COMPEL(1)

Name
compel - Execute parasitic code within another process.

Synopsis

compel hgen [option ...]

compel plugins [PLUGIN_NAME ...]

compel [--compat] includes | cflags | ldflags

compel [--compat] [--static] libs

DESCRIPTION

compel is a utility to execute arbitrary code, also called parasite code, in the context of a foreign process. The parasitic code, once compiled with compel flags and packed, can be executed in the context of other tasks. Currently there is only one way to load the parasitic blob into victim task using libcompel.a, called c-header.

ARGUMENTS

  Positional Arguments

hgen
    create a header from the .po file, which is the parasite binary.

plugins
    prints the plugins available.

ldflags
    prints the ldflags available to compel during linking of parasite code.

cflags
    prints the compel cflags to be used during compilation of parasitic code.

includes
    prints list of standard include directories.

libs
    prints list of static or dynamic libraries that compel can link with.

OPTIONS

-f, --file FILE
    Path to the binary file, FILE, which compel must turn into a header

-o, --output FILE
    Path to the header file, FILE, where compel must write the resulting header.

-p, --prefix NAME
    Specify prefix for var names

-l, --log-level NUM
    Default log level of compel.

-h, --help
    Prints usage and exits.

-V, --version
    Prints version number of compel.

SOURCE EXAMPLES

  Parasitic Code

#include <compel/plugins/std.h>

int parasite_trap_cmd(int cmd, void *args); //gets called by compel_run_in_thread()

int parasite_daemon_cmd(int cmd, void *arg); // gets called by compel_rpc_call() and compel_rpc_call_sync()

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

  Infecting code

The parasitic code is compiled and converted to a header using compel, and included here.

#include <compel/infect.h>

#include "parasite.h"

Following steps are performed to infect the victim process:

    • stop the task: int compel_stop_task(int pid);

    • prepare infection handler: struct parasite_ctl *compel_prepare(int pid, bool handle_rseq);

    • execute system call: int compel_syscall(ctl, int syscall_nr, long *ret, int arg ...);

    • infect victim: int compel_infect(ctl, nr_thread, size_of_args_area);

    • cure the victim: int compel_cure(ctl); //ctl pointer is freed by this call

    • Resume victim: int compel_resume_task(pid, orig_state, state) or int compel_resume_task_sig(pid, orig_state, state, stop_signo). //compel_resume_task_sig() could be used in case when victim is in stopped state. stop_signo could be read by calling compel_parse_stop_signo().

ctl must be configured with blob information by calling PREFIX_setup_c_header(), with ctl as its argument. PREFIX is the argument given to -p when calling hgen, else it is deduced from file name.

EXAMPLES

To generate a header file(.h) from a parasite binary file(.po) use:

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

parasite.po file is obtained by compiling the parasite source with compel flags and linking it with the compel plugins.

AUTHOR

The CRIU team.