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