Re: [PATCH 4/4] seccomp: add the synchronous mode for seccomp_unotify

Andrei Vagin
2022-08-30T21:23:50Z

On Tue, Aug 30, 2022 at 3:43 AM Christian Brauner <brauner@kernel.org> wrote:
>
> On Mon, Aug 29, 2022 at 06:43:56PM -0700, Andrei Vagin wrote:
> > seccomp_unotify allows more privileged processes does actions on behalf
> > of less privileged processes.
> >
> > In many cases, the workflow is fully synchronous. It means a target
> > process triggers a system call and passes controls to a supervisor
> > process that handles the system call and returns controls to the target
> > process. In this context, "synchronous" means that only one process is
> > running and another one is waiting.
> >
> > There is the WF_CURRENT_CPU flag that is used to advise the scheduler to
> > move the wakee to the current CPU. For such synchronous workflows, it
> > makes context switches a few times faster.
> >
> > Right now, each interaction takes 12µs. With this patch, it takes about
> > 3µs.
>
> Seems like a nice idea though I leave it to the sched people to judge
> whether this is sane or not. So the supervisor which gets woken will be
> moved to the current cpu in this synchronous scenario.
>
> I have no strong opinions on this patch. There are two things I wonder
> about. First, how meaningful is that speed up given that the supervisor
> will most often do a lot of heavy-handed things anyway.

I would not use the "most often" phrase in this case;). It is true for LXC-like
use cases when we need to handle rare syscalls. In this case, the performance
of this interface doesn't play a big role. But my use case is very different. I
have a prototype of the gVisor platform, where seccomp is used to trap
guest system calls. In this case, the difference between 12µs and 3µs is
tremendous.

The idea of WF_CURRENT_CPU is not mine. I spied it from the umcg series.
I took the second patch from that series without any changes.

>
> Second, this flag is a very specific thing and I wonder how much
> userspace will really use this and what's more use this correctly.
>
> Just to note that LXD - one of the biggest user of this feature - isn't
> synchronous iiuc for example. Each container gets a separate seccomp
> supervisor thread (well, go routine but whatever) which exposes a socket
> that the container manager connects to and sends the seccomp
> notifications it received from its payload according to an api we
> established. And each notification is handled in a separate thread
> (again, go routine but whatever).

It could be synchronous if seccomp events had been handled in [lxc monitor]. But
right now, [lxc monitor] is just a proxy. In this case, you are right, lxc will
not get any benefits by setting this flag. But we can look at this from another
side. If we add these changes, we will have another big user of the interface. I
think the number of gVisor containers that are started each day is comparable
with the number of LXC/LXD containers.

>
> >
> > This change introduce the SECCOMP_USER_NOTIF_FD_SYNC_WAKE_UP flag that
> > it used to enable the sync mode.
> >
> > Signed-off-by: Andrei Vagin <avagin@gmail.com>
> > ---
> > include/uapi/linux/seccomp.h | 4 ++++
> > kernel/seccomp.c | 35 +++++++++++++++++++++++++++++++++--
> > 2 files changed, 37 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
> > index 0fdc6ef02b94..dbfc9b37fcae 100644
> > --- a/include/uapi/linux/seccomp.h
> > +++ b/include/uapi/linux/seccomp.h
> > @@ -115,6 +115,8 @@ struct seccomp_notif_resp {
> > __u32 flags;
> > };
> >

<snip>

> >
> > #ifdef SECCOMP_ARCH_NATIVE
> > @@ -1117,7 +1120,10 @@ static int seccomp_do_user_notification(int this_syscall,
> > INIT_LIST_HEAD(&n.addfd);
> >
> > atomic_add(1, &match->notif->requests);
> > - wake_up_poll(&match->wqh, EPOLLIN | EPOLLRDNORM);
> > + if (match->notif->flags & SECCOMP_USER_NOTIF_FD_SYNC_WAKE_UP)
> > + wake_up_poll_on_current_cpu(&match->wqh, EPOLLIN | EPOLLRDNORM);
> > + else
> > + wake_up_poll(&match->wqh, EPOLLIN | EPOLLRDNORM);
>
> (We're accumulating a lot of conditional wake primitives in the notifier.)
>

I am not sure that I understand what you mean here.

Thanks,
Andrei.