Re: [PATCH v6 4/5] seccomp: add support for passing fds via USER_NOTIF

[Andy Lutomirski <luto@amacapital.net>]

> On Sep 19, 2018, at 2:55 AM, Tycho Andersen <tycho@tycho.ws> wrote:
> 
>> On Wed, Sep 12, 2018 at 04:52:38PM -0700, Andy Lutomirski wrote:
>>> On Thu, Sep 6, 2018 at 8:28 AM, Tycho Andersen <tycho@tycho.ws> wrote:
>>> The idea here is that the userspace handler should be able to pass an fd
>>> back to the trapped task, for example so it can be returned from socket().
>>> 
>>> I've proposed one API here, but I'm open to other options. In particular,
>>> this only lets you return an fd from a syscall, which may not be enough in
>>> all cases. For example, if an fd is written to an output parameter instead
>>> of returned, the current API can't handle this. Another case is that
>>> netlink takes as input fds sometimes (IFLA_NET_NS_FD, e.g.). If netlink
>>> ever decides to install an fd and output it, we wouldn't be able to handle
>>> this either.
>> 
>> An alternative could be to have an API (an ioctl on the listener,
>> perhaps) that just copies an fd into the tracee.  There would be the
>> obvious set of options: do we replace an existing fd or allocate a new
>> one, and is it CLOEXEC.  Then the tracer could add an fd and then
>> return it just like it's a regular number.
>> 
>> I feel like this would be more flexible and conceptually simpler, but
>> maybe a little slower for the common cases.  What do you think?
> 
> I'm just implementing this now, and there's one question: when do we
> actually do the fd install? Should we do it when the user calls
> SECCOMP_NOTIF_PUT_FD, or when the actual response is sent? It feels
> like we should do it when the response is sent, instead of doing it
> right when SECCOMP_NOTIF_PUT_FD is called, since if there's a
> subsequent signal and the tracer decides to discard the response,
> we'll have to implement some delete mechanism to delete the fd, but it
> would have already been visible to the process, etc. So I'll go
> forward with this unless there are strong objections, but I thought
> I'd point it out just to avoid another round trip.
> 
> 

Can you do that non-racily?  That is, you need to commit to an fd *number* right away, but what if another thread uses the number before you actually install the fd?

Do we really allow non-“kill” signals to interrupt the whole process?  It might be the case that we don’t really need to clean up from signals if there’s a guarantee that the thread dies.