Re: [PATCH v2 3/4] backup: Make sure that source and target size match

[Kevin Wolf <kwolf@redhat.com>]

Am 06.05.2020 um 08:07 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 05.05.2020 13:03, Kevin Wolf wrote:
> > Am 30.04.2020 um 20:21 hat Vladimir Sementsov-Ogievskiy geschrieben:
> > > 30.04.2020 17:27, Kevin Wolf wrote:
> > > > Since the introduction of a backup filter node in commit 00e30f05d, the
> > > > backup block job crashes when the target image is smaller than the
> > > > source image because it will try to write after the end of the target
> > > > node without having BLK_PERM_RESIZE. (Previously, the BlockBackend layer
> > > > would have caught this and errored out gracefully.)
> > > > 
> > > > We can fix this and even do better than the old behaviour: Check that
> > > > source and target have the same image size at the start of the block job
> > > > and unshare BLK_PERM_RESIZE. (This permission was already unshared
> > > > before the same commit 00e30f05d, but the BlockBackend that was used to
> > > > make the restriction was removed without a replacement.) This will
> > > > immediately error out when starting the job instead of only when writing
> > > > to a block that doesn't exist in the target.
> > > > 
> > > > Longer target than source would technically work because we would never
> > > > write to blocks that don't exist, but semantically these are invalid,
> > > > too, because a backup is supposed to create a copy, not just an image
> > > > that starts with a copy.
> > > > 
> > > > Fixes: 00e30f05de1d19586345ec373970ef4c192c6270
> > > > Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1778593
> > > > Cc: qemu-stable@nongnu.org
> > > > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > > 
> > > I'm OK with it as is, as it fixes bug:
> > > 
> > > Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> > > 
> > > still, some notes below
> > > 
> > > 
> > > > ---
> > > >    block/backup-top.c | 14 +++++++++-----
> > > >    block/backup.c     | 14 +++++++++++++-
> > > >    2 files changed, 22 insertions(+), 6 deletions(-)
> > > > 
> > > > diff --git a/block/backup-top.c b/block/backup-top.c
> > > > index 3b50c06e2c..79b268e6dc 100644
> > > > --- a/block/backup-top.c
> > > > +++ b/block/backup-top.c
> > > > @@ -148,8 +148,10 @@ static void backup_top_child_perm(BlockDriverState *bs, BdrvChild *c,
> > > >             *
> > > >             * Share write to target (child_file), to not interfere
> > > >             * with guest writes to its disk which may be in target backing chain.
> > > > +         * Can't resize during a backup block job because we check the size
> > > > +         * only upfront.
> > > >             */
> > > > -        *nshared = BLK_PERM_ALL;
> > > > +        *nshared = BLK_PERM_ALL & ~BLK_PERM_RESIZE;
> > > >            *nperm = BLK_PERM_WRITE;
> > > >        } else {
> > > >            /* Source child */
> > > > @@ -159,7 +161,7 @@ static void backup_top_child_perm(BlockDriverState *bs, BdrvChild *c,
> > > >            if (perm & BLK_PERM_WRITE) {
> > > >                *nperm = *nperm | BLK_PERM_CONSISTENT_READ;
> > > >            }
> > > > -        *nshared &= ~BLK_PERM_WRITE;
> > > > +        *nshared &= ~(BLK_PERM_WRITE | BLK_PERM_RESIZE);
> > > >        }
> > > >    }
> > > > @@ -192,11 +194,13 @@ BlockDriverState *bdrv_backup_top_append(BlockDriverState *source,
> > > >    {
> > > >        Error *local_err = NULL;
> > > >        BDRVBackupTopState *state;
> > > > -    BlockDriverState *top = bdrv_new_open_driver(&bdrv_backup_top_filter,
> > > > -                                                 filter_node_name,
> > > > -                                                 BDRV_O_RDWR, errp);
> > > > +    BlockDriverState *top;
> > > >        bool appended = false;
> > > > +    assert(source->total_sectors == target->total_sectors);
> > > 
> > > May be better to error-out, just to keep backup-top independent. Still, now it's not
> > > really needed, as we have only one caller. And this function have to be refactored
> > > anyway, when publishing this filter (open() and close() should appear, so this code
> > > will be rewritten anyway.)
> > 
> > Yes, the whole function only works because it's used in this restricted
> > context today. For example, we only know that total_sectors is up to
> > date because the caller has called bdrv_getlength() just a moment ago.
> > 
> > I think fixing this would be beyond the scope of this patch, but
> > certainly a good idea anyway.
> > 
> > > And the other thought: the permissions we declared above, will be activated only after
> > > successful bdrv_child_refresh_perms(). I think some kind of race is possible, so that
> > > size is changed actual permission activation. So, may be good to double check sizes after
> > > bdrv_child_refresh_perms().. But it's a kind of paranoia.
> > 
> > We're not in coroutine context, so we can't yield. I don't see who could
> > change the size in parallel (apart from an external process, but an
> > external process can mess up anything).
> > 
> > When we make backup-top an independent driver, instead of double
> > checking (what would you do on error?), maybe we could move the size
> > initialisation (then with bdrv_getlength()) to after
> > bdrv_child_refresh_perms().
> > 
> > > Also, third thought: the restricted permissions doesn't save us from resizing
> > > of the source through exactly this node, does it? Hmm, but your test works somehow. But
> > > (I assume) it worked in a previous patch version without unsharing on source..
> > > 
> > > Ha, but bdrv_co_truncate just can't work on backup-top, because it doesn't have file child.
> > > But, if we fix bdrv_co_truncate to skip filters, we'll need to define .bdrv_co_truncate in
> > > backup_top, which will return something like -EBUSY.. Or just -ENOTSUP, doesn't matter.
> > 
> > Maybe this is a sign that bdrv_co_truncate shouldn't automatically skip
> > filters because filters might depend on a fixed size?
> > 
> > Or we could make the automatic skipping depend on having BLK_PERM_RESIZE
> > for the child. If the filter doesn't have the permission, we must not
> > call truncate for its child (it would crash). Then backup-top and
> > similar filters must just be careful not to take RESIZE permissions.
> > 
> 
> Hmm this should work.. Still it's a workaround, seems out of the
> concept of permission system..

I'm not sure about this. I see the problem more with unconditionally
skipping the filter without checking whether the operation is even
allowed on the filtered child.

> I think, that the problem is that .bdrv_top_child_perm can't return an
> error.  The handler answers the question:
> 
> - Hi, we are your owners and we want the following cumulative
> permissions on you. Then, which permissions do you want on your child?
> 
> And the handler can't answer: "Hi, you guys want too much, I refuse to
> play by your rules"..

It can implement .bdrv_check_perm to do that. It's just a bit more work
for each driver to do that.

Kevin