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-sta...@nongnu.org > > Signed-off-by: Kevin Wolf <kw...@redhat.com> > > I'm OK with it as is, as it fixes bug: > > Reviewed-by: Vladimir Sementsov-Ogievskiy <vsement...@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. Kevin
