Re: [libvirt PATCH] qemu: allow passt to self-daemonize

[Stefano Brivio <sbrivio@redhat.com>]

On Thu, 9 Feb 2023 09:52:00 +0100
Michal Prívozník <mprivozn@redhat.com> wrote:

> On 2/9/23 00:13, Laine Stump wrote:
> > I initially had the passt process being started in an identical
> > fashion to the slirp-helper - libvirt was daemonizing the new process
> > and recording its pid in a pidfile. The problem with this is that,
> > since it is daemonized immediately, any startup error in passt happens
> > after the daemonization, and thus isn't seen by libvirt - libvirt
> > believes that the process has started successfully and continues on
> > its merry way. The result was that sometimes a guest would be started,
> > but there would be no passt process for qemu to use for network
> > traffic.
> > 
> > Instead, we should be starting passt in the same manner we start
> > dnsmasq - we just exec it as normal (along with a request that passt
> > create the pidfile, which is just another option on the passt
> > commandline) and wait for the child process to exit; passt then has a
> > chance to parse its commandline and complete all the setup prior to
> > daemonizing itself; if it encounters an error and exits with a non-0
> > code, libvirt will see the code and know about the failure. We can
> > then grab the output from stderr, log that so the "user" has some idea
> > of what went wrong, and then fail the guest startup.
> > 
> > Signed-off-by: Laine Stump <laine@redhat.com>
> > ---
> >  src/qemu/qemu_passt.c | 9 ++++-----
> >  1 file changed, 4 insertions(+), 5 deletions(-)
> > 
> > diff --git a/src/qemu/qemu_passt.c b/src/qemu/qemu_passt.c
> > index 0f09bf3db8..f640a69c00 100644
> > --- a/src/qemu/qemu_passt.c
> > +++ b/src/qemu/qemu_passt.c
> > @@ -141,24 +141,23 @@ qemuPasstStart(virDomainObj *vm,
> >      g_autofree char *passtSocketName = qemuPasstCreateSocketPath(vm, net);
> >      g_autoptr(virCommand) cmd = NULL;
> >      g_autofree char *pidfile = qemuPasstCreatePidFilename(vm, net);
> > +    g_autofree char *errbuf = NULL;
> >      char macaddr[VIR_MAC_STRING_BUFLEN];
> >      size_t i;
> >      pid_t pid = (pid_t) -1;
> >      int exitstatus = 0;
> >      int cmdret = 0;
> > -    VIR_AUTOCLOSE errfd = -1;
> >  
> >      cmd = virCommandNew(PASST);
> >  
> >      virCommandClearCaps(cmd);
> > -    virCommandSetPidFile(cmd, pidfile);
> > -    virCommandSetErrorFD(cmd, &errfd);
> > -    virCommandDaemonize(cmd);
> > +    virCommandSetErrorBuffer(cmd, &errbuf);
> >  
> >      virCommandAddArgList(cmd,
> >                           "--one-off",
> >                           "--socket", passtSocketName,
> >                           "--mac-addr", virMacAddrFormat(&net->mac, macaddr),
> > +                         "--pid", pidfile,  
> 
> The only problem with this approach is that our virPidFile*() functions
> rely on locking the very first byte. And when reading the pidfile, we
> try to lock the file and if we succeeded it means the file wasn't locked
> which means the process holding the lock died and thus the pid in the
> pidfile is stale.
> 
> Now, I don't see passt locking the pidfile at all. So effectively, after
> this patch qemuPasstStop() would do nothing (well, okay, it'll remove
> the pidfile), qemuPasstSetupCgroup() does nothing, etc.

And it doesn't need to do anything, actually! passt is started with the
--one-off option:

       -1, --one-off
              Quit  after  handling  a single client connection, that is, once
              the client closes the socket, or once we get a socket error.

well, removing the PID file is nice (passt can't do it as it won't see
the filesystem after starting up), but that's about it.

-- 
Stefano