Re: [PATCH v2 1/2] block/null: Report DATA if not reading zeroes

[Markus Armbruster <armbru@redhat.com>]

Markus Armbruster <armbru@redhat.com> writes:

> Nir Soffer <nirsof@gmail.com> writes:
>
>> If read-zeroes is not set, we did not report BDRV_BLOCK_DATA or
>> BDRV_BLOCK_ZERO. This is not consistent with other drivers and can
>> confuse users or other programs:
>>
>>     % qemu-img map --output json "json:{'driver': 'raw', 'file': {'driver': 'null-co', 'size': '1g'}}"
>>     [{ "start": 0, "length": 1073741824, "depth": 0, "present": false, "zero": false, "data": false, "compressed": false}]
>>
>>     % qemu-nbd "json:{'driver': 'raw', 'file': {'driver': 'null-co', 'size': '1g'}}" &
>>
>>     % nbdinfo --map nbd://127.0.0.1
>>              0  1073741824    1  hole
>>
>> With this change we report DATA in this case:
>>
>>     % ./qemu-img map --output json "json:{'driver': 'raw', 'file': {'driver': 'null-co', 'size': '1g'}}"
>>     [{ "start": 0, "length": 1073741824, "depth": 0, "present": true, "zero": false, "data": true, "compressed": false, "offset": 0}]
>>
>>     % ./qemu-nbd "json:{'driver': 'raw', 'file': {'driver': 'null-co', 'size': '1g'}}" &
>>
>>     % nbdinfo --map nbd://127.0.0.1
>>              0  1073741824    0  data
>>
>> Signed-off-by: Nir Soffer <nirsof@gmail.com>
>> ---
>>  block/null.c         | 4 +---
>>  qapi/block-core.json | 5 +++--
>>  2 files changed, 4 insertions(+), 5 deletions(-)
>>
>> diff --git a/block/null.c b/block/null.c
>> index dc0b1fdbd9..7ba87bd9a9 100644
>> --- a/block/null.c
>> +++ b/block/null.c
>> @@ -239,9 +239,7 @@ static int coroutine_fn null_co_block_status(BlockDriverState *bs,
>>      *map = offset;
>>      *file = bs;
>>  
>> -    if (s->read_zeroes) {
>> -        ret |= BDRV_BLOCK_ZERO;
>> -    }
>> +    ret |= s->read_zeroes ? BDRV_BLOCK_ZERO : BDRV_BLOCK_DATA;
>>      return ret;
>>  }
>>  
>> diff --git a/qapi/block-core.json b/qapi/block-core.json
>> index b1937780e1..7c95c9e36a 100644
>> --- a/qapi/block-core.json
>> +++ b/qapi/block-core.json
>> @@ -3293,8 +3293,9 @@
>>  #     requests.  Default to zero which completes requests immediately.
>>  #     (Since 2.4)
>>  #
>> -# @read-zeroes: if true, reads from the device produce zeroes; if
>> -#     false, the buffer is left unchanged.
>> +# @read-zeroes: if true, emulate a sparse image, and reads from the
>> +#     device produce zeroes; if false, emulate an allocated image but
>> +#     reads from the device leave the buffer unchanged.
>>  #     (default: false; since: 4.1)
>>  #
>>  # Since: 2.9
>
> Possibly dumb question: how is this doc change related to the code fix?
>
> Suggest to split the sentence for easier reading:
>
>    # @read-zeroes: If true, emulate a sparse image, and reads from the
>    #     device produce zeroes.  If false, emulate an allocated image,
>    #     but reads from the device leave the buffer unchanged.

false is a security hazard, as secure-coding-practices.rst points out.
I think it should be pointed out right here as well.  Especially since
"security hazard" is the default!

I'd do it in a separate patch, but I'm a compulsive patch splitter.