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

From: Aleksa Sarai
Date: Thu Sep 13 2018 - 05:41:44 EST

On 2018-09-12, Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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?

When we first discussed this I sent a (probably somewhat broken) patch
for "dup4" which would let you inject a file descriptor into a different
process -- I still think that having a method for injecting a file
descriptor without needing ptrace (and SCM_RIGHTS) shenanigans would be
generally useful. (With "dup4" you have a more obvious API for flags and
whether you allocate a new fd or use a specific one.)

Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH

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