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

[Stefano Brivio <sbrivio@redhat.com>]

On Tue, 14 Feb 2023 09:01:39 +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(-)  
> 
> 
> OOOPS, somehow I've accidentally merged this. Let me post follow up patches.
> 
> > 
> > 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",  
> 
> BTW: we definitely need something better than this. IF, something goes
> wrong after we've executed passt but before we execute QEMU, then passt
> just hangs there. This is because passt clone()-s itself (i.e. creates a
> child process), but QEMU that would connect to the socket never comes
> around. Thus, the child process never sees the EOF on the socket and
> just hangs in there thinking there will be somebody connecting, soon.

Okay, I see the point now -- I thought libvirtd would start passt only
once it knows for sure that the guest will connect to it.

> I thought this could be solved by just killing the whole process group,
> but the child process calls setsid(), which creates its own process
> group. I've managed to work around this by passing --foreground, but I'm
> unclear about the consequences. Though, it looks like it's still
> dropping caps, creating its own namespaces, etc. So this may actually be
> the way to go.

I wouldn't recommend that: --foreground is really intended for
interactive usage and we won't be able, for example, to spawn a child
in a new PID namespace, which is a nice security feature, I think.

I already suggested this to Laine offline: can libvirt just connect() to
the socket and close() it, in case QEMU doesn't start? Then passt will
terminate.

It should be a few (~5) lines of code, instead of all the complexity
potentially involved in tracking PIDs and avoiding related races, and
design-wise it looks clean to me (libvirtd plays for a moment the QEMU
role, because QEMU is not around).

-- 
Stefano