Fix supervisor hang on child exit

[jserv]

Commit 6080aad changed the supervisor poll timeout from 100ms to -1 and removed the per-iteration check_child(), relying solely on POLLHUP from the seccomp listener FD to detect child exit. This hangs when the child exits without a pending notification.

This replaces the single-FD poll with two-FD poll: the seccomp listener plus a signalfd for SIGCHLD. The kernel delivers SIGCHLD the instant the child exits, waking poll() without any per-iteration waitpid overhead.

Signal mask lifecycle:

• Block SIGCHLD before fork() (race-free: no window to lose the signal)
• Save/restore the caller's original mask on all parent return paths
• Restore in the child before exec() (guest sees clean signal state)

The signalfd is owned by supervise_loop with a single goto-cleanup exit path; the mask is owned by kbox_run_supervisor.

Also pin AUTO syscall mode to seccomp on all architectures: the aarch64 rewrite fast path hangs for dynamically-linked binaries (separate bug in the rewrite runtime's interpreter dispatch).
--syscall-mode=rewrite remains available for explicit use.

Close #24

Change-Id: I09bccef6e2ca7a08a34c86b075f3ca9cdfa29b9c

---

Summary by cubic

Fixes a supervisor hang when the child exits without a pending seccomp notification by waking poll() with SIGCHLD. Also pins AUTO syscall mode to seccomp to avoid rewrite-path hangs; --syscall-mode=rewrite remains available. Closes #24.

• Bug Fixes

  • Poll both the seccomp listener and a signalfd for SIGCHLD to wake immediately on child exit, avoiding per-iteration waitpid().
  • Block SIGCHLD before fork(), then restore the original signal mask in both parent and child; own and clean up the signalfd in the supervisor loop.

• Migration

  • AUTO now maps to seccomp on all architectures.
  • Use --syscall-mode=rewrite to force the rewrite path when needed.

^{Written for commit 9cc4b97. Summary will update on new commits.}

[cubic-dev-ai]

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