== What CRtools is == '''CRtools''' is an utility to checkpointSee main article here [http:/restore process tree. Unlike checkpoint/restore implemented completely in kernel space,it tries to achieve the same target mostly in user space. === Agenda === # Basic design (checkpoint == proc + SEIZE, restore == syscalls + execve)# What's required from kernel == Basic design == === Checkpoint === The checkpoint procedure relies heavily on '''/proc''' file system (it's a general place where crtools takes all the information it needs).Which includes * Files descriptors information (via '''/proc/$pid/fd''' and '''/proc/$pid/fdinfo''').* Pipes parameters.* Memory maps (via '''/proc/$pid/maps'''). The process dumper (lets call it simply the dumper further) does the following steps during checkpoint stage # A '''$pid''' of a process group leader is obtained from the command line.# By using this '''$pid''' the dumper walks though '''/proc/$pid/status''' and gathers children '''$pids''' recursively. At the end we will have a process tree.# Then it takes every '''$pid''' from a process tree, sends ''SIGSTOP'' to every process found, and performs the following steps on each '''$pid'''.#* Collects VMA areas by parsing '''/proc/$pid/maps'''criu.#* Seizes a task via relatively new ptrace interface. Seizing a task means to put it into a special state when the task have no idea if it's being operated by ptrace.#* Core parameters of a task (such as registers and friends) are being dumped via ptrace interface and parsing '''org/proc/$pid/stat''' entry.#* The dumper injects a parasite code into a task via ptrace interface. This allows us to dump pages of a task right from within the task's address space. An injection procedure is pretty simple - the dumper scans executable VMA areas of a task (which were collected previously) and tests if there a place for <code>syscall</code> call, then (by ptrace as well) it substitutes an original code with <code>syscall</code> instructions and creates a new VMA area inside process address space. Finally parasite code get copied into the new VMA and the former code which was modified during parasite bootstrap procedure get restored.#* Then (by using a parasite code) the dumper flushes contents of a task's pages to the file. And pulls out parasite code block completely, since we don't need it anymore.#* Once parasite removed a task get unseized via ptrace call but it remains stopped still.#* The dumper writes out files and pipes parameter and data.# The procedure continues for every '''$pid'''.CR_tools]
