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James Tucker
de16943cbf
Add the one-shot "be-child ssh-fwd" children that perform forwarded socket work, and the monitor and client plumbing for the socket ops. The conn and session flows do not use the ops yet. The next commit routes them. - fwdchild.go: a child re-execed by the monitor, born unprivileged at clone. The monitor applies the connection's bound identity as SysProcAttr.Credential, so the kernel's DAC sees the connection user's permissions. It dials a unix socket, or creates one with umask-at-bind mode and optional unlink-if-owned, or binds a TCP listener, or creates the agent-forwarding socket in a user-owned directory. The result returns to the monitor over an inherited socketpair as one status line with at most one descriptor attached. - monitor.go: handleDial and handleListen run the ops on one-shot children with a spawn-to-result deadline, monFwdOpDeadline. On expiry the child's group is SIGKILLed. killChild reaps such a child with a bounded non-blocking wait. A child left in uninterruptible sleep is abandoned in its own group rather than blocking the loop goroutine. The dial and listen placeholder replies are gone, and their not-implemented test with them. - monitor_client.go: Dial, Listen, and ListenAgent over the existing request-ID plumbing. - incubator.go, monitor_linux.go, monitor_other.go: registration and per-platform spawn attributes for the fwd children. Tests drive real children across uid boundaries. They cover dial and bind round trips, unix and TCP listens, agent-listen directory hygiene, unlink-if-owned, peer-credential assertions, and the no-identity-keys-on-the-wire shape of every request. Forwarding children spawn with an empty environment through spawnRPCChild: they take everything from argv and the result socket, and the daemon's environment must not reach a child running as the connection's user. Updates tailscale/corp#50380 Signed-off-by: James Tucker <james@tailscale.com>