Difference between revisions of "Compel"

From CRIU

Latest revision as of 14:43, 29 August 2018

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 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.

Writing parasite code[edit]

Execution of parasite code always starts with a function in compel std plugin that should be linked with parasite binary (see below). From the parasite code these symbols should be available for libcompel to work

parasite_trap_cmd(int cmd, void *arg);: This routine gets called by compel_run_in_thread()

parasite_daemon_cmd(int cmd, void *arg);: This routine gets called by compel_rpc_call() and compel_rpc_call_sync(). The arg points to the memory with arguments, see the #Arguments passing section below.

parasite_cleanup(void);: This gets called on parasite unload by compel_cure()

Compiling and packing[edit]

Compile the source of your parasite code with compel flags:

$ gcc -c foo1.c -o foo1.o $(compel cflags)

Then link the parasite binary. Include all the .o files needed.

$ ld $(compel ldflags) foo1.o foo2.o $(compel plugins) -o parasite.po

The .po blob can now be loaded as parasite.

Loading blob[edit]

Using CLI[edit]

This functionality is in plans and not implemented yet.

Using libcompel.a library[edit]

Currently there is only one way to load the blob into victim task using libcompel.a, called c-header ^[1]. First you should make a header out of your .po file with the hgen action of compel tool:

compel hgen -f parasite.po -o parasite.h

Options meaning is the following:

-f tells which binary to turn into header
-o tells where to write the resulting header

Once parasite.h file is ready, it should be included into the infecting program source code to be compiled with it.

Running parasite code[edit]

So, in order to infect a task with parasite one must do the following.

Stop the task. This is done by calling compel_stop_task(int pid). Its return value should be saved in case it's positive (it contains the task state).
Prepare the infection handler. This is done by calling compel_prepare(int pid). The return value is an opaque pointer to struct parasite_ctl.
Run the remote code:
- Execute a system call with compel_syscall(ctl, int syscall_nr, long *ret, int arg ...) (all 6 parameters)
- Infect the victim with the parasite blob by calling compel_infect(ctl, nr_thread, size_of_args_area)
Cure the victim by calling compel_cure(ctl). Note that ctl pointer is freed by the call so it should not be used thereafter.
Resume the task by calling compel_resume_task(pid, orig_state, state) with the saved state value from the first step.

Infecting[edit]

Infecting the victim with a parasite blob needs some special treatment.

First, the ctl should be configured with the blob information. For that, you should call PREFIX_setup_c_header() function with ctl as an argument. Here PREFIX is the same as the argument given to -p option to compel hgen, and if not given, it is derived from the input file name, dropping the path and the suffix (in the above example it's parasite).

Second, when infecting a parasite one should specify the amount of threads it will mess with (1, if only the thread leader will be accessed) and the maximum size of the memory area shared between infecting task and parasite used for arguments/result passing.

Arguments passing[edit]

To pass arguments between the infecting code and parasite, one may call compel_parasite_args(ctl, type) and get the pointer where it can put data. Subsequent calls to compel_rpc_call[_sync]() will result in this data visible in void *arg address of the parasite_daemon_cmd().

@@ Line 1: / Line 1: @@
-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/checkpoint-restore/criu/tree/criu-dev/compel the criu-dev branch of CRIU repo, subdirectory compel].
-Now compel sits in the main criu repo at https://github.com/xemul/criu/tree/criu-dev/compel.
-== 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.
-An utility to execute code in foreign process address space. The code should be compiled with compel flags and packed, then it can be executed in other task's context. The parasite code executes in an environment w/o glibc, thus you cannot call the usual stdio/stdlib/etc. functions. Compel provides a set of plugins for your convenience. Plugins get linked to parasite binary on the pack stage (see below).
+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 parasite code always starts with a function in compel std plugin that should be linked with parasite binary (see below). From the parasite code these symbols should be available for libcompel to work
-<pre>
+; <code>parasite_trap_cmd(int cmd, void *arg);</code>
-int main(void *arg_p, unsigned int arg_s);
+: This routine gets called by <code>compel_run_in_thread()</code>
-</pre>
+; <code>parasite_daemon_cmd(int cmd, void *arg);</code>
+: This routine gets called by <code>compel_rpc_call()</code> and <code>compel_rpc_call_sync()</code>. The <code>arg</code> points to the memory with arguments, see the [[#Arguments passing]] section below.
-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.
+; <code>parasite_cleanup(void);</code>
+: This gets called on parasite unload by <code>compel_cure()</code>
-=== Compile the sources and pack the binary ===
+== Compiling and packing ==
-Take a program on C and compile it with compel flags
+Compile the source of your parasite code with compel flags:
 <pre>
-$ gcc -c foo.c -o foo.o $(compel cflags)
+$ gcc -c foo1.c -o foo1.o $(compel cflags)
 </pre>
-To combine the foo.o out of many sources, they should all be linked with compel flags as well
+Then link the parasite binary. Include all the .o files needed.
 <pre>
-$ ld foo_1.o foo_2.o -o foo.o $(compel ldflags)
+$ ld $(compel ldflags) foo1.o foo2.o $(compel plugins) -o parasite.po
 </pre>
-Pack the binary. Packing would link the object file with compel plugins (see below)
+The .po blob can now be loaded as parasite.
-<pre>
+== Loading blob ==
-$ compel pack foo.o -o foo.compel.o [-l plugin]
-</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.
+=== Using CLI ===
-The foo.compel.o is ready for remote execution (foo.o was not).
+This functionality is in plans and not implemented yet.
-=== Execute the code remotely ===
+=== Using libcompel.a library ===
-Using CLI like this
+Currently there is only one way to load the blob into victim task using libcompel.a, called ''c-header'' <ref>This is done for historical reasons. It was the most handy way to load [[parasite code]] by CRIU. In plans we have loading the .po ELF file itself</ref>. First you should make a header out of your .po file with the <code>hgen</code> action of compel tool:
-<pre>
+ compel hgen -f parasite.po -o parasite.h
-$ compel run -f foo.compel.o -p $pid [args]
-</pre>
-Or, you can link with libcompel.so and use
+Options meaning is the following:
+* <code>-f</code> tells which binary to turn into header
+* <code>-o</code> tells where to write the resulting header
-<pre>
+Once <code>parasite.h</code> file is ready, it should be included into the infecting program source code to be compiled with it.
-libcompel_exec() libcompel_exec_start()/libcompel_exec_end()
-</pre>
-calls.
+== Running parasite code ==
-The library calls require binary argument that will get copied into parasite context and passed to it via arg_p/arg_s pair. When run from CLI the arguments are packed in argc/argv manner.
+So, in order to infect a task with parasite one must do the following.
-=== How to communicate to parasite code ===
+* Stop the task. This is done by calling <code>compel_stop_task(int pid)</code>. Its return value should be saved in case it's positive (it contains the task state).
+* Prepare the infection handler. This is done by calling <code>compel_prepare(int pid)</code>. The return value is an opaque pointer to <code>struct parasite_ctl</code>.
+* Run the remote code:
+** Execute a system call with <code>compel_syscall(ctl, int syscall_nr, long *ret, int arg ...)</code> (all 6 parameters)
+** Infect the victim with the parasite blob by calling <code>compel_infect(ctl, nr_thread, size_of_args_area)</code>
+* Cure the victim by calling <code>compel_cure(ctl)</code>. Note that <code>ctl</code> pointer is freed by the call so it should not be used thereafter.
+* Resume the task by calling <code>compel_resume_task(pid, orig_state, state)</code> with the saved state value from the first step.
-There are several ways for doing this.
+=== Infecting ===
-If you run the parasite binary from CLI, the tail command line arguments are passed into the parasite main() function.
+Infecting the victim with a parasite blob needs some special treatment.
-<pre>
+First, the <code>ctl</code> should be configured with the blob information. For that,
-$ compel run -f foo.compel.o -p 123 -- arg1 arg2 arg3
+you should call <code>''PREFIX''_setup_c_header()</code> function
-</pre>
+with <code>ctl</code> as an argument. Here <code>''PREFIX''</code>
+is the same as the argument given to <code>-p</code> option
+to <code>compel hgen</code>, and if not given, it is derived
+from the input file name, dropping the path and the suffix
+(in the above example it's <code>parasite</code>).
-In the main() common argc and argv are accessed using the
+Second, when infecting a parasite one should specify the amount of threads it will mess with (1, if only the thread leader will be accessed) and the maximum size of the memory area shared between infecting task and parasite used for arguments/result passing.
-<pre>
+=== Arguments passing ===
-argc = std_argc(arg_p); argv = std_argv(arg_p, argc);
-</pre>
-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.
+To pass arguments between the infecting code and parasite, one may call <code>compel_parasite_args(ctl, type)</code> and get the pointer where it can put data. Subsequent calls to <code>compel_rpc_call[_sync]()</code> will result in this data visible in <code>void *arg</code> address of the <code>parasite_daemon_cmd()</code>.
 == See also ==
 * [[Compel/Usage_scenarios]]
-* [[Compel/Plugins]]
+* [[Compel plugins]]
+* Examples of working code available [https://github.com/checkpoint-restore/criu/tree/criu-dev/compel/test on github] <ref>Clone the repo, build the project by running <code>make</code>, then go to <code>compel/test/$test_name</code> directory and run <code>make</code> there. Running <code>spy</code> bynary runs the example. Then -- RTFS or talk to us on [https://lists.openvz.org/mailman/listinfo/criu criu@openvz.org] :)</ref>
+* Info about CRIU [[code blobs]] in which the same technology is used
+-----
+<references/>
 [[Category:Compel]]
+[[Category:Editor help needed]]