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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'''.#*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 '''/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, the former modified code get restored.
