Re: [libvirt] [PATCH] qemu_capabilities: Pass pointer to va_list in virQEMUCapsSetVAList
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 '¶meter_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.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org