Port of upstream grok-build 9684fa3 (1.0.11 window) `child_net.rs`.
The per-child seccomp filter matched seven socket syscalls by number and
allowed everything else, so a restricted child could still reach the
network through `sendmmsg` or an io_uring submission queue, and a
syscall issued through another arch table (x32 on x86_64) never matched
the deny list at all.
- Gate on the audit arch before matching `nr` and reject x32 numbers on
x86_64 (foreign-table syscalls now get EPERM instead of passing).
- Add `sendmmsg`, `io_uring_setup`, `io_uring_enter`, `io_uring_register`.
- Build the program once (`prebuilt_child_network_filter`) and hand the
slice to the `pre_exec` installer; the old code allocated the Vec after
`fork`, which is not async-signal-safe. All three callers updated.
- Tests evaluate the BPF program over synthetic `seccomp_data`.
Claude-Session: https://claude.ai/code/session_01RUqV8HVGXcAETyYbZ2tPQ4
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