[libvirt] [PATCH 1/2] virTypedParamsSerialize: minor fixes

Marc Hartmayer mhartmay at linux.ibm.com
Tue Jul 3 10:32:03 UTC 2018


On Thu, Jun 21, 2018 at 03:47 PM +0200, Marc Hartmayer <mhartmay at linux.ibm.com> wrote:
> On Wed, Jun 13, 2018 at 03:11 PM +0200, John Ferlan <jferlan at redhat.com> wrote:
>> On 06/07/2018 08:17 AM, Marc Hartmayer wrote:
>>> On Wed, May 09, 2018 at 09:51 PM +0200, John Ferlan <jferlan at redhat.com> wrote:
>>>> On 05/07/2018 11:24 AM, Marc Hartmayer wrote:
>>>>> On Mon, Apr 30, 2018 at 03:20 PM +0200, John Ferlan <jferlan at redhat.com> wrote:
>>>>>> On 04/25/2018 11:55 AM, Marc Hartmayer wrote:
>>>>>>> 1. Don't allocate if there is nothing that needs to be
>>>>>>>    allocated. Especially as the result of calling calloc(0, ...) is
>>>>>>>    implementation-defined.
>>>>
>>>> uh oh - another memory recollection challenge ;-)
>>>>
>>>>>>
>>>>>> Following VIR_ALLOC_N one finds :
>>>>>>
>>>>>> VIR_ALLOC_N(params_val, nparams)
>>>>>>
>>>>>> which equates to
>>>>>>
>>>>>> # define VIR_ALLOC_N(ptr, count) \
>>>>>>          virAllocN(&(ptr), sizeof(*(ptr)), (count), true, \
>>>>>>                    VIR_FROM_THIS, __FILE__, __FUNCTION__, __LINE__)
>>>>>>
>>>>>> or
>>>>>>
>>>>>> virAllocN(&params_val, sizeof(params_val), nparams, true, ...)
>>>>>>
>>>>>> and eventually/essentially
>>>>>>
>>>>>> *params_val = calloc(sizeof(params_val), nparams)
>>>>>>
>>>>>> If the returned value is NULL then we error w/ OOM (4th param=true).
>>>>>>
>>>>>> So, unless @params_val had no elements, it won't be calloc(0,...) and
>>>>>
>>>>> I'm talking about the case that nparams == 0 => calloc(sizeof(…), 0).
>>>>>
>>>>
>>>> And I was thinking is there a specific consumer/caller of
>>>> virTypedParamsSerialize that was passing something incorrectly.
>>>>
>>>>>> thus the question becomes is there a more specific path you are
>>>>>> referencing here?
>>>>>
>>>>> It’s a “generic” serializer so it should work for every (valid)
>>>>> case. What happens, for example, if params == NULL and nparams == 0? In
>>>>> that case I would expect *remote_params_val = NULL and
>>>>> *remote_params_len = 0.
>>>>>
>>>>
>>>> Going through the callers to virTypedParamsSerialize can/does anyone
>>>> pass params == NULL and nparams == 0?  Would that be reasonable and/or
>>>> expected?
>>>>
>>>> My look didn't find any - either some caller checks that the passed
>>>> @params != NULL (usually via virCheckNonNullArgGoto in the external API
>>>> call) or @params is built up on the fly and wouldn't be NULL because
>>>> there'd be an error causing failure beforehand. Although I'll admit the
>>>> migration ones are also crazy to look at.
>>>>
>>>> I could have made a mistake too; hence, the is there a specific instance
>>>> that I missed? Or perhaps this is a result of some branched/private code
>>>> that had that error which I don't have access to?
>>>
>>> It was the result of private code but actually it was intended and no
>>> error :)
>>>
>>>>
>>>> To answer your "What happens, for example, if params == NULL and nparams
>>>> == 0?", well supposedly "VIR_ALLOC_N(params_val, 0)" should return NULL,
>>>> so params_val and thus *remote_params_val == NULL.
>>>
>>> Did you try what 'VIR_ALLOC_N(params_val, 0)' actually returns? At least
>>> for me, it doesn’t return a NULL pointer.
>>>
>>
>> I think I already determined that NULL isn't returned; otherwise, we
>> would have failed w/ OOM. I do recall trying this and seeing a non NULL
>> value returned.
>>
>> IIRC: I found no way into [de]serialize w/ a 0 parameter, so having that
>> guard didn't seem necessary,
>
> I used the libvirt-python APIs for some testing. I called an (own) API
> with 'None' as argument and this resulted in 0 parameters.
>
>> but it's been 2 months since I looked at
>> this and that level of detail has long been flushed out of main memory
>> cache. ;-)
>>
>>> The man page for calloc-posix reads:
>>>
>>> “If either nelem or elsize is 0, then either a null pointer or a unique
>>> pointer value that can be successfully passed to free() shall be
>>> returned.” [1]
>>>
>>> [1] https://www.unix.com/man-page/POSIX/3posix/calloc/
>>>
>>
>> perhaps then we avoid this conundrum and take Daniel's advice in his
>> response:
>>
>>    "If there is any problem with this, then we should just change
>> bootstrap.conf to use calloc-gnu instead of calloc-posix, which
>> basically turns calloc(0) into calloc(1) for compat with glibc
>> behaviour."
>
> This would still require this patch, no? At least if we agree that the
> following example should work:

Okay, the example wouldn’t even compile… but I hope the overall message
is clear :)

>
> virTypedParameterPtr params;
> int nparams;
>
> virTypedParamsDeserialize(NULL, 0, &params, &nparams);
> assert(params == NULL && assert nparams == 0);
>
> Do we?
>

Polite ping.

[…ping]

--
Beste Grüße / Kind regards
   Marc Hartmayer

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