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| − | == What CRtools is ==
| + | See main article here [http://criu.org/CR_tools] |
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| − | '''CRtools''' is an utility to checkpoint/restore process tree. Unlike checkpoint/restore implemented completely in kernel space,
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| − | it tries to achieve the same target mostly in user space.
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| − | === Agenda ===
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| − | # Basic design (checkpoint == proc + SEIZE, restore == syscalls + execve)
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| − | # What's required from kernel
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| − | == Basic design ==
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| − | === Checkpoint ===
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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).
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| − | Which includes
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| − | * Files descriptors information (via '''/proc/$pid/fd''' and '''/proc/$pid/fdinfo''').
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| − | * Pipes parameters.
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| − | * Memory maps (via '''/proc/$pid/maps''').
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| − | The process dumper (lets call it simply the dumper further) does the following steps during checkpoint stage
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| − | # A '''$pid''' of a process group leader is obtained from the command line.
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| − | # By using this '''$pid''' the dumper walks though '''/proc/$pid/status''' and gathers children '''$pids''' recursively. At the end we will have a process tree.
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| − | # Then it takes every '''$pid''' from a process tree, sends ''SIGSTOP'' to every process found, and performs the following steps on each '''$pid'''.
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| − | #* Collects VMA areas by parsing '''/proc/$pid/maps'''.
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| − | #* 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.
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| − | #* Core parameters of a task (such as registers and friends) are being dumped via ptrace interface and parsing '''/proc/$pid/stat''' entry.
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| − | #* The dumper injects a parasite code into a task via ptrace interface. This allows us to dump pages of a task right from within the task's address space.
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| − | #** An injection procedure is pretty simple - the dumper scans executable VMA areas of a task (which were collected previously) and tests if there a place for <code>syscall</code> call, then (by ptrace as well) it substitutes an original code with <code>syscall</code> instructions and creates a new VMA area inside process address space.
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| − | #** Finally parasite code get copied into the new VMA and the former code which was modified during parasite bootstrap procedure get restored.
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| − | #* Then (by using a parasite code) the dumper flushes contents of a task's pages to the file. And pulls out parasite code block completely, since we don't need it anymore.
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| − | #* Once parasite removed a task get unseized via ptrace call but it remains stopped still.
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| − | #* The dumper writes out files and pipes parameter and data.
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| − | # The procedure continues for every '''$pid'''.
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| − | === Restore ===
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| − | The restore procedure (aka restorer) proceed in the following steps:
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| − | # A process tree has been read from a file.
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| − | # Every process started with saved (i.e. original) '''$pid''' via <code>clone()</code> call with new <code>CLONE_CHILD_USEPID</code> flag.
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| − | # Files and pipes are restored (by restored it's meant - they are opened and positioned).
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| − | # 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).
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| − | # Finally execve with new Elf file as an argument is executed, which initiate the kernel's stage of restore procedure.
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