Editing CR tools

== What CRtools is ==

'''CRtools''' is an utility to checkpoint/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/$pid/maps'''.
#* Seizes a task via relatively new ptrace interface. Seizing a task means to put it into a special state when the task have no idea if it's being operated by ptrace.
#* Core parameters of a task (such as registers and friends) are being dumped via ptrace interface and parsing '''/proc/$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 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'''.