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

[Kevin Wolf]

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: address@hidden
> > Signed-off-by: Kevin Wolf <address@hidden>
> 
> I'm OK with it as is, as it fixes bug:
> 
> Reviewed-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> 
> 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