[libvirt] PATCH: 4/5: Support daemonizing child & env variables

[Jim Meyering]

"Daniel P. Berrange" <berrange at redhat.com> wrote:
> Some of the existing usage of fork/exec in libvirt is done such that the
> child process is daemonized. In particular the libvirt_proxy and the
> auto-spawned  libvirtd for qemu:///session. Since we want to switch these
> to use virExec, we need to suport a daemon mode.
>
> This patch removes the two alternate virExec and virExecNonBlock functions
> and renames the internal __virExec to virExec. It then gains a 'flags'
> parameter which can be used to specify non-blocking mode, or daemon mode.
>
> We also add the ability to pass in a list of environment variables which
> get passed on to execve(). We also now explicitly close all file handles.
> Although libvirt code is careful to set O_CLOSEXEC on all its file handles,
> in multi-threaded apps there is a race condition between opening a FD and
> setting O_CLOSEXEC. Furthermore, we can't guarentee that all applications
> using libvirt are careful to set O_CLOSEXEC. Leaking FDs to a child is a
> potential security risk and often causes SELinux AVCs to be raised. Thus
> explicitely  closing all FDs is a important safety net.

How about closing those FDs in the child instead, right after the fork?
Then, if a virExec caller has an open socket or file descriptor,
it won't be closed behind its back.

> diff -r 28ddf9f5791c src/util.c
...
> +    for (i = 3; i < openmax; i++)
> +        if (i != infd &&
> +            i != null &&
> +            i != childout &&
> +            i != childerr)
> +            close(i);
> +
> +    if (flags & VIR_EXEC_DAEMON) {
> +        if (setsid() < 0) {
> +            ReportError(conn, VIR_ERR_INTERNAL_ERROR,
> +                        _("cannot become session leader: %s"),
> +                        strerror(errno));
> +            _exit(1);
> +        }
> +
> +        if (chdir("/") < 0) {
> +            ReportError(conn, VIR_ERR_INTERNAL_ERROR,
> +                        _("cannot change to root directory: %s"),
> +                        strerror(errno));
> +            _exit(1);
> +        }
> +
> +        pid = fork();
...