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https://github.com/yuvalino/tembleos
Forkless virtual machine and multi-threaded ssh+shell server
https://github.com/yuvalino/tembleos
dropbear forkless linux mksh posix shell ssh tembel tvm vm xnu
Last synced: about 2 months ago
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Forkless virtual machine and multi-threaded ssh+shell server
- Host: GitHub
- URL: https://github.com/yuvalino/tembleos
- Owner: yuvalino
- Created: 2024-09-21T09:56:47.000Z (3 months ago)
- Default Branch: main
- Last Pushed: 2024-10-27T22:03:42.000Z (about 2 months ago)
- Last Synced: 2024-10-28T01:15:38.994Z (about 2 months ago)
- Topics: dropbear, forkless, linux, mksh, posix, shell, ssh, tembel, tvm, vm, xnu
- Language: C
- Homepage:
- Size: 122 KB
- Stars: 2
- Watchers: 2
- Forks: 0
- Open Issues: 1
-
Metadata Files:
- Readme: README.md
Awesome Lists containing this project
README
# 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](https://github.com/mkj/dropbear) SSH and [mksh](https://github.com/MirBSD/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
To build, just run `$ make`.
To run, `$ ./forkless` and an SSH server at port 2222 will start running with user:password `user:alpine`.
You may also ensure tests are passing by running `$ make test && ./test_forkless`.
## Current Features
### Programs
- `ssh` - dropbear
- `scp` - dropbear
- `sh` - MirBSD Korn Shell
- builtins (`echo`, `export`, `cd`, etc..)
Soon:
- busybox tools
### Platforms
- Linux x86-64
Soon:
- 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:
```c
int main(int argc, char **argv)
{
tvm_init();
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](https://simple.wikipedia.org/wiki/TempleOS). Temble is hebrew for "dummy", which also is a name of an Israeli [hat](https://en.wikipedia.org/wiki/Tembel_hat) popular in mid-20th century.