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

[Daniel P. Berrange]

On Tue, Aug 19, 2008 at 10:51:29PM +0200, Jim Meyering wrote:
> "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.

This is being done after a fork(). The diff context in this patch is
a little misleading. The fork() shown here is the 2nd optional fork() done
in daemon mode. The first fork() is higher up before we close the FDs.

> > 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();
> ...

Regards,
Daniel
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