Checkpoint/Restore

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

1. A $pid of a process group leader is obtained from the command line.
2. 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.
3. 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 syscall call, then (by ptrace as well) it substitutes an original code with syscall 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.
4. The procedure continues for every $pid.

Restore

The restore procedure (aka restorer) proceed in the following steps

1. A process tree has been read from a file.
2. Every process started with saved (i.e. original) $pid via clone() call.
3. Files and pipes are restored (by restored it's meant - they are opened and positioned).
4. A new memory map is created, filled with data the program had at checkpoint time.
5. Finally the program is kicked to start with rt_sigreturn system call.