seccomp: Delay filter activation

[Sargun Dhillon]

We've run into a problem where attaching a filter can be quite messy
business because the filter itself intercepts sendmsg, and other
syscalls related to exfiltrating the listener FD. I believe that this
problem set has been brought up before, and although there are
"simpler" methods of exfiltrating the listener, like clone3 or
pidfd_getfd, but these are still less than ideal.

One of the ideas that's been talked about (I want to say back at LSS
NA) is the idea of "delayed activation". I was thinking that it might
be nice to have a mechanism to do delayed attach, either activated on
execve / fork, or an ioctl on the listenerfd to activate the filter
and have a flag like SECCOMP_FILTER_FLAG_NEW_LISTENER_INACTIVE, which
indicates that the listener should be setup, but not enforcing, and
another ioctl to activate it.

The later approach is preferred due to simplicity, but I can see a
situation where you could accidentally get into a state where the
filter is not being enforced. Additionally, this may have unforeseen
implications with CRIU.

I'm curious whether this is a problem others share, and whether any of
the aforementioned approaches seem reasonable.

-Thanks,
Sargun