1 unstable release

0.1.0 Oct 9, 2023


1.5K SLoC


Traces programs, so you can debug them without breakpoints. Breakpoints are annoying.

Logs all ran cpu instructions, register writes, memory writes, and syscalls.

Per now it only works with linux programs, both x86_64 and aarch64. Programs are traced inside docker, so you can trace them from any operating system. This also means you can trace different architectures, because docker supports emulation through qemu. This also makes a lot of stuff work out of the libc, wrt. libc and other libraries. It does mean you have up to two layers of emulation.


Most is not documented, if you find anything that's not obvious, please just send me an email or open an issue!

Build the docker container(s)

We use an augmented qemu to trace binaries, so compile it:

cd tools

You can also invoke the commands manually:

docker build --platform linux/arm64 . -t rebg:arm64
docker build --platform linux/amd64 . -t rebg:amd64


cargo install --path .

You can also just use cargo r -r -- instead of rebg in the following commands, if you do not want to install rebg yet.

Start the web UI

You probably wanna start the web UI and leave it running.

cd web
npm i
npm run dev


Debug a binary inside docker

The last argument specifies if you want to run qemu inside docker, or native, which you can do if you have installed our modded qemu globally.

$ rebg ./memory-arm64 docker
stat: 4919
dyn: 52428
arr[]: 0x411040
main: 0x40069c
sp: 0x5500800c50
printf: 0x4004e0

You can now visit the web ui, to debug the execution in the past.

Don't start web server

If you don't want rebg to start its websocket server for the UI, you can make it just quit when the execution is done.

$ rebg ./memory-arm64 -q docker
stat: 4919
dyn: 52428
arr[]: 0x411040
main: 0x40069c
sp: 0x5500800c50
printf: 0x4004e0

Debug info

If something is not working properly, you can enable debug output by setting the verbosity to trace, debug, info, warn, or error like so:

$ RUST_LOG="rebg=info" rebg ./memory-arm64 docker
2023-11-02T11:24:16.916549Z  INFO rebg::host::docker: Starting qemu
2023-11-02T11:24:16.917392Z  INFO rebg::tracer::qemu: Waiting for connection...
2023-11-02T11:24:17.071363Z  INFO rebg::tracer::qemu: Connected! TcpStream { addr: [::ffff:]:1337, peer: [::ffff:]:59211, fd: 5 }
stat: 4919
dyn: 52428
arr[]: 0x411040
main: 0x40069c
sp: 0x5500800c50
printf: 0x4004e0
2023-11-02T11:24:18.498586Z  WARN rebg::analyzer::dump: Error decoding syscall: BadFormat
2023-11-02T11:24:18.500490Z  INFO rebg::serve: Execution done, starting WS server.

Performance on MacOS

If you're using macos and tracing linux/amd64, you can cut runtime by about 60% by using rosetta with docker. In Docker Desktop you can find it under "Features in development".

Future work

  • PIN: Linux & Windows tracing.
  • Efficently storing memory history.


We have a UI now! :D

You can use basic vim bindings: j/k for down/up. I'm not sure if we want to override the arrow keys, but we'll probably do it eventually.

Recording all state allows to introduce some bindings which look like things you may be familiar with from gdb:

next / last instruction

  • Shift-J: equivalent to gdb ni
  • Shift-K: the same, but in reverse (time-wise)

step to next call / parent

  • h: step to whatever called this (back in time)
  • l: step to the next call
    • if there is no next call, step to ret

note to self

this feels so fucking great to step this way, i think just showing this shows how great debugging using this could be

Developing QEMU in docker

To spawn a container you can build and run qemu inside, you can use the following commands

Start container

cd tools
docker compose up --build -d

Exec into container

docker compose exec develop /bin/bash

Configure & compile

root@54541497458c:~/qemu# ./configure --with-git-submodules=ignore --enable-tcg-interpreter --target-list=aarch64-linux-user,x86_64-linux-user
root@54541497458c:~/qemu# make -j $(nproc)

Run & test

root@54541497458c:~/qemu# ./build/aarch64-linux-user/qemu-aarch64 /bin/ls

Test with nc

First, spawn a listener. For some reason ipv4 requests to v6 listener is automatically translated to ipv6.

root@94772de42933:~/qemu# while true; do echo -e '\n\n===='; nc -6 -lnvp 1337; done
Listening on :: 1337
Connection received on ::ffff: 48382

Then, somewhere else, build and run!

root@94772de42933:~/qemu# make -j $(nproc) && ./build/aarch64-linux-user/qemu-aarch64 -rebgtcp localhost:1337 -rebglog /dev/null /bin/ls

This mounts the folder in qemu so you can edit in your normal editor and build it and test it in the docker.

Developer debugging tips

Use a computer or VPS with a lot of cores to debug qemu, it's going to compile a lot faster. At least spawn a persistent docker if you you're not natively on linux, so you don't have to recompile everything for every little change.

QEMU has a lot of weird indirection, and the patches I've made are quite hacky. You can debug it in GDB though (again, use a VPS if you don't have x86_64 at home, ptrace doesn't work in qemu-system, which docker uses). For instance debugging what printed something to stderr:

(gdb) b write if $rdi==2
Breakpoint 2 at 0x7ffff7c4ba20: file ../sysdeps/unix/sysv/linux/write.c, line 25.
(gdb) r
Thread 1 "qemu-x86_64" hit Breakpoint 2, __GI___libc_write (fd=2, buf=0x7fffffffb5f0, nbytes=1) at ../sysdeps/unix/sysv/linux/write.c:25
(gdb) bt
#0  __GI___libc_write (fd=2, buf=0x7fffffffb5f0, nbytes=1) at ../sysdeps/unix/sysv/linux/write.c:25
#8  0x000055555569e10d in vfprintf (__ap=0x7fffffffd630, __fmt=0x5555556d9e6a "{", __stream=0x7ffff7d516a0 <_IO_2_1_stderr_>) at /usr/include/x86_64-linux-gnu/bits/stdio2.h:135
#9  qemu_log (fmt=fmt@entry=0x5555556d9e6a "{") at ../util/log.c:153
#10 0x0000555555665b85 in thunk_print (arg=0x4002823fe0, type_ptr=0x55555578416c <ioctl_entries+268>, type_ptr@entry=0x555555784164 <ioctl_entries+260>)
    at ../linux-user/thunk.c:417
#11 0x0000555555650368 in print_syscall_ret_ioctl (cpu_env=<optimized out>, name=<optimized out>, ret=0, arg0=<optimized out>, arg1=<optimized out>, arg2=274919997408, 
    arg3=274922302144, arg4=180, arg5=0) at ../linux-user/strace.c:959
#12 0x000055555565135a in print_syscall_ret (cpu_env=cpu_env@entry=0x555555836660, num=num@entry=16, ret=ret@entry=0, arg1=arg1@entry=1, arg2=arg2@entry=21523, 
    arg3=arg3@entry=274919997408, arg4=274922302144, arg5=180, arg6=0) at ../linux-user/strace.c:4153
#13 0x00005555556650aa in do_syscall (cpu_env=cpu_env@entry=0x555555836660, num=16, arg1=1, arg2=21523, arg3=274919997408, arg4=274922302144, arg5=180, arg6=0, arg7=0, arg8=0)
    at ../linux-user/syscall.c:13379
#14 0x000055555558f844 in cpu_loop (env=env@entry=0x555555836660) at ../linux-user/x86_64/../i386/cpu_loop.c:233
#15 0x000055555558b78d in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at ../linux-user/main.c:968

Edited for brievety, but this lets you trace down stuff real quick.


~2M SLoC