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-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. > > > > 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
