[libvirt] [PATCH for-4.2?] block/qcow2-bitmap: fix crash bug in qcow2_co_remove_persistent_dirty_bitmap

[Eric Blake]

On 12/5/19 1:30 PM, Vladimir Sementsov-Ogievskiy wrote:
> Here is double bug:
> 
> First, return error but not set errp. This may lead to:
> qmp block-dirty-bitmap-remove may report success when actually failed
> 
> block-dirty-bitmap-remove used in a transaction will crash, as
> qmp_transaction will think that it returned success and will cal

call

> block_dirty_bitmap_remove_commit which will crash, as state->bitmap is
> NULL
> 
> Second (like in anecdote), this case is not an error at all. As it is
> documented in the comment above bdrv_co_remove_persistent_dirty_bitmap
> definition, absence of bitmap is not an error, and similar case handled
> at start of qcow2_co_remove_persistent_dirty_bitmap, it returns 0 when
> there is no bitmaps at all..

double .

> 
> But when there are some bitmaps, but not the requested one, it return
> error with errp unset.
> 
> Fix that.
> 
> Fixes: b56a1e31759b750
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov at virtuozzo.com>
> ---
> 
> Hi all!
> 
> Ohm, suddenly we faced this bug. It's a regression in 4.2. I'm very
> sorry for introducing it, and it sad that it's found so late..
> 
> Personally, I think that this one worth rc5, as it makes new bitmap
> interfaces unusable. But the decision is yours.
> 
> Last minute edit: hmm, actually, transaction action introduced in
> 4.2, so crash is not a regression, only broken block-dirty-bitmap-remove
> command is a regression... Maybe it's OK for stable.

Libvirt REALLY wants to use transaction bitmap management (and require 
qemu 4.2) for its incremental backup patches that Peter is almost ready 
to merge in.  I'm trying to ascertain:

When exactly can this bug hit?  Can you give a sequence of QMP commands 
that will trigger it for easy reproduction?  Are there workarounds (such 
as checking that a bitmap exists prior to attempting to remove it)?  If 
it does NOT get fixed in rc5, how will libvirt be able to probe whether 
the fix is in place?

> 
>   block/qcow2-bitmap.c | 9 ++++++---
>   1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/block/qcow2-bitmap.c b/block/qcow2-bitmap.c
> index 8abaf632fc..c6c8ebbe89 100644
> --- a/block/qcow2-bitmap.c
> +++ b/block/qcow2-bitmap.c
> @@ -1469,8 +1469,10 @@ int coroutine_fn qcow2_co_remove_persistent_dirty_bitmap(BlockDriverState *bs,
>       Qcow2BitmapList *bm_list;
>   
>       if (s->nb_bitmaps == 0) {
> -        /* Absence of the bitmap is not an error: see explanation above
> -         * bdrv_remove_persistent_dirty_bitmap() definition. */
> +        /*
> +         * Absence of the bitmap is not an error: see explanation above
> +         * bdrv_co_remove_persistent_dirty_bitmap() definition.
> +         */
>           return 0;
>       }
>   
> @@ -1485,7 +1487,8 @@ int coroutine_fn qcow2_co_remove_persistent_dirty_bitmap(BlockDriverState *bs,
>   
>       bm = find_bitmap_by_name(bm_list, name);
>       if (bm == NULL) {
> -        ret = -EINVAL;
> +        /* Absence of the bitmap is not an error, see above. */
> +        ret = 0;
>           goto out;
>       }
>   
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org