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Created page with "== 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)..."
== 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/$pid/maps''').
The process dumper (lets call it a dumper further) does the following steps during checkpoint stage
# A '''$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) '''$pid''' via <code>clone()</code> call.
# Files and pipes are restored (by restored it's meant - they are opened and positioned).
# A new memory map is created, filled with data the program had at checkpoint time.
# Finally the program is kicked to start with rt_sigreturn system call.
=== Checkpoint ===
The checkpoint procedure relies heavily on '''/proc''' file system (it's a general place where crtools takes all the information it needs).
Which includes
* Files descriptors information (via '''/proc/$pid/fd''' and '''/proc/$pid/fdinfo''').
* Pipes parameters.
* Memory maps (via '''/proc/$pid/maps''').
The process dumper (lets call it a 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.
# Files and pipes are restored (by restored it's meant - they are opened and positioned).
# A new memory map is created, filled with data the program had at checkpoint time.
# Finally the program is kicked to start with rt_sigreturn system call.