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