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The process dumper (lets call it a dumper further) does the following steps during checkpoint stage

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.

# '''$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.

# By using this '''$pid''' the dumper walks though '''/proc/$pid/task/$tid/children''' and gathers children '''$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'''.

# Then we take every '''$pid''' from a process tree, seize and them with ptrace ''PTRACE_SEIZE'' call (which put tasks into seized state, where tasks do not know that they are actually stopped and someone does nasty things with them :), and performs the following steps on each '''$pid'''.

#* Collects VMA areas by parsing '''/proc/$pid/maps'''.

# Collect 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.

# Collect file descriptor numbers the task has via '''/proc/$pid/fd'''.

#* Core parameters of a task (such as registers and friends) are being dumped via ptrace interface and parsing '''/proc/$pid/stat''' entry.

# 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.

# The dumper injects a parasite code into a task via ptrace interface. This is done in two steps - at first we inject only a few bytes for ''mmap'' syscall at CS:IP the task has at moment of seizing. Then ptrace allow us to run an injected syscall and we allocate enough memory for a parasite code chunk we need for dumping. After that the parasite code is copied into new place inside dumpee address space and CS:IP set respectively to point to our parasite code.

#** 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.

# After everything dumped (such as memory pages, which can be written out only from inside dumpee address space) we use ptrace facility again and cure dumpee by dropping out all our parasite code and restoring original code.

#** 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'''.

# The procedure continues for every '''$pid'''.