Re: [PATCH v4 4/4] seccomp: add support for passing fds via USER_NOTIF

[Tycho Andersen <tycho@tycho.ws>]

On Fri, Jun 22, 2018 at 01:34:18AM +0200, Jann Horn wrote:
> On Fri, Jun 22, 2018 at 12:05 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().
> [...]
> > +Userspace can also return file descriptors. For example, one may decide to
> > +intercept ``socket()`` syscalls, and return some file descriptor from those
> > +based on some policy. To return a file descriptor, the ``return_fd`` member
> > +should be non-zero, the ``fd`` argument should be the fd in the listener's
> > +table to send to the tracee (similar to how ``SCM_RIGHTS`` works), and
> > +``fd_flags`` should be the flags that the fd in the tracee's table is opened
> > +with (e.g. ``O_EXCL`` or similar).
> 
> fd_flags only contains file descriptor flags (meaning only O_CLOEXEC).
> O_EXCL is a file creation flag, so setting it here wouldn't make sense.
> Setting file status flags like O_APPEND does make sense, but those are
> stored in the `struct file` and don't need to be passed separately;
> the caller can e.g. set them via fcntl(fd, F_SETFD, flags) or on
> open().
> (The fcntl.2 manpage explains these.)

Ugh, yes, O_CLOEXEC is what I meant. Thanks, I'll clarify.

Tycho