Re: [Qemu-devel] [PATCH v4] qemu-img: Check for backing image if specified during create

[Eric Blake]

On 05/12/2017 01:07 PM, Max Reitz wrote:
> On 2017-05-11 20:27, John Snow wrote:
>> Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1213786
>>
>> Or, rather, force the open of a backing image if one was specified
>> for creation. Using a similar -unsafe option as rebase, allow qemu-img
>> to ignore the backing file validation if possible.
>>

>> +++ b/block.c
>> @@ -4275,37 +4275,37 @@ void bdrv_img_create(const char *filename, const 
>> char *fmt,
>>      // The size for the image must always be specified, with one exception:
>>      // If we are using a backing file, we can obtain the size from there
>>      size = qemu_opt_get_size(opts, BLOCK_OPT_SIZE, 0);
>> -    if (size == -1) {
> 
> "Hang on, why should this be -1 when the defval is 0? Where does the -1
> come from?"
> "..."
> "Oh, the option exists and is set to -1? Why is that?"
> "..."
> "Oh, because this function always sets it itself, and because @img_size
> is set to (uint64_t)-1."

I had pretty much the same conversation on my v1 review.
https://lists.gnu.org/archive/html/qemu-devel/2017-05/msg01097.html

> 
> First, I won't start with how signed integer overflow is
> implementation-defined in C because I hope you have thrashed that out
> with Eric (I hope that "to thrash out" is a good translation for
> "auskaspern" (lit. "to buffoon out").).

Sounds like a reasonable choice of words, even if I don't speak the
counterpart language to validate your translation.

(uint64_t)-1 is well-defined in C (so I think we're just fine here). But
(int64_t)UINT64_MAX is where signed integer overflow does indeed throw
wrinkles at you.

I seem to recall that qemu has chosen to use compiler flags and/or
assumptions that we are using 2s-complement arithmetic with sane
behavior (that is, tighter behavior than the bare minimum that C
requires), because it was easier than auditing our code for strict C
compliance on border cases of conversions from unsigned to signed that
trigger undefined behavior.  But again, I don't think it affects this
patch (where our conversion is only from signed to unsigned, and that is
well-defined behavior).


> 
> Second, well, at least we should put -1 as the default value here, then.

Indeed, now that two reviewers have tripped on it,
qemu_opt_get_size(,,-1) would be nicer.

> 
> Not strictly your fault or something that you need to fix, but it is
> just a single line in the vicinity...
> 
> Let me know if you want to address this, for now I'll leave a
> 
> Reviewed-by: Max Reitz <address@hidden>
> 
> here if you don't want to.

I'm okay whether you want to squash that fix into this patch, or whether
you do it as a separate followup patch.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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