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Compel temporary GIT repo is at http://github.com/xemul/compel.
'''Compel''' is a utility to execute arbitrary code in a context of a foreign process. Compel is part of CRIU, and its sources are available from [https://github.com/xemul/criu/tree/criu-dev/compel the criu-dev branch of CRIU repo, subdirectory compel].
The code to be executed is called '''parasite code'''. Once compiled with compel flags and packed, it can be executed in other task's context. Note the code is run in environment without glibc, thus it can not call the usual stdio/stdlib/etc. functions.
A set of [[compel plugins]] are provided for your convenience. Plugins get linked to the parasite binary during the pack stage.
== How we want it to look like ==
== Writing parasite code ==
Execution of the parasite code starts with the function
Execution of the parasite code starts with the function
that should be present in your code. The <code>arg_p</code> and <code>arg_s</code> is the binary argument that will get delivered to parasite code by complel <code>_start()</code> call (see below). Sometimes this binary argument can be treated as CLI arguments argc/argv.
that should be present in your code. The <code>arg_p</code> and <code>arg_s</code> is the binary argument that will get delivered to parasite code by complel <code>_start()</code> call (see below). Sometimes this binary argument can be treated as CLI arguments argc/argv.
=== Compile the sources and pack the binary ===
=== Compiling and packing ===
Take a program on C and compile it with compel flags
Take the program and compile it with compel flags:
<pre>
<pre>
$ gcc -c foo.c -o foo.o $(compel cflags)
$ gcc -c foo1.c -o foo1.o $(compel cflags)
</pre>
</pre>
To combine the foo.o out of many sources, they should all be linked with compel flags as well
To combine the foo.o out of many sources, they should all be linked with compel linker flags:
<pre>
<pre>
$ ld foo_1.o foo_2.o -o foo.o $(compel ldflags)
$ ld foo1.o foo2.o -o foo.o $(compel ldflags)
</pre>
</pre>
Pack the binary. Packing would link the object file with compel plugins (see below)
Pack the binary. Packing would link the object file with [[compel plugins]].
<pre>
<pre>
</pre>
</pre>
The <code>-l</code> option specifies plugins to be added to the packed binary. Plugin <code>std</code> is added all the time w/o any options.
The <code>-l</code> option specifies plugins to be added to the packed binary. The <code>std</code> plugin is added by default, i.e. it does not require any flags.
Now, the <code>foo.compel.o</code> is ready for remote execution (note that <code>foo.o</code> was not).
=== Execute the code remotely ===
=== Execute the code ===
Using CLI like this
Using CLI, the compiled and packed parasite code can be executed like this:
<pre>
<pre>
$ compel run -f foo.compel.o -p $pid [args]
$ compel run -f foo.compel.o -p ''pid'' [args ...]
</pre>
</pre>
Alternatively, you can link it with the <code>libcompel.so</code> and use the following calls:
<source lang="C">
<source lang="C">
</source>
</source>
calls.
These library calls require binary argument that is copied into the parasite context and is available from it via arg_p/arg_s pair. When run from CLI, the arguments are packed in the usual way (argc/argv).
=== Communicate to parasite code ===
=== How to communicate to parasite code ===
There are several ways for doing this.
There are several ways for doing this.
If you run the parasite binary from CLI, the tail command line arguments are passed into the parasite main() function.
If you run the parasite binary from CLI, the command line arguments after <code>--</code> are passed into the parasite's <code>main()</code> function.
<pre>
<pre>
</pre>
</pre>
In the main() common argc and argv are accessed using the
In <code>main()</code>, common argc and argv can be accessed using the following code:
<source lang="C">
<source lang="C">
</source>
</source>
calls available in <code>argv</code> [[Compel/Plugins|plugin]]. Then use argc and argv as you would use them in normal C program run from shell.
These calls are available in <code>argv</code> [[compel plugins|plugin]]. Now, argc and argv can be handled as usual.
== See also ==
== See also ==
* [[Compel/Usage_scenarios]]
* [[Compel/Usage_scenarios]]
* [[Compel/Plugins]]
* [[Compel plugins]]
[[Category:Compel]]
[[Category:Compel]]