On Mon, Jul 24, 2023 at 09:36:28AM -0700, Linus Torvalds wrote:
On Mon, 24 Jul 2023 at 09:19, Christian Brauner brauner@kernel.org 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.