unp_pcb_owned_lock2() has some sharp edges and forces callers to deal
with a bunch of cases. Simplify it:
- Rename to unp_pcb_lock_peer().
- Return the connected peer instead of forcing callers to load it
beforehand.
- Handle self-connected sockets.
- In unp_connectat(), just lock the accept socket directly. It should
not be possible for the nascent socket to participate in any other
lock orders.
- Get rid of connect_internal(). It does not provide any useful
checking anymore.
- Block in unp_connectat() when a different thread is concurrently
attempting to lock both sides of a connection. This provides simpler
semantics for callers of unp_pcb_lock_peer().
- Make unp_connectat() return EISCONN if the socket is already
connected. This fixes a race[1] when multiple threads attempt to
connect() to different addresses using the same datagram socket.
Upper layers will disconnect a connected datagram socket before
calling the protocol connect's method, but there is no synchronization
between this and protocol-layer code.
Reported by: syzkaller [1]
Tested by: pho
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26299
ccdadf1a9b