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

On Thu, Jun 21, 2018 at 03:47 PM +0200, Marc Hartmayer <mhartmay(a)linux.ibm.com&gt; wrote: > On Wed, Jun 13, 2018 at 03:11 PM +0200, John Ferlan <jferlan(a)redhat.com&gt; wrote: >> On 06/07/2018 08:17 AM, Marc Hartmayer wrote: >>> On Wed, May 09, 2018 at 09:51 PM +0200, John Ferlan <jferlan(a)redhat.com&gt; wrote: >>>> On 05/07/2018 11:24 AM, Marc Hartmayer wrote: >>>>> On Mon, Apr 30, 2018 at 03:20 PM +0200, John Ferlan <jferlan(a)redhat.com&gt; 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]