Re: [Qemu-devel] [PATCH v2 04/16] qcow2: Keep unknown extra snapshot data

[Max Reitz]

On 19.08.19 21:23, Eric Blake wrote:
> On 8/19/19 1:55 PM, Max Reitz wrote:
>> The qcow2 specification says to ignore unknown extra data fields in
>> snapshot table entries.  Currently, we discard it whenever we update the
>> image, which is a bit different from "ignore".
>>
>> This patch makes the qcow2 driver keep all unknown extra data fields
>> when updating an image's snapshot table.
>>
>> Signed-off-by: Max Reitz <address@hidden>
>> ---
>>  block/qcow2.h          |  5 ++++
>>  block/qcow2-snapshot.c | 61 +++++++++++++++++++++++++++++++++++-------
>>  2 files changed, 56 insertions(+), 10 deletions(-)
>>
> 
>> @@ -162,7 +184,7 @@ static int qcow2_write_snapshots(BlockDriverState *bs)
>>          sn = s->snapshots + i;
>>          offset = ROUND_UP(offset, 8);
>>          offset += sizeof(h);
>> -        offset += sizeof(extra);
>> +        offset += MAX(sizeof(extra), sn->extra_data_size);
> 
> Why would we ever have less than sizeof(extra) bytes to write on output,
> since we always produce the fields on creation and synthesize the
> missing fields of extra on read?  Can't you rewrite this as:
> 
> assert(sn->extra_data_size >= sizeof(extra));
> offset += sn->extra_data_size;

Hm, but I don’t prop up extra_data_size to be at least sizeof(extra).  I
can do that, but it would add a few extra lines here and there.

> Otherwise,
> 
> Reviewed-by: Eric Blake <address@hidden>

In any case, thanks for reviewing again :-)

Max

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