[libvirt] [PATCH] qemu_capabilities: Pass pointer to va_list in virQEMUCapsSetVAList

[Michal Privoznik]

On 3/27/19 2:46 PM, Eric Blake wrote:
> On 3/27/19 7:02 AM, Andrea Bolognani wrote:
>> On Wed, 2019-03-27 at 10:50 +0100, Michal Privoznik wrote:
>>> There is one specific caller (testInfoSetArgs() in
>>> qemuxml2argvtest.c) which expect the  va_list argument to change
>>
>> s/  / /
>>
>>> after returning from the virQEMUCapsSetVAList() function.
>>> However, since we are passing plain va_list this is not
>>> guaranteed. The man page of stdarg(3) says:
>>>
>>>    If ap is passed to a function that uses va_arg(ap,type), then
>>>    the value of ap is undefined after the return of that function.
>>>
>>> (ap is a variable of type va_list)
> 
>> however I'd like to hear Eric's opinion before this gets merged.
> 
> Passing a va_list *arg as a parameter is doable, but it comes with odd
> effects. That is because the C standard permits both of the following
> implementation styles:
> 
> typedef struct __something *va_list;
> typedef struct __something va_list[];
> 
> but you get different semantics on what happens if you try to take the
> address of a va_list parameter, (C parameter lists undergo pointer
> decay, and taking the address of a parameter results in either the
> address of a pointer or the address of the first member of an array -
> which are different levels of dereferencing and thus different types).
> The type 'pointer to va_list' is sane, and the computation of
> '&local_va_list' is sane; it is only '&parameter_va_list' that gets you
> in trouble.  Here's where I demonstrated it further for the qemu list:
> 
> https://lists.gnu.org/archive/html/qemu-devel/2017-10/msg00171.html
> 
> The only portable way to take va_list from one function to pass to a
> va_list * of another function is to va_copy() from the parameter into a
> local va_list, then take the address of the local.  But if you are the
> original owner of a local va_list (because your function took ...
> instead of va_list), you don't have to worry about the hoop-jumping
> involved to stay portable.
> 
>>
>> @@ -1680,7 +1680,7 @@ virQEMUCapsSetList(virQEMUCapsPtr qemuCaps, ...)
>>       va_list list;
>>   
>>       va_start(list, qemuCaps);
>> -    virQEMUCapsSetVAList(qemuCaps, list);
>> +    virQEMUCapsSetVAList(qemuCaps, &list);
>>       va_end(list);
> 
> This usage is safe, because it is the address of a local variable. I
> don't see any instance where you are taking the address of a parameter,
> so while the patch is unusual, it doesn't look wrong.
> 

Okay, thanks for detailed explanation. I'll probably go with what Dan 
sugested and make virQEMUCapsSetVAList inline then. It looks safer and 
more portable.

Michal