criu

Foundational Technologies

CRIU relies on a wide array of advanced Linux kernel features and userspace libraries to perform transparent checkpoint and restore.

Kernel Technologies

Core C/R Capabilities

kcmp(): A system call used to identify shared resources (files, memory mappings, namespaces) between processes by performing internal kernel pointer comparisons.
clone3(): A modern process creation interface that allows CRIU to atomically request specific PIDs and TIDs, even across nested PID namespaces.
prctl() Extensions:
- PR_SET_MM: Allows the restorer to reconstruct a process’s original memory layout (code, data, heap, etc.).
- PR_GET_TID_ADDRESS: Captures the address used for set_tid_address.
- PR_SET_THP_DISABLE: Preserves the status of Transparent Huge Pages.
ptrace() Extensions:
- PTRACE_SEIZE & PTRACE_INTERRUPT: Enables non-disruptive task stopping.
- PTRACE_GETSIGMASK & PTRACE_SETSIGMASK: Captures and restores thread signal masks.
- PTRACE_PEEKSIGINFO: Reads pending signal queues without delivering them.
- PTRACE_GET_RSEQ_CONF: Retrieves Restartable Sequences (rseq) registration details.

Resource Introspection

/proc Filesystem:
- /proc/$pid/map_files: Provides stable handles to files mapped into a process’s memory.
- /proc/$pid/fdinfo: Exposes internal state for file descriptors, including positions, flags, and socket handles.
- ioctl(PAGEMAP_SCAN): Efficiently identifies dirty and present pages in large address spaces.
sock_diag: A netlink-based interface used to retrieve detailed protocol-level state for sockets (TCP, UDP, Unix, etc.).

Advanced Subsystems

TCP Repair Mode: A specialized socket state that allows CRIU to capture and restore the full internal state of TCP connections without sending network packets.
Userfaultfd: Enables Lazy Migration by allowing CRIU to handle page faults in userspace and load memory pages on-demand.
Mount V2 APIs: Uses fsopen(), fsmount(), open_tree(), and move_mount() to robustly reconstruct complex filesystem hierarchies and propagation groups.
Netfilter (nftables/iptables): Used to “lock” network connections during migration to prevent state changes.

Userspace Technologies

Compel

Compel is a dedicated sub-project that provides the infrastructure for Parasite Injection. It allows CRIU to execute self-contained code (PIE) within the context of a target process to capture internal state.

Google Protocol Buffers (protobuf)

CRIU uses Protocol Buffers as the standard serialization format for all image files. This ensures a structured, extensible, and cross-version compatible way to store process state.

ZDTM (Zero-Downtime Migration)

ZDTM is CRIU’s comprehensive test suite. It includes hundreds of tests that verify the functional correctness of C/R across various architectures and kernel versions.