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

On Tue, 14 Feb 2023 09:01:39 +0100 Michal Prívozník <mprivozn(a)redhat.com&gt; 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(a)redhat.com&gt; >> --- >> 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).