pasta: Wait for tap to be set up before spawning command

Adapted from a patch by Paul Holzinger: when pasta spawns a command,
operating without a pre-existing user and network namespace, it needs
to wait for the tap device to be configured and its handler ready,
before the command is actually executed.

Otherwise, something like:
  pasta --config-net nslookup passt.top

usually fails as the nslookup command is issued before the network
interface is ready.

We can't adopt a simpler approach based on SIGSTOP and SIGCONT here:
the child runs in a separate PID namespace, so it can't send SIGSTOP
to itself as the kernel sees the child as init process and blocks
the delivery of the signal.

We could send SIGSTOP from the parent, but this wouldn't avoid the
possible condition where the child isn't ready to wait for it when
the parent sends it, also raised by Paul -- and SIGSTOP can't be
blocked, so it can never be pending.

Use SIGUSR1 instead: mask it before clone(), so that the child starts
with it blocked, and can safely wait for it. Once the parent is
ready, it sends SIGUSR1 to the child. If SIGUSR1 is sent before the
child is waiting for it, the kernel will queue it for us, because
it's blocked.

Reported-by: Paul Holzinger <pholzing@redhat.com>
Fixes: 1392bc5ca002 ("Allow pasta to take a command to execute")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/passt.c b/passt.c
index 8b2c50d..d957e14 100644
--- a/passt.c
+++ b/passt.c
@@ -301,6 +301,9 @@ int main(int argc, char **argv)
 	else
 		write_pidfile(pidfile_fd, getpid());
 
+	if (pasta_child_pid)
+		kill(pasta_child_pid, SIGUSR1);
+
 	isolate_postfork(&c);
 
 	timer_init(&c, &now);