Difference between revisions of "CR tools"

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== What CRtools is ==
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See main article here [http://criu.org/CR_tools]
 
 
'''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'''.
 
 
 
=== Restore ===
 
 
 
The restore procedure (aka restorer) proceed in the following steps:
 
 
 
# A process tree has been read from a file.
 
# Every process started with saved (i.e. original) '''$pid''' via <code>clone()</code> call with new <code>CLONE_CHILD_USEPID</code> flag.
 
# Files and pipes are restored (by restored it's meant - they are opened and positioned).
 
# A new file generated. The file has an Elf format but with modified executable and program header types (telling the kernel that this particular file is not a regular Elf'oid but rather the kernel needs to handle it in a slightly different way).
 
# Finally execve with new Elf file as an argument is executed, which initiate the kernel's stage of restore procedure.
 

Latest revision as of 20:31, 17 December 2011

See main article here [1]