I underestimated the dangers of Rust FFI

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As a continuation of Newsboat rewrite series, here’s an example of just how attentive you have to be when writing FFI code.

FFI stands for “foreign function interface”. It’s a way for Rust code to call other languages, and to be called from other languages as well. FFI relies on C as a lingua franca, a common language that defines how things are laid out in memory, and how functions are invoked.

Working with FFI is dangerous. You’re responsible for upholding invariants that are usually taken care of by Rust. You must get your types right, and you need to make sure there is no way for you to overwrite someone else’s memory.

Here’s an abridged version of the code I was working on the other day (also available on Rust Playground):

// This code is BROKEN — see below for a correct one.
use std::ffi::CString;

fn obtain_answer() -> Option<String> {
    let format = "The answer ... is: %llu"; // ❶
    const BUF_SIZE: usize = 1024;
    let buffer = Vec::<u8>::with_capacity(BUF_SIZE);
    unsafe {
        CString::new(buffer).ok().and_then(|buffer| {
            CString::new(format).ok().and_then(|format| {
                let buffer_ptr = buffer.into_raw();
                libc::snprintf( // ❷
                    BUF_SIZE as libc::size_t,
                    format.as_ptr() as *const libc::c_char,
                let buffer = CString::from_raw(buffer_ptr);
                buffer.into_string().ok() // ❸

It takes a format string ❶, passes it into C function snprintf ❷ to replace “%llu” with std::u64::MAX, and collects the result as a Rust String ❸. Pretty innocuous, isn’t it? And you’d expect that if there were a bug, it’d be somewhere inside that unsafe block, right?

But the bug is right between Vec::with_capacity and the unsafe keyword. Nothing there? Exactly!

This is not documented in CString’s docs in any way yet, but its new method calls Vec::into_boxed_slice, which sheds unused capacity. So here’s what happens when the code is ran:

  1. we create an empty Vec that has 1024 bytes reserved already;
  2. we pass that Vec into CString::new, which appends a null byte (increasing vector size to 1, and decreasing reserve to 1023);
  3. CString::new then calls Vec::into_boxed_slice, which allocates just enough space to hold that one byte, copies it there, and frees the original buffer of 1024 bytes;
  4. we convert the CString into a pointer to its one-byte buffer using into_raw;
  5. unlike Rust slices, C pointers don’t store the size of the chunk they point to—that’s why we also pass BUF_SIZE to snprintf. However, in our case this size no longer matches the size of the buffer (we pass 1024 but it’s actually 1);
  6. snprintf trusts us, and writes all over the 1023 bytes that follow our buffer.

This is a classic buffer overrun, the staple on which numerous bugs subsist. The behaviour of this code differs by platform, compiler version, and what other code is ran afterwards. It will most probably work just fine—the first time around, anyway. In the Playground, you’ll get a timeout, coredump, and a message saying “realloc(): invalid next size”. On my Linux machine, I saw tests hang in jemalloc (with pre-1.32 Rust, i.e. before Rust transitioned to the system allocator).

A solution that sorta works

Once found, the problem is easy to fix: resize the vector to BUF_SIZE-1, leaving that one last byte unused for CString::new to insert the terminating null byte. Use anything but zero for initialization value, as new checks for them and will return an Err if vector contains any zeroes already.

const BUF_SIZE: usize = 1024;
let buffer = Vec::<u8>::with_capacity(BUF_SIZE);
buffer.resize(BUF_SIZE - 1, 1);
unsafe {
    CString::new(buffer).ok().and_then(|buffer| {
        // `buffer` definitely has `BUF_SIZE` bytes in it

But as commenters on lobste.rs point out, that’s still a wrong way to do it. CString is meant to own a C string that we loan to FFI; it’s not suited for buffers that we ask the FFI to fill. So let’s finally get to…

The correct solution

Just use Vec. Preallocate as much space as you need, use FFI, reconstruct the Vec, simultaneously resizing it to match how many bytes were written. In production we’d also check if snprintf couldn’t fit everything into the buffer, and allocate a larger one if need be. But this is a blog post, so let’s skip that part for brevity:

let mut buffer = Vec::<u8>::with_capacity(BUF_SIZE);
let buffer_ptr = buffer.as_mut_ptr();
unsafe {
    // We're passing the buffer to C, so let's make
    // Rust forget about it for a while.

    let bytes_written = libc::snprintf(
        buffer_ptr as *mut libc::c_char,
        buf_size as libc::size_t,
        format_cstring.as_ptr() as *const libc::c_char,
    ) as usize;

    let buffer = Vec::from_raw_parts(

Avoid FFI.

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