Forkless TembleOS Virtual Machine

Background

iOS does not allow fork(2) semantics.

Applications and daemons cannot create processes themselves and may not use the system(3) function. Instead, only launchd (equivalent to Linux' init) may create processes using posix_spawn(3).

But what if we wanted to do it anyway? Maybe even run an sshd server with an interactive shell?

What started as a small project to statically compile dropbear SSH and mksh, glue them together and refactor the fork(2)s to pthread_create(3)s, ended up writing an entire POSIX abstraction!

Build and run

macOS: You need gnu-sed (to build toybox), just run $ brew install gnu-sed.

To build, just run $ make.

To run, $ ./forkless sshd and an SSH server at port 2222 will start running with user:password user:alpine.

Tests

• tvm tests: for testing the vm specifically, build and run with $ make test && ./test_tvm.
• forkless tests: for testing dropbear+mksh+toybox over the tvm with pytest. You need to $ pip install pytest pexpect and then run $ make && pytest test_forkless.py.

Current Features

Programs

• ssh - dropbear
• scp - dropbear
• sh - MirBSD Korn Shell
  • builtins (echo, export, cd, etc..)
• toybox - ls, cat, id, grep etc
• tvm - tvm diagnostics
  • tvm ps - internal process list
  • tvm lsof - internal file-descriptor tables

Platforms

• Linux x86-64
• XNU arm64

TVM

• fork() creates a new task.
• Each task has its own file-descriptor table (with 0, 1, 2 pointing to task-specific files under the hood)
• All I/O operations (read(2) / fwrite(3) / perror(3) / printf / getc) operate on the task-specific file-descriptors / streams.
• Calls to sigaction(2) change the signal disposition only for the calling task.
• Full kill(2) / waitpid(2) support.
• Copy-on-write for each task.
• execve(2) and exec(3) with pre-registered programs.
• Userspace device drivers framework
  • TTY device (no real pty/tty is opened in host OS).
• Task-specific environment.
• Task-specific getopt(3) support.

Soon:

• pthread_create(3) with shared globals and private TLS (coming soon).

Adding custom programs

As you can see in main.c, adding a new program is easy:

int main(int argc, char **argv)
{
    tvm_init("forkless");
    tvm_register_program("/bin/sh", (main_func_t) mksh_main);
    tvm_register_program("/usr/bin/scp", (main_func_t) scp_main);

    // ...
}

Notice mksh_main and scp_main are the main functions of each program.

Overview

TembleOS is a fully-fledged OS-layer abstraction that creates a fork(2)/exec(2)-enabled environment without ever using these functions, with the most minimal source code modifications.

The entire code is implemented in tvm.c (with a small API in tvm.h), and must be statically compiled with the rest of the program (dropbear and mksh in our case).

One basic concept is the "task". A task is essentially a tvm-process. It is a group of threads that together act like a POSIX-compliant process with getpid(2), getppid(2) and other process-wide attributes being the same.

Tests

Custom testing framework was developed, compiled with $ make test.

It has a similar API to pytest:

$ ./test_forkless -h
USAGE: ./test_forkless [OPTION]...
Run tests

  -l             print test list and exit
  -k=TEST        only run TEST (can be passed multiple times)
  -v             verbose output
  -g             debug with gdb
  -h             display this help and exit

Etymology

TembleOS is a play on a few things. First and foremost a tribute to Terry A. Davis, the developer of TempleOS. Temble is hebrew for "dummy", which also is a name of an Israeli hat popular in mid-20th century.