On Mon, Jul 24, 2023 at 09:36:28AM -0700, Linus Torvalds wrote:
> On Mon, 24 Jul 2023 at 09:19, Christian Brauner <brauner@xxxxxxxxxx> wrote:
> >
> > SCM_RIGHTS which have existed since 2.1 or sm allow you to do the same
> > thing just cooperatively. If you receive a bunch of fds from another
> > task including sockets and so on they refer to the same struct file.
>
> Yes, but it has special synchronization rules, eg big comment in
> commit cbcf01128d0a ("af_unix: fix garbage collect vs MSG_PEEK").
>
> There are magic rules with "total_refs == inflight_refs", and that
> total_refs thing is very much the file count, ie
>
> total_refs = file_count(u->sk.sk_socket->file);
>
> where we had some nasty bugs with files coming back to life.
It's a bit of a shame that this isn't documented anywhere as that's
pretty sensitive stuff and it seems a high bar to assume that everyone
has that historical knowledge. :/ So we should definitely document this
in Documentation/ going forward.
This means pidfd_getfd() needs the same treatment as MSG_PEEK for sockets.
