mirrord internals - hooking libc functions in Rust and fixing bugs

Posted July 4, 2022 by Mehul Arora ‐ 14 min read

“Is mirrord some kind of ptrace magic?”, that’s what I exactly thought when I was introduced to this idea of “mirroring traffic”. To my surprise, the idea and design behind mirrord are based on simple concepts implemented in a novel way! This is what I want to discuss in this blog post along with my experience as a Junior Engineer learning how to tackle bugs working on this badass project.

What is mirrord? 🪞

mirrord lets you run a local process in the context of a cloud service, which means we can test our code on staging, without actually deploying it there. This leads to shorter feedback loops (you don’t have to wait on long CI processes to test your code in staging conditions) and a more stable staging environment (since untested services aren’t being deployed there). There is a detailed overview of mirrord and what we strive to achieve with it in this blog post.

mirrord-layer + LD_PRELOAD = ❤️

mirrord-layer, shipped as a dynamic library, is responsible for “overriding” or “hooking” libc functions through LD_PRELOAD.


LD_PRELOAD1, available as an environment variable, is a feature provided by dynamic linkers like ld.so that lets us load a shared library into a process before the process loads anything else. In our case, we use LD_PRELOAD to load mirrord-layer, which overrides libc functions with a custom implementation. By overriding file and socket functions, we can then transparently plug the process into the remote pod, having it read and write files and traffic remotely without changing a single line of code. Overriding these libc functions on different systems would have been a difficult task and this is where Frida-gum comes to save the day through its inline hooking interceptor.

Let’s go over a quick example of how we can hook the open system call by finding and replacing libc symbols through Frida’s Rust bindings.

  • Get a reference to the Frida Gum runtime.
lazy_static! {
    static ref GUM: Gum = unsafe { Gum::obtain() };
  • Write a detour for open.
unsafe extern "C" fn open_detour(
    name: *const c_char, 
    flags: c_int,
) -> c_int {
    println!("open_detour: {}", std::ffi::CStr::from_ptr(name).to_str().unwrap());
    let res = libc::open(name, flags);

Create an interceptor.

Find the exported symbol from other shared libraries for open and replace it with our detour through the interceptor.

fn init() {
    let mut interceptor = Interceptor::obtain(&GUM);
    let open = Module::find_export_by_name(None, "open").unwrap();
    interceptor.replace(open, NativePointer(open_detour as *mut c_void), NativePointer(0 as *mut c_void)).unwrap();

The complete crate for the example above is available here.

After cargo +nightly build, let’s LD_PRELOAD our shared library and run the unix utility called cat on our very cool sample file.

[email protected]:~/mirrord$ LD_PRELOAD=target/debug/libmirrord.so cat file.txt
open_detour: file.txt
boots and cats

[email protected]:~/mirrord$ echo "look at the statement before "boots and cats" is printed!"
look at the statement before "boots and cats" is printed!

Awesome! we are able to override the functionality of libc’s system call wrappers and replace them with our custom code.

Mirroring network traffic & web servers 💻

I want to do a quick walkthrough of how a simple webserver would work when run with mirrord and how this led me to finding my first bug! So, in general, web servers implement the flow of creating a socket and accepting connections on it by making the following system calls sequentially - socket, bind, listen, accept2.

Referring to the notes on the Linux manual for listen, we discuss these system calls in detail and how mirrord handles them.

[1] socket

socket returns a socket descriptor referring to a communication endpoint. When mirrord hooks a process’ socket call, it maintains that original behavior, but also keeps a record of the new socket in an internal data structure. To describe this data structure and what’s going on behind the scenes I will refer to these diagrams below -

  • The local process calls socket, which then tries to find the socket symbol in libc from the shared library dependencies.
mirrord socket detour
  • Frida’s interceptor replaced (in-place) the libc’s socket wrapper with our detour, so the socket call goes to our detour 😉.
mirrord socket detour
  • Inside the detour, we call libc’s socket wrapper and store the returned descriptor in a hashmap called SOCKETS that maps the socket to its related metadata and “initialized” state.
mirrord socket detour
pub(crate) static SOCKETS: LazyLock<Mutex<HashMap<RawFd, Arc<Socket>>>> =
    LazyLock::new(|| Mutex::new(HashMap::new()));
  • In the end, we just return the socket descriptor returned by the call to libc to the local process.
mirrord socket detour

Note: The words “hook” and “detour” are used interchangeably as they refer to the same idea, but “detour” is more formal as it is used in the codebase.

[2] bind

To bind an address to the socket descriptor returned by the socket system call, bind is called. Our detour for bind doesn’t really do much because all the juicy stuff happens in listen. However, it puts the socket in a Bound state if it exists in our SOCKETS hashmap along with the address supplied by the process through the sockaddr struct.

mirrord bind image

Structs for Socket metadata and its states:

pub struct Socket {
    domain: c_int,
    type_: c_int,
    protocol: c_int,
    pub state: SocketState,

pub enum SocketState {

[3] listen

To start accepting connections on our socket, we have to mark the socket as passive using the listen system call. There are quite a few things happening in our “little” detour here, so let’s take it step by with the help of these diagrams below -

  • Change the socket state from Bound to Listening in our SOCKETS hashmap.
mirrord listen detour
  • Call libc’s bind with address port as 0, which looks something like sockaddr_in.port = 0 at a lower level in C. This makes the - OS assign a port to our address, without us having to check for any available ports.
  • Call libc’s getsockname to get the port that was assigned to our address. We call this our “fake port”.
  • Call libc’s listen to qualify as an endpoint open to accepting new connections.
  • Send a message to mirrord-agent, with information including the “real” and “fake” port, that a new “peer” has connected to the agent to receive network traffic on the “real” port.
mirrord listen detour

Long story short, mirrord-layer listens on the “fake” port bound to the address specified by the user. For example, if a user calls bind on port 80, mirrord-layer will create a port like 3424 and call listen on it by binding the address to it. This also means that we don’t need sudo to run our web server when listening on a special port like 80 since it is never actually bound. In parallel, mirrord-agent forwards traffic to this fake port giving us the illusion that our process is running on the remote pod. We will talk about how mirrord-agent works in another blog post!

mirrord listen detour

[4] accept

Now we just need to handle new connections! Every time accept is called in our local process, we call libc’s accept and get a new socket descriptor referring to that connection/socket passed to accept, but that’s just not it because under the hood we also maintain an internal connection queue for pending connections. This means that every time we receive a new connection request from the agent pod we enqueue that in our CONNECTION_QUEUE. Each socket descriptor has its own unique queue.

Furthermore in our detour for accept, we do the following -

  • Is there a socket in Listening state in our SOCKETS hashmap, matching the socket passed to the parameters to accept?
  • If yes, we get the pending connection from our CONNECTION_QUEUE for our original socket descriptor.
  • Add the new socket descriptor to our SOCKETS hashmap in the Connected state.
  • Modify the pointer to the sockaddr struct to implicitly return the address of the new connection.
mirrord accept detour

Alright then, we have all our detours in place. Everything should work smoothly! Or so I thought. Let’s test it out by rolling back to the commit with only these detours in place.

git checkout d8b4de6

That’s the commit before the patch I made for the bug I discovered. We don’t need to explicitly build and load the agent image in our cluster because the image is already hardcoded in the agent specification. So let’s get rolling?

MIRRORD_IMPERSONATED_POD_NAME=http-echo-deployment-77fddcdc49-6z22r LD_PRELOAD=/home/mehula/mirrord/target/debug/libmirrord.so node sample/app.js

The YAML file for the http-echo deployment is available here.

Let’s look at some logs of our web server running with mirrord. I won’t dump all of them here, but I’ll just pick what’s important. All the logs are available here.

2022-06-23T20:06:12.011931Z DEBUG mirrord: socket called
2022-06-23T20:06:12.012145Z DEBUG mirrord: bind called
2022-06-23T20:06:12.012473Z DEBUG mirrord: listen called
server listening to {"address":""}
2022-06-23T20:06:16.638872Z DEBUG mirrord: send message to client 80

Alright, so our web server is up and running, waiting to accept new connections! And just like we talked about the system calls involved before, all of socket, bind, and listen were called. But now let’s test out if sending a GET request to our remote pod mirrors the traffic to our local process.


2022-06-23T20:41:19.082404Z DEBUG mirrord: send message to client 80
2022-06-23T20:41:21.901455Z DEBUG mirrord: new connection id: 0
2022-06-23T20:41:21.901647Z DEBUG mirrord: No socket found for connection_id: 0
      throw er; // Unhandled 'error' event

Error: accept EINVAL
    at TCP.onconnection (net.js:1497:24)
Emitted 'error' event at:
    at TCP.onconnection (net.js:1497:10)

Looks like even though a connection was enqueued in our CONNECTION_QUEUE, it was never dequeued and no new socket descriptor was inserted in our SOCKETS hashmap.

Note: All references made are in the context of the present version of mirrord, not commit d8b4de6.

That is weird, why was accept never called? Let’s debug our node process and see what’s going on!

mirrord - try to debug node process

Well, good luck debugging that and I won’t waste your time trying to figure out how to step into listen() and other related functions to look at the underlying function calls. Instead, we will look at the underlying system calls with strace.

Let’s run the node server with strace and send a GET request to it.

[email protected]:~/mirrord$ strace -c node sample/app.js
server listening to {"address":"::","family":"IPv6","port":8080}
new client connection from ::ffff:
connection data from ::ffff: {"type":"Buffer","data":[71,69,84,32,47,32,72,84,84,80,47,49,46,49,13,10,72,111,115,116,58,32,108,111,99,97,108,104,111,115,116,58,56,48,56,48,13,10,85,115,101,114,45,65,103,101,110,116,58,32,99,117,114,108,47,55,46,54,56,46,48,13,10,65,99,99,101,112,116,58,32,42,47,42,13,10,13,10]}
connection from ::ffff: closed
^Cstrace: Process 285853 detached
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 80.95    0.018595           5      3361           mprotect
  4.74    0.001088           9       113           mmap
  3.56    0.000817           3       266           brk
  1.17    0.000268           5        51           futex
  0.94    0.000215           7        30         8 openat
  0.89    0.000204           9        22           fstat
  0.79    0.000182           5        31        10 ioctl
  0.71    0.000163           8        20           close
  0.68    0.000156           7        20           read
  0.56    0.000129          11        11           getgid
  0.54    0.000125          20         6           clone
  0.50    0.000114          10        11           geteuid
  0.45    0.000104           9        11           getegid
  0.45    0.000103           9        11           getuid
  0.44    0.000101          14         7           prlimit64
  0.42    0.000096          12         8           pread64
  0.41    0.000094           3        26           munmap
  0.34    0.000079           6        13           getpid
  0.33    0.000075          10         7           rt_sigaction
  0.24    0.000054          18         3           pipe2
  0.15    0.000034           4         7           rt_sigprocmask
  0.13    0.000031          15         2           eventfd2
  0.10    0.000024          12         2           epoll_create1
  0.09    0.000021           3         6           madvise
  0.07    0.000016           2         7           write
  0.07    0.000015           7         2         1 arch_prctl
  0.05    0.000012          12         1           set_robust_list
  0.04    0.000010           1         6           epoll_ctl
  0.04    0.000010          10         1           getrandom
  0.04    0.000009           9         1           set_tid_address
  0.02    0.000005           1         4         1 epoll_wait
  0.02    0.000004           0        11         8 stat
  0.02    0.000004           2         2           setsockopt
  0.01    0.000003           3         1           socket
  0.01    0.000003           3         1           listen
  0.01    0.000003           1         2         1 accept4
  0.01    0.000002           2         1           bind
  0.01    0.000002           2         1           getsockname
  0.00    0.000000           0         1         1 access
  0.00    0.000000           0         1           getpeername
  0.00    0.000000           0         1           execve
  0.00    0.000000           0         2           fcntl
  0.00    0.000000           0         2           getcwd
  0.00    0.000000           0         4           readlink
  0.00    0.000000           0         2           dup3
  0.00    0.000000           0         7           statx
------ ----------- ----------- --------- --------- ----------------
100.00    0.022970                  4106        30 total

It looks like accept is never called and the only system call closest to accept we can see on this list is accept4. According to the Linux manual page, accept and accept4 are essentially the same except for the flags parameter, which we probably don’t care about right now. So we will hook accept4 the same way as accept and pray that things go well this time.

2022-06-24T16:22:59.983321Z DEBUG mirrord: accept4 hooked
2022-06-24T16:23:00.371721Z DEBUG mirrord: socket called
2022-06-24T16:23:00.371935Z DEBUG mirrord: bind called
2022-06-24T16:23:00.372050Z DEBUG mirrord: listen called
server listening to {"address":""}
2022-06-24T16:23:04.983632Z DEBUG mirrord: send message to client 80
2022-06-24T16:23:22.756866Z DEBUG mirrord: new connection id: 0
2022-06-24T16:23:22.758080Z DEBUG mirrord: No socket found for connection_id: 0
      throw er; // Unhandled 'error' event

Error: accept EINVAL
    at TCP.onconnection (net.js:1497:24)
Emitted 'error' event at:
    at TCP.onconnection (net.js:1497:10)

Hah, didn’t take long for things to south, the exact same error again 😔. We hooked the libc wrapper for accept4 but it was never called?

Here are a few reasons that I can think of why this could not be working:

  • Node is probably into some sorcery and has decided to screw with me this time.
  • Maybe Node never even calls accept, but instead something else to accept new connections.

I don’t believe in sorcery, so I will dig into the second reasoning here.

strace only shows us the underlying system calls made by a process. So let’s do some static analysis and look for some functions similar to accept or accept4.

I will be using Ghidra here, a reverse engineering toolkit that comes in super handy when decompiling a binary. So let’s load our node binary into Ghidra and analyze it!

mirrord - use ghidra to search symbols

So looks like we won’t find anything useful unless we import some more relevant shared objects used by our node binary.

mirrord - use ghidra to find imports

Finding paths for shared library dependencies can be a bit painful with find, so instead, I will use ldd here.

[email protected]:~/mirrord$ which node
[email protected]:~/mirrord$ ldd /usr/bin/node
        linux-vdso.so.1 (0x00007fffda938000)
        libnode.so.64 => /lib/x86_64-linux-gnu/libnode.so.64 (0x00007f9934a00000)
        libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f99349dd000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f99347eb000)
        libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007f99347cf000)
        libuv.so.1 => /lib/x86_64-linux-gnu/libuv.so.1 (0x00007f993479e000)
        libcares.so.2 => /lib/x86_64-linux-gnu/libcares.so.2 (0x00007f993478a000)
        libnghttp2.so.14 => /lib/x86_64-linux-gnu/libnghttp2.so.14 (0x00007f993475f000)
        libcrypto.so.1.1 => /lib/x86_64-linux-gnu/libcrypto.so.1.1 (0x00007f9934489000)
        libssl.so.1.1 => /lib/x86_64-linux-gnu/libssl.so.1.1 (0x00007f99343f6000)
        libicui18n.so.66 => /lib/x86_64-linux-gnu/libicui18n.so.66 (0x00007f99340f7000)
        libicuuc.so.66 => /lib/x86_64-linux-gnu/libicuuc.so.66 (0x00007f9933f11000)
        libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f9933f0b000)
        libstdc++.so.6 => /lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f9933d27000)
        libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f9933bd8000)
        libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f9933bbd000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f9935fcb000)
        libicudata.so.66 => /lib/x86_64-linux-gnu/libicudata.so.66 (0x00007f99320fc000)

Let’s start with libnode and look for the accept like symbols/functions again.

mirrord - use ghidra to find imports

That gives us some hope! And probably a good lead to follow -

A quick Google search tells me that the uv__accept function belongs to libuv which is also listed as a node dependency here. Let’s load libuv and carry on our search!

mirrord - use ghidra to find uv__accept

Here’s a decompiled version of uv__accept which clearly shows it makes calls to either uv__accept4 or accept. We already have our hook for accept in place, so we probably don’t need to worry about that, but let’s look into uv__accept4.

mirrord - use ghidra to check decompilation

AH! This is it. It all makes sense now. uv__accept4 is directly making the syscall instead of using the libc wrapper. So let’s hook uv__accept4 to behave the same as our hook for accept/accept4.

#[cfg(target_os = "linux")]
unsafe extern "C" fn accept4_detour(
    sockfd: i32,
    address: *mut sockaddr,
    address_len: *mut socklen_t,
    flags: i32,
) -> i32 {
    let accept_fd = libc::accept4(sockfd, address, address_len, flags);

    if accept_fd == -1 {
    } else {
        accept(sockfd, address, address_len, accept_fd)

Yet another hopeful GET request -


2022-06-24T18:44:55.391978Z DEBUG mirrord: uv__accept4 hooked
2022-06-24T18:44:55.392238Z DEBUG mirrord: accept4 hooked
2022-06-24T18:44:55.392321Z DEBUG mirrord: accept hooked
2022-06-24T18:44:55.722728Z DEBUG mirrord: socket called
2022-06-24T18:44:55.722935Z DEBUG mirrord: bind called
2022-06-24T18:44:55.723112Z DEBUG mirrord: listen called
server listening to {"address":""}
2022-06-24T18:45:00.392698Z DEBUG mirrord: send message to client 80
2022-06-24T18:45:02.962967Z DEBUG mirrord: new connection id: 0
2022-06-24T18:45:02.963693Z DEBUG mirrord: No socket found for connection_id: 0
2022-06-24T18:45:02.963787Z DEBUG mirrord: Accept called with sockfd 28, addr 0x0, addrlen 0x0
2022-06-24T18:45:02.963905Z DEBUG mirrord: Accepted connection from read_fd:30, write_sock:SocketpairStream { raw_fd: 31 }
2022-06-24T18:45:02.963949Z DEBUG mirrord: writing pending data for connection_id: 0
new client connection from
2022-06-24T18:45:02.965490Z DEBUG mirrord: Accept called with sockfd 28, addr 0x0, addrlen 0x0

Conclusion 🤠

Time to celebrate? Yes! We were finally able to find the correct function to hook and make accept work the way want it to work in the context of mirrord. Writing hooks is not easy - not only does it take an extensive amount of time, but also a ton of research. That’s why we try to follow a feature guide which lets us work on new features/hooks based on real use cases and needs so that we don’t end up wasting time on something that no one would actually use.

Hope you enjoyed reading the post! Please feel free to reach out to me with feedback at [email protected]/Discord, or provide any suggestions/open issues/PRs on our website.

Credits 🐻

On a personal note, these past two months working at MetalBear on mirrord have not only been an amazing learning experience but have also given me a chance to work with some extremely talented engineers and Rust enthusiasts. Just want to take a moment and thank my team for their guidance and mentorship with this little meme -

senior junior meme

  1. Available as DYLD_INSERT_LIBRARIES on OSX. ↩︎

  2. Webservers also make use of select between listen and accept↩︎