[PATCH v11 04/13] copy-on-read: pass overlay base node name to COR driver

[Vladimir Sementsov-Ogievskiy]

14.10.2020 19:08, Andrey Shinkevich wrote:
> On 14.10.2020 14:09, Max Reitz wrote:
>> On 12.10.20 19:43, Andrey Shinkevich wrote:
>>> We are going to use the COR-filter for a block-stream job.
>>> To limit COR operations by the base node in the backing chain during
>>> stream job, pass the name of overlay base node to the copy-on-read
>>> driver as base node itself may change due to possible concurrent jobs.
>>> The rest of the functionality will be implemented in the patch that
>>> follows.
>>>
>>> Signed-off-by: Andrey Shinkevich <andrey.shinkevich at virtuozzo.com>
>>> ---
>>>   block/copy-on-read.c | 14 ++++++++++++++
>>>   1 file changed, 14 insertions(+)
>>
>> Is there a reason why you didn’t add this option to QAPI (as part of a
>> yet-to-be-created BlockdevOptionsCor)?  Because I’d really like it there.
>>
> 
> I agree that passing a base overlay under the base option looks clumsy. We could pass the base node name and find its overlay ourselves here in cor_open(). In that case, we can use the existing QAPI.

Actually, there is no existing QAPI: if you don't modify qapi/*.json, user is not able to pass the option through QAPI. It's still possible to pass the option through command-line, or when create the filter internally (like we are going to do in block-stream), but not through QAPI. So, it's better to make a new QAPI parameter, to make the new option available for QMP interface.

> The reason I used the existing QAPI is to make it easier for a user to operate with the traditional options and to keep things simple. So, the user shouldn't think what overlay or above-base node to pass.
> If we introduce the specific BlockdevOptionsCor, what other options may come with?
> 
>>> diff --git a/block/copy-on-read.c b/block/copy-on-read.c
>>> index bcccf0f..c578b1b 100644
>>> --- a/block/copy-on-read.c
>>> +++ b/block/copy-on-read.c
>>> @@ -24,19 +24,24 @@
>>>   #include "block/block_int.h"
>>>   #include "qemu/module.h"
>>>   #include "qapi/error.h"
>>> +#include "qapi/qmp/qerror.h"
>>>   #include "qapi/qmp/qdict.h"
>>>   #include "block/copy-on-read.h"
>>>   typedef struct BDRVStateCOR {
>>>       bool active;
>>> +    BlockDriverState *base_overlay;
>>>   } BDRVStateCOR;
>>>   static int cor_open(BlockDriverState *bs, QDict *options, int flags,
>>>                       Error **errp)
>>>   {
>>> +    BlockDriverState *base_overlay = NULL;
>>>       BDRVStateCOR *state = bs->opaque;
>>> +    /* We need the base overlay node rather than the base itself */
>>> +    const char *base_overlay_node = qdict_get_try_str(options, "base");
>>
>> Shouldn’t it be called base-overlay or above-base then?
>>
> 
> The base_overlay identifier is used below as the pointer to BS. The base_overlay_node stands for the name of the node. I used that identifier to differ between the types. And the above_base has another meaning per block/stream.c - it can be a temporary filter with a JSON-name.
> 
>>>       bs->file = bdrv_open_child(NULL, options, "file", bs, &child_of_bds,
>>>                                  BDRV_CHILD_FILTERED | BDRV_CHILD_PRIMARY,
>>> @@ -52,7 +57,16 @@ static int cor_open(BlockDriverState *bs, QDict *options, int flags,
>>>           ((BDRV_REQ_FUA | BDRV_REQ_MAY_UNMAP | BDRV_REQ_NO_FALLBACK) &
>>>               bs->file->bs->supported_zero_flags);
>>> +    if (base_overlay_node) {
>>> +        qdict_del(options, "base");
>>> +        base_overlay = bdrv_lookup_bs(NULL, base_overlay_node, errp);
>>
>> I think this is a use-after-free.  The storage @base_overlay_node points
>> to belongs to a QString, which is referenced only by @options; so
>> deleting that element of @options should free that string.
>>
>> Max
>>
> 
> I will swap those two function calls (bdrv_lookup_bs(); qdict_del();).
> Thank you.
> 
> Andrey
> 
>>> +        if (!base_overlay) {
>>> +            error_setg(errp, QERR_BASE_NOT_FOUND, base_overlay_node);
>>> +            return -EINVAL;
>>> +        }
>>> +    }
>>>       state->active = true;
>>> +    state->base_overlay = base_overlay;
>>>       /*
>>>        * We don't need to call bdrv_child_refresh_perms() now as the permissions
>>>
>>
>>


-- 
Best regards,
Vladimir