fork(), but for somebody else's process

Boxd introduces a Linux system call, `import_cow_range`, to rapidly clone a running virtual machine’s guest memory into an independently running VMM process using fork-style copy-on-write semantics. Rather than copying gigabytes of RAM, writing snapshots to disk, forking and then losing memory through `exec()`, or handling page faults in userspace via `userfaultfd`, the new mechanism shares the parent’s memory pages while copying only page-table entries and write-protecting both mappings. When either machine modifies a shared page, the kernel handles the fault and creates a private page copy, while untouched pages remain shared. The design uses pidfds to avoid process-ID reuse races, requires guest RAM at the same fixed virtual address in both processes, and relies on existing ptrace permission checks. Cloning requires the source VM to remain alive and briefly paused while its mapping is imported, but can create a running machine in under 200 milliseconds without disk I/O or initial RAM copying, enabling live machines to serve as branchable templates for parallel tasks and experiments.

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