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

[Jann Horn <jannh@google.com>]

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.)