On 2/14/23 11:08, Stefano Brivio wrote: > On Tue, 14 Feb 2023 09:01:39 +0100 > Michal Prívozník <mprivozn@xxxxxxxxxx> 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@xxxxxxxxxx> >>> --- >>> 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'm failing to see how that would be possible. Starting a guest involves many actions, each one of can fail. From defensive coding POV it's better we have the option to kill passt. > >> 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. Well, it's clone() that brings all the problems (well, in combination with setsid()). > > 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. That relies on the fact that passt isn't stuck and responds to the EOF. We certainly can do that if passt needs graceful shutdown, but mustn't rely on that. > > 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). > Well, we can place all these helper processes into a CGroup and let it trace PIDs. That should be race free. Michal
