>> +     */
>> +    if (!is_filtered_child(parent_bs, to_replace_bs) &&
>> +        !is_filtered_child(to_replace_bs, parent_bs))
>> +    {
>> +        error_setg(errp, "The node to be replaced must be connected to the "
>> +                   "source through filter nodes only");
> 
> "and the one on top must be a filter, too" not mentioned in the error..

Well, unless the source node is the node to be replaced.  Hm...  This
gets very hard to express.  I think I’d prefer to keep this as it is,
even though it is not quite correct, unless you have a better suggestion
of what to report. :-/

>>           to_replace_bs = NULL;
>>           goto out;
>>       }
>> diff --git a/blockdev.c b/blockdev.c
>> index 4e72f6f701..758e0b5431 100644
>> --- a/blockdev.c
>> +++ b/blockdev.c
>> @@ -3887,7 +3887,7 @@ static void blockdev_mirror_common(const char *job_id, BlockDriverState *bs,
>>       }
>>   
>>       if (has_replaces) {
>> -        BlockDriverState *to_replace_bs;
>> +        BlockDriverState *to_replace_bs, *backing_bs;
>>           AioContext *replace_aio_context;
>>           int64_t bs_size, replace_size;
>>   
>> @@ -3897,7 +3897,37 @@ static void blockdev_mirror_common(const char *job_id, BlockDriverState *bs,
>>               return;
>>           }
>>   
>> -        to_replace_bs = check_to_replace_node(bs, replaces, errp);
>> +        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:
>> +                backing_bs = NULL;
>> +                break;
>> +
>> +            case MIRROR_SYNC_MODE_TOP:
>> +                backing_bs = bdrv_filtered_cow_bs(bdrv_skip_rw_filters(bs));
> 
> why not  bdrv_backing_chain_next(bs) like in mirror_start?

Good question.  I suppose it should be
bdrv_filtered_cow_bs(bdrv_backing_chain_next(bs)) in mirror_start()?
Because with sync=top, we just want to remove the topmost COW node (and
filters on top), but keep filters behind it.

Max

>> +                break;
>> +
>> +            case MIRROR_SYNC_MODE_NONE:
>> +                backing_bs = bs;
>> +                break;
>> +
>> +            default:
>> +                abort();
>> +            }
>> +        } else {
>> +            assert(backing_mode == MIRROR_LEAVE_BACKING_CHAIN);
>> +            backing_bs = bdrv_filtered_cow_bs(bdrv_skip_rw_filters(target));
>> +        }
>> +
>> +        to_replace_bs = check_to_replace_node(bs, backing_bs, replaces, errp);
>>           if (!to_replace_bs) {
>>               return;
>>           }
>>
> 
> 



[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 488 bytes --]

  reply index

Thread overview: 136+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-09 16:13 [Qemu-devel] [PATCH v6 00/42] block: Deal with filters Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 01/42] block: Mark commit and mirror as filter drivers Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 02/42] copy-on-read: Support compressed writes Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 03/42] throttle: " Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 04/42] block: Add child access functions Max Reitz
2019-08-09 16:56   ` Eric Blake
2019-09-04 16:16   ` Kevin Wolf
2019-09-09  7:56     ` Max Reitz
2019-09-09  9:36       ` Kevin Wolf
2019-09-09 14:04         ` Max Reitz
2019-09-09 16:13           ` Kevin Wolf
2019-09-10  9:14             ` Max Reitz
2019-09-10 10:47               ` Kevin Wolf
2019-09-10 11:36                 ` Max Reitz
2019-09-10 12:48                   ` Kevin Wolf
2019-09-10 12:59                     ` Max Reitz
2019-09-10 13:10                       ` Kevin Wolf
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 05/42] block: Add chain helper functions Max Reitz
2019-08-09 17:01   ` Eric Blake
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 06/42] qcow2: Implement .bdrv_storage_child() Max Reitz
2019-08-09 17:07   ` Eric Blake
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 07/42] block: *filtered_cow_child() for *has_zero_init() Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 08/42] block: bdrv_set_backing_hd() is about bs->backing Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 09/42] block: Include filters when freezing backing chain Max Reitz
2019-08-10 13:32   ` Vladimir Sementsov-Ogievskiy
2019-08-12 12:56     ` Max Reitz
2019-09-05 13:05   ` Kevin Wolf
2019-09-09  8:02     ` Max Reitz
2019-09-09  9:40       ` Kevin Wolf
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 10/42] block: Drop bdrv_is_encrypted() Max Reitz
2019-08-10 13:42   ` Vladimir Sementsov-Ogievskiy
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 11/42] block: Add bdrv_supports_compressed_writes() Max Reitz
2019-09-05 13:11   ` Kevin Wolf
2019-09-09  8:09     ` Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 12/42] block: Use bdrv_filtered_rw* where obvious Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 13/42] block: Use CAFs in block status functions Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 14/42] block: Use CAFs when working with backing chains Max Reitz
2019-08-10 15:19   ` Vladimir Sementsov-Ogievskiy
2019-09-05 14:05   ` Kevin Wolf
2019-09-09  8:25     ` Max Reitz
2019-09-09  9:55       ` Kevin Wolf
2019-09-09 14:08         ` Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 15/42] block: Re-evaluate backing file handling in reopen Max Reitz
2019-08-10 16:05   ` Vladimir Sementsov-Ogievskiy
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 16/42] block: Flush all children in generic code Max Reitz
2019-08-10 15:36   ` Vladimir Sementsov-Ogievskiy
2019-08-12 12:58     ` Max Reitz
2019-09-05 16:24       ` Kevin Wolf
2019-09-09  8:31         ` Max Reitz
2019-09-09 10:01           ` Kevin Wolf
2019-09-09 14:15             ` Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 17/42] block: Use CAFs in bdrv_refresh_limits() Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 18/42] block: Use CAFs in bdrv_refresh_filename() Max Reitz
2019-08-10 16:22   ` Vladimir Sementsov-Ogievskiy
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 19/42] block: Use CAF in bdrv_co_rw_vmstate() Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 20/42] block/snapshot: Fix fallback Max Reitz
2019-08-10 16:34   ` Vladimir Sementsov-Ogievskiy
2019-08-12 13:06     ` Max Reitz
2019-09-10 11:56   ` Kevin Wolf
2019-09-10 12:04     ` Max Reitz
2019-09-10 12:49       ` Kevin Wolf
2019-09-10 13:06         ` Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 21/42] block: Use CAFs for debug breakpoints Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 22/42] block: Fix bdrv_get_allocated_file_size's fallback Max Reitz
2019-08-10 16:41   ` Vladimir Sementsov-Ogievskiy
2019-08-12 13:09     ` Max Reitz
2019-08-12 17:14       ` Vladimir Sementsov-Ogievskiy
2019-08-12 19:15         ` Max Reitz
2019-09-10 14:52   ` Kevin Wolf
2019-09-11  6:20     ` Max Reitz
2019-09-11  6:55       ` Kevin Wolf
2019-09-11  7:37         ` Max Reitz
2019-09-11  8:27           ` Kevin Wolf
2019-09-11 10:00             ` Max Reitz
2019-09-11 10:31               ` Kevin Wolf
2019-09-11 11:00                 ` Max Reitz
2019-09-12 10:34                   ` Kevin Wolf
2019-11-14 13:11                   ` Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 23/42] blockdev: Use CAF in external_snapshot_prepare() Max Reitz
2019-09-10 15:02   ` Kevin Wolf
2019-09-11  6:21     ` Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 24/42] block: Use child access functions for QAPI queries Max Reitz
2019-08-10 16:57   ` Vladimir Sementsov-Ogievskiy
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 25/42] mirror: Deal with filters Max Reitz
2019-08-12 11:09   ` Vladimir Sementsov-Ogievskiy
2019-08-12 13:26     ` Max Reitz
2019-08-14 15:17       ` Vladimir Sementsov-Ogievskiy
2019-08-31  9:57   ` Vladimir Sementsov-Ogievskiy
2019-09-02 14:35     ` Max Reitz
2019-09-03  8:32       ` Vladimir Sementsov-Ogievskiy
2019-09-09  7:41         ` Max Reitz
2019-09-13 12:55   ` Kevin Wolf
2019-09-16 10:26     ` Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 26/42] backup: " Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 27/42] commit: " Max Reitz
2019-08-31 10:44   ` Vladimir Sementsov-Ogievskiy
2019-09-02 14:55     ` Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 28/42] stream: " Max Reitz
2019-08-12 11:55   ` Vladimir Sementsov-Ogievskiy
2019-09-13 14:16   ` Kevin Wolf
2019-09-16  9:52     ` Max Reitz
2019-09-16 14:47       ` Kevin Wolf
2019-12-11 12:52       ` Max Reitz
2019-12-11 15:52         ` Kevin Wolf
2019-12-11 16:12           ` Max Reitz
2019-12-11 16:35             ` Kevin Wolf
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 29/42] nbd: Use CAF when looking for dirty bitmap Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 30/42] qemu-img: Use child access functions Max Reitz
2019-08-12 12:14   ` Vladimir Sementsov-Ogievskiy
2019-08-12 13:28     ` Max Reitz
2019-08-14 16:04   ` Vladimir Sementsov-Ogievskiy
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 31/42] block: Drop backing_bs() Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 32/42] block: Make bdrv_get_cumulative_perm() public Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 33/42] blockdev: Fix active commit choice Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 34/42] block: Inline bdrv_co_block_status_from_*() Max Reitz
2019-08-09 16:14 ` [Qemu-devel] [PATCH v6 35/42] block: Fix check_to_replace_node() Max Reitz
2019-08-15 15:21   ` Vladimir Sementsov-Ogievskiy
2019-08-15 17:01     ` Max Reitz [this message]
2019-08-16 11:01       ` Vladimir Sementsov-Ogievskiy
2019-08-16 13:30         ` Max Reitz
2019-08-16 14:24           ` Vladimir Sementsov-Ogievskiy
2019-08-09 16:14 ` [Qemu-devel] [PATCH v6 36/42] iotests: Add tests for mirror @replaces loops Max Reitz
2019-08-09 16:14 ` [Qemu-devel] [PATCH v6 37/42] block: Leave BDS.backing_file constant Max Reitz
2019-08-16 16:16   ` Vladimir Sementsov-Ogievskiy
2019-08-09 16:14 ` [Qemu-devel] [PATCH v6 38/42] iotests: Let complete_and_wait() work with commit Max Reitz
2019-08-23  5:59   ` Vladimir Sementsov-Ogievskiy
2019-08-09 16:14 ` [Qemu-devel] [PATCH v6 39/42] iotests: Add filter commit test cases Max Reitz
2019-08-31 11:41   ` Vladimir Sementsov-Ogievskiy
2019-09-02 15:06     ` Max Reitz
2019-08-31 12:35   ` Vladimir Sementsov-Ogievskiy
2019-09-02 15:09     ` Max Reitz
2019-08-09 16:14 ` [Qemu-devel] [PATCH v6 40/42] iotests: Add filter mirror " Max Reitz
2019-08-31 12:35   ` Vladimir Sementsov-Ogievskiy
2019-08-09 16:14 ` [Qemu-devel] [PATCH v6 41/42] iotests: Add test for commit in sub directory Max Reitz
2019-08-09 16:14 ` [Qemu-devel] [PATCH v6 42/42] iotests: Test committing to overridden backing Max Reitz
2019-09-03  9:18   ` Vladimir Sementsov-Ogievskiy

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=39c41687-e192-e63a-984f-6d60b075df1a@redhat.com \
    --to=mreitz@redhat.com \
    --cc=kwolf@redhat.com \
    --cc=qemu-block@nongnu.org \
    --cc=qemu-devel@nongnu.org \
    --cc=vsementsov@virtuozzo.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

QEMU-Devel Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/qemu-devel/0 qemu-devel/git/0.git
	git clone --mirror https://lore.kernel.org/qemu-devel/1 qemu-devel/git/1.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 qemu-devel qemu-devel/ https://lore.kernel.org/qemu-devel \
		qemu-devel@nongnu.org
	public-inbox-index qemu-devel

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.nongnu.qemu-devel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git