Re: [PATCH v3 1/1] qemu-img: Add --target-is-zero to convert

[Eric Blake <eblake@redhat.com>]

On 2/4/20 7:59 AM, Vladimir Sementsov-Ogievskiy wrote:

>> I understand that it is safer to have restrictions now and lift them 
>> later, than to allow use of the option at any time and leave room for 
>> the user to shoot themselves in the foot with no way to add safety 
>> later.  The argument against no backing file is somewhat 
>> understandable (technically, as long as the backing file also reads as 
>> all zeroes, then the overall image reads as all zeroes - but why have 
>> a backing file that has no content?); the argument requiring -n is a 
>> bit weaker (if I'm creating an image, I _know_ it reads as all zeroes, 
>> so the --target-is-zero argument is redundant, but it shouldn't hurt 
>> to allow it).
> 
> I know that it reads as all zeroes, only if this format provides zero 
> initialization..
> 
>>
>>> +++ b/qemu-img.c
>>
>>> @@ -2247,6 +2256,11 @@ static int img_convert(int argc, char **argv)
>>>           warn_report("This will become an error in future QEMU 
>>> versions.");
>>>       }
>>> +    if (s.has_zero_init && !skip_create) {
>>> +        error_report("--target-is-zero requires use of -n flag");
>>> +        goto fail_getopt;
>>> +    }
>>
>> So I think we could drop this hunk with no change in behavior.
> 
> I think, no we can't. If we allow target-is-zero, with -n, we'd better 
> to check that what we are creating is zero-initialized (format has 
> zero-init), and if not we should report error.
> 

Good call.  Yes, if we allow --target-is-zero without -n, we MUST insist 
that bdrv_has_zero_init() returns 1 (or, after my followup series, 
bdrv_known_zeroes() includes BDRV_ZERO_CREATE).  I can work that into v2 
of my series, if desired.

>>
>>> +
>>>       s.src_num = argc - optind - 1;
>>>       out_filename = s.src_num >= 1 ? argv[argc - 1] : NULL;
>>> @@ -2380,6 +2394,11 @@ static int img_convert(int argc, char **argv)
>>>       }
>>>       s.target_has_backing = (bool) out_baseimg;
>>> +    if (s.has_zero_init && s.target_has_backing) {
>>> +        error_report("Cannot use --target-is-zero with a backing 
>>> file");
>>> +        goto out;
>>> +    }
>>> +
>>
>> while keeping this one makes sense.  At any rate, typo fixes are 
>> minor, and whether or not we drop the hunk I claim is a needless 
>> restriction against redundancy,
>>
>> Reviewed-by: Eric Blake <eblake@redhat.com>
>>
> 
> 

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