Accept an empty `sun_path` when `fd` is not `AT_FDCWD`: the descriptor
then names the peer unix socket directly, instead of being the starting
directory for a pathname lookup. The held file reference keeps the peer
PCB stable, playing the role `unp_vp_mtxpool` plays in the pathname
path.
The descriptor must carry `CAP_CONNECTAT` and refer to an `AF_UNIX`
socket (`EPROTOTYPE` otherwise, `ENOTSOCK` for non-sockets). As with a
pathname, a stream/seqpacket peer must be listening. No filesystem
permission or MAC vnode check applies on this path: possession of the
descriptor is the authorization, as with descriptor passing.
Note this makes it possible to connect a datagram socket to an unbound
peer, which no pathname could previously name.
`connect(2)` and the implicit-connect send path pass `AT_FDCWD` and
still reject an empty path with `EINVAL`.
The `unp_sun_path()` call is hoisted out of `unp_connectat()` because the
early exit conditions for the two system calls (`connect(2)` and
`connectat(2)`) are slightly different.
Additionally, support `/dev/fd/<N>`. In a world with `connectat(2)`,
this is largely overkill, but this also allows me to add support for
direct peer connections with plain `connect(2)`. I think that is a wise
choice because this will allow me to propose this functionality for
Linux too without a new system call (saving that conversation for
later). Ultimately, I want to see multiple operating systems support
this to foster broader userland adoption, which should benefit everyone
including FreeBSD --- it's nicer if more 3rd party in addition to 1st
party software uses the new kernel functionality. Therefore, I hope this
additional feature is also acceptable.
Signed-off-by: John Ericson <[email protected]>
Assisted-by: Claude Code (Claude Opus 4.8 and Fable 5)
Reviewed by: markj
MFC after: 2 months
Differential Revision: https://reviews.freebsd.org/D58405
John Ericson
6563dcb6b1