Re: [PATCH] pasta: wait for netns setup before calling exec

[Stefano Brivio <sbrivio@redhat.com>]

On Fri, 3 Feb 2023 15:44:40 +0100
Paul Holzinger <pholzing@redhat.com> wrote:

> On 02/02/2023 11:25, Stefano Brivio wrote:
> > On Wed,  1 Feb 2023 19:01:16 +0100
> > Paul Holzinger <pholzing@redhat.com> wrote:
> >  
> >> When a user spawns a command with pasta they expect the network to be
> >> ready. Currently this does not work because pasta will fork/exec
> >> before it will setup the network config.
> >>
> >> This patch fixes it by using a pipe to sync parent and child. The child
> >> will now block reading from this pipe before the exec call. The parent
> >> will then unblock the child only after the netns was configured.  
> > Thanks for the patch! I'm reviewing this in a bit.
> >
> > A few considerations meanwhile:
> >
> > - there's actually a bigger issue (you're fixing here) than the
> >    namespace configuration (via netlink) itself: the tap device isn't
> >    ready (tap_sock_init() hasn't been called yet) when we spawn the
> >    command in the new namespace. Oops.
> >
> >    If you're wondering: we can't just reorder things, because to complete
> >    the configuration phase (conf()) we need the namespace to be set up,
> >    and we can't initialise the tap device before it's set up
> >
> > - pipes are more commonly used to transfer data around (hence the whole
> >    code you need to open a communication channel, check it, close it).
> >    Did you try with a signal? Or is there a reason why it wouldn't work?
> >
> >    You could simply SIGSTOP the child, from the child itself:
> >
> > 	kill(getpid(), SIGSTOP);
> >
> >    and send a SIGCONT to it (we already store the PID of the child in
> >    pasta_child_pid) once we're ready.
> >
> >    SIGCONT is special in that it doesn't need CAP_KILL or the processes
> >    to run under the same UID -- just in the same session, so it wouldn't
> >    risk interfering with the isolation_*() calls.
> >
> >    I haven't tested this but I think it should lead to simpler code.
>
> Thinking about this more STOP/CONT will not work reliably, it could stop
> the child forever when the parent sends SIGCONT before the child
> SIGSTOPs itself. While this is unlikely we have no control over how both
> processes are scheduled.
> 
> With this pipe version there is no problem when the parent closes the fd
> before the child calls read, read will simply return EOF and the child can
> continue, thus it will work correctly in all cases.

Ah, right, nice catch. Still, you could probably use pause() or
sigsuspend() instead of the SIGSTOP. Let me try a quick stand-alone
experiment and I'll get back to you (probably early next week), unless
you manage to get it working before.

If that doesn't work, fine, let's go with the pipe.

Unfortunately CLONE_STOPPED (flag for clone) has gone away a long time
ago.

-- 
Stefano