The FFI boundary as a firewall: validate and narrow

By now you’ve written extern "C" functions in both directions, seen bindgen generate Rust code and cheadergen generate C headers.

On paper that is everything you need and it’s tempting to just go ahead and rewrite your project now. Take your C API and translate it one-for-one into Rust.

The problem is: often you can’t — and just as often, you shouldn’t. Real-world C is complicated, idiosyncratic, and (let’s admit it!) full of skeletons. You’re likely thinking about a Rust rewrite not just to address security problems or performance issues; rewriting in Rust is a chance to clean up your codebase: re-establish module boundaries, drop the legacy assumptions, write the module you wished you had.

At the same time big-bang rewrites (where you replace the entire codebase at once) famously never work. Which leaves a problem: you have a clean Rust codebase, a messy legacy codebase, and a need to keep both working together for some time¹.

During this transitionary period the FFI layer is where those worlds meet. It has to bridge unsafe-everything-goes C and the borrow checker — and bridge your new design and the old one.

One major misconception many people have is that FFI is about type translation. That is correct, of course, but it’s like saying programming is about typing words into a computer; it is missing the point. The main job of your FFI interface is establishing confidence. In a legacy codebase you rarely know with 100% certainty that reality matches your assumptions: some caller might actually pass a nullptr, the *const c_char that used to be UTF-8 might now hold binary data.

The FFI boundary is your chance to turn legacy uncertainty into known-good state before it crosses into your new system. Uncertainty leaks. Your FFI must be a vigilant firewall against it.

To this end we at Mainmatter have identified a number of rules that we nowadays live by.

Always validate your inputs

Validate every assumption at the FFI boundary. Null checks, length checks, UTF-8 checks. Panic or return an error if anything is even slightly off. In web servers the HTTP handler function is your primary interface to the “chaos world” of the internet, think of FFI functions the same way: they are your primary interface to the legacy-C “chaos world”.

You should also lean on Rust’s type system so “forgot to check” isn’t even an option. For example: at Mainmatter, we encourage contributors to use Option<NonNull<T>> instead of *mut T as much as possible, for the simple reason that a *mut T can be dereferenced directly (*ptr), which triggers UB if the pointer is null. You would have to add a manual if ptr.is_null() {} check before every pointer dereference. With Option<NonNull<T>> on the other hand, the type system forces you to handle the None case explicitly. Even the laziest .unwrap() will result in a loud panic instead of potentially silent UB.

// `*mut T` silently accepts null. You'll remember to check. Until you don't.
pub extern "C" fn bm_do_thing(input: *mut Thing) -> BmResult { /* … */
}

// `Option<NonNull<T>>` encodes null as `None` — the compiler forces the branch.
pub extern "C" fn bm_do_thing(input: Option<NonNull<Thing>>) -> BmResult {
    let Some(input) = input else {
        return BmResult::ErrInvalid;
    };
    // `input: NonNull<Thing>`, statically non-null.
    /* … */
}

Don’t overload primitives

Legacy C code has a habit of packing several meanings into one return type. POSIX read() is a good example: -1 means error (check errno), 0 means end-of-file, anything positive is the byte count. It is very easy to mess this up. Rust gives you better tools — bool for yes/no, an enum for branching outcomes, an out-parameter for the count. These types usually translate quite well into C headers too, so use them!

#[repr(C)]
pub enum BmReadStatus { Ok, Eof, IoError }

pub extern "C" fn bm_read(/* … */, out_bytes: Option<NonNull<usize>>) -> BmReadStatus;

Head to the exercise

Head to the exercise. We’ll continue with the porting work by taking a look at bm_normalize_url, found here exercises/_bm/src/bookmark.c. It will normalize a given URL by lowercasing it and writing the normalized string into the provided buffer. The exercise already contains our “first draft” Rust translation: a naïve transliteration from C to Rust that has a number of issues.

Let’s apply what we learned above and fix the implementation!

Hint: check the exercises tests to check the expected behaviour of bm_normalize_url

Exercise

The exercise for this section is located in 01_intro/06_validate_and_narrow

1. The exact amount of time of course varies with the scale of your codebase, but let me tell you from hard-won experience: measure this in years. C codebase migrations have a funny habit of taking much, much longer than you think. As a rule of thumb: take your worst case estimate, double that and add a year. Really. ↩