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

["Michal Prívozník" <mprivozn@redhat.com>]

On 2/14/23 11:08, Stefano Brivio wrote:
> 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'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