[libvirt] using clang

[Jim Meyering]

Some people have expressed an interest in setting up
a libvirt autobuilder using clang.  If you do that, be aware that
you'll see an ugly looking NULL-deref problem when using the clang
that comes with F12 or F13, but if you're using a new enough version
(rawhide), it's gone.  The offending code is in qemu_monitor.c around
line 377 and involves the CMSG_* macros:

    365	 memset(&msg, 0, sizeof(msg));
    366
    367	iov[0].iov_base = (void *)data;
    368	iov[0].iov_len = len;
    369
    370	msg.msg_iov = iov;
    371	msg.msg_iovlen = 1;
    372
    373	msg.msg_control = control;
    374	msg.msg_controllen = sizeof(control);
    375
    376	cmsg = CMSG_FIRSTHDR(&msg)((size_t) (&msg)->msg_controllen >= sizeof (struct cmsghdr
    ) ? (struct cmsghdr *) (&msg)->msg_control : (struct cmsghdr *) 0);

*** 2 Null pointer value stored to 'cmsg'

    377	cmsg->cmsg_len = CMSG_LEN(sizeof(int))((((sizeof (struct cmsghdr)) + sizeof (size_t) - 1) & (size_t) ~(sizeof (size_t) - 1)) + (sizeof(int)));

*** 3 Dereference of null pointer

Presuming this code is actually exercised, it's obviously
not dereferencing NULL, so it would be a false positive.
I looked at a few usage examples and it seems like glibc's
own tests do allocate more storage.  If this code is *not*
currently being used, we should model it after the glibc test code.

-----------------------------------------
FYI, anyone can (and all developers should) run clang.
It's simple, but does require a configure-from-scratch,
so it's good to keep a separate
Build like this:

    scan-build -o clang ./autogen.sh --enable-compile-warnings=maximum &&
    scan-build -o clang make

The "-o clang" tells it to put results in a new directory named "clang".