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/* This Source Code Form is subject to the terms of the Mozilla Public * License, v. 2.0. If a copy of the MPL was not distributed with this * file, You can obtain one at http://mozilla.org/MPL/2.0/. */ #![deny(missing_docs)] //! # FFI Support //! //! This crate implements a support library to simplify implementing the patterns that the //! `mozilla/application-services` repository uses for it's "Rust Component" FFI libraries. //! //! It is *strongly encouraged* that anybody writing FFI code in this repository read this //! documentation before doing so, as it is a subtle, difficult, and error prone process. //! //! ## Terminology //! //! For each library, there are currently three parts we're concerned with. There's no clear correct //! name for these, so this documentation will attempt to use the following terminology: //! //! - **Rust Component**: A Rust crate which does not expose an FFI directly, but may be may be //! wrapped by one that does. These have a `crate-type` in their Cargo.toml (see //! https://doc.rust-lang.org/reference/linkage.html) of `lib`, and not `staticlib` or `cdylib` //! (Note that `lib` is the default if `crate-type` is not specified). Examples include the //! `fxa-client`, and `logins` crates. //! //! - **FFI Component**: A wrapper crate that takes a Rust component, and exposes an FFI from it. //! These typically have `ffi` in the name, and have `crate-type = ["lib", "staticlib", "cdylib"]` //! in their Cargo.toml. For example, the `fxa-client/ffi` and `logins/ffi` crates (note: //! paths are subject to change). When built, these produce a native library that is consumed by //! the "FFI Consumer". //! //! - **FFI Consumer**: This is a low level library, typically implemented in Kotlin (for Android) //! or Swift (for iOS), that exposes a memory-safe wrapper around the memory-unsafe C API produced //! by the FFI component. It's expected that the maintainers of the FFI Component and FFI Consumer //! be the same (or at least, the author of the consumer should be completely comfortable with the //! API exposed by, and code in the FFI component), since the code in these is extremely tightly //! coupled, and very easy to get wrong. //! //! Note that while there are three parts, there may be more than three libraries relevant here, for //! example there may be more than one FFI consumer (one for Android, one for iOS). //! //! ## Usage //! //! This library will typically be used in both the Rust component, and the FFI component, however //! it frequently will be an optional dependency in the Rust component that's only available when a //! feature flag (which the FFI component will always require) is used. //! //! The reason it's required inside the Rust component (and not solely in the FFI component, which //! would be nice), is so that types provided by that crate may implement the traits provided by //! this crate (this is because Rust does not allow crate `C` to implement a trait defined in crate //! `A` for a type defined in crate `B`). //! //! In general, examples should be provided for the most important types and functions //! ([`call_with_result`], [`IntoFfi`], //! [`ExternError`], etc), but you should also look at the code of //! consumers of this library. //! //! ### Usage in the Rust Component //! //! Inside the Rust component, you will implement: //! //! 1. [`IntoFfi`] for all types defined in that crate that you want to return //! over the FFI. For most common cases, the [`implement_into_ffi_by_pointer!`] and //! [`implement_into_ffi_by_json!`] macros will do the job here, however you can see that trait's //! documentation for discussion and examples of implementing it manually. //! //! 2. Conversion to [`ExternError`] for the error type(s) exposed by that //! rust component, that is, `impl From<MyError> for ExternError`. //! //! ### Usage in the FFI Component //! //! Inside the FFI component, you will use this library in a few ways: //! //! 1. Destructors will be exposed for each types that had [`implement_into_ffi_by_pointer!`] called //! on it (using [`define_box_destructor!`]), and a destructor for strings should be exposed as //! well, using [`define_string_destructor`] //! //! 2. The body of every / nearly every FFI function will be wrapped in either a //! [`call_with_result`] or [`call_with_output`]. //! //! This is required because if we `panic!` (e.g. from an `assert!`, `unwrap()`, `expect()`, from //! indexing past the end of an array, etc) across the FFI boundary, the behavior is undefined //! and in practice very weird things tend to happen (we aren't caught by the caller, since they //! don't have the same exception behavior as us). //! //! If you don't think your program (or possibly just certain calls) can handle panics, you may //! also use the versions of these functions in the [`abort_on_panic`] module, which //! do as their name suggest. //! //! Additionally, c strings that are passed in as arguments may be converted to rust strings using //! helpers such as [`rust_str_from_c`], [`opt_rust_str_from_c`], [`rust_string_from_c`], //! [`opt_rust_string_from_c`], etc. //! use std::{panic, thread}; mod error; pub mod handle_map; mod into_ffi; mod macros; mod string; pub use crate::error::*; pub use crate::into_ffi::*; pub use crate::macros::*; pub use crate::string::*; // We export most of the types from this, but some constants // (MAX_CAPACITY) don't make sense at the top level. pub use crate::handle_map::{ConcurrentHandleMap, Handle, HandleError, HandleMap}; /// Call a callback that returns a `Result<T, E>` while: /// /// - Catching panics, and reporting them to C via [`ExternError`]. /// - Converting `T` to a C-compatible type using [`IntoFfi`]. /// - Converting `E` to a C-compatible error via `Into<ExternError>`. /// /// This (or [`call_with_output`]) should be in the majority of the FFI functions, see the crate /// top-level docs for more info. /// /// If your function doesn't produce an error, you may use [`call_with_output`] instead, which /// doesn't require you return a Result. /// /// ## Example /// /// A few points about the following example: /// /// - This function *must* be unsafe, as it reads from a raw pointer. If you made it safe, then safe /// Rust could cause memory safety violations, which would be very bad! (However, FFI functions /// that don't read from raw pointers don't need to be marked `unsafe`! Sadly, most of ours need /// to take strings, and so we're out of luck...) /// /// - We need to mark it as `#[no_mangle] pub extern "C"`. /// /// - We prefix it with a unique name for the library (e.g. `mylib_`). Foreign functions are not /// namespaced, and symbol collisions can cause a large number of problems and subtle bugs, /// including memory safety issues in some cases. /// /// ```rust,no_run /// # use ffi_support::{ExternError, ErrorCode}; /// # use std::os::raw::c_char; /// /// # #[derive(Debug)] /// # struct BadEmptyString; /// # impl From<BadEmptyString> for ExternError { /// # fn from(e: BadEmptyString) -> Self { /// # ExternError::new_error(ErrorCode::new(1), "Bad empty string") /// # } /// # } /// /// #[no_mangle] /// pub unsafe extern "C" fn mylib_print_string( /// // Strings come in as a null terminated C string. This is certainly not ideal but it simplifies /// // the "FFI consumer" code, which is trickier code to get right, as it typically has poor /// // support for interacting with native libraries. /// thing_to_print: *const c_char, /// // Note that taking `&mut T` and `&T` is both allowed and encouraged, so long as `T: Sized`, /// // (e.g. it can't be a trait object, `&[T]`, a `&str`, etc). Also note that `Option<&T>` and /// // `Option<&mut T>` are also allowed, if you expect the caller to sometimes pass in null, but /// // that's the only case when it's currently to use `Option` in an argument list like this). /// error: &mut ExternError /// ) { /// // You should try to to do as little as possible outside the call_with_result, /// // to avoid a case where a panic occurs. /// ffi_support::call_with_result(error, || { /// let s = ffi_support::rust_str_from_c(thing_to_print); /// if s.len() == 0 { /// // This is a silly example! /// return Err(BadEmptyString); /// } /// println!("{}", s); /// Ok(()) /// }) /// } /// ``` /// /// ## Unwind (panic) Safety /// /// Internally, this function wraps it's argument in a /// [`AssertUnwindSafe`](std::panic::AssertUnwindSafe). That means it doesn't attempt to force you /// to mark things as [`UnwindSafe`](std::panic::UnwindSafe). Effectively, we're saying that every /// caller to this function is automatically panic safe, which is a lie. This is not ideal, but it's /// unclear what the right call here would be. /// /// To be clear, making the wrong choice here has no bearing on memory safety, unless there are /// exisiting memory safety holes in the code. That means by using `AssertUnwindSafe`, we end up in /// a position closer to the position we'd be in if we were working in a language with exceptions, /// which typically provides little-to-no assistance in terms of program correctness in the case of /// something `throw`ing. /// /// Anyway, if we *were* to require `F: UnwindSafe`, the implementer of the FFI component would need /// to use `AssertUnwindSafe` on every FFI binding that wraps a method that needs to call something /// on a `&mut T` (note that this is *not* true for `*mut T`, which we want to discourage). The use /// of this seems likely to be frequent enough in this FFI that I have an extremely hard time /// believing it would be used with consideration, so while the strategy of "assume everything is /// panic-safe" is clearly not great, it seems likely to be what happens anyway. /// /// There are, of course, other options: /// /// 1. Abort on panic (e.g. only expose the implementations in `abort_on_panic`), which is bad /// for obvious reasons, and seems even worse given our position as libraries. /// 2. Poison on panic (as [`std::sync::Mutex`] does, for example). This is a valid option, but /// seems wrong for all cases. /// 3. Re-initialize on panic (e.g. reopen the DB connection). /// /// 2 and 3 are promising, and allowing users of `ffi-support` to make these choices with a low /// amount of boilerplate is something we'd like to investigate in the future, but currently this /// is where we've landed. pub fn call_with_result<R, E, F>(out_error: &mut ExternError, callback: F) -> R::Value where F: FnOnce() -> Result<R, E>, // It would be nice to only require std::fmt::Debug if the `log_backtraces` // feature is on, but there's not really a way to do that in stable rust (at least // not in a way that wouldn't add more work for consumers of this lib). E: Into<ExternError> + std::fmt::Debug, R: IntoFfi, { call_with_result_impl(out_error, callback, false) } /// Call a callback that returns a `T` while: /// /// - Catching panics, and reporting them to C via [`ExternError`] /// - Converting `T` to a C-compatible type using [`IntoFfi`] /// /// Note that you still need to provide an [`ExternError`] to this function, to report panics. /// /// See [`call_with_result`] if you'd like to return a `Result<T, E>` (Note: `E` must /// be convertible to [`ExternError`]). /// /// This (or [`call_with_result`]) should be in the majority of the FFI functions, see /// the crate top-level docs for more info. pub fn call_with_output<R, F>(out_error: &mut ExternError, callback: F) -> R::Value where F: FnOnce() -> R, R: IntoFfi, { // We need something that's `Into<ExternError>`, even though we never return it, so just use // `ExternError` itself. call_with_result(out_error, || -> Result<_, ExternError> { Ok(callback()) }) } fn call_with_result_impl<R, E, F>( out_error: &mut ExternError, callback: F, abort_on_panic: bool, ) -> R::Value where F: FnOnce() -> Result<R, E>, E: Into<ExternError> + std::fmt::Debug, R: IntoFfi, { *out_error = ExternError::success(); // It's not ideal to handle unwind safety this way, however I'm not sure we can reasonably // expect the FFI code to think about this in a meaningful way. That said, you cannot cause // memory safety violations by breaking unwind safety (note that this function is not `unsafe`), // short of bugs in unsafe code elsewhere, so this isn't the *worst* thing we could be doing. let res: thread::Result<(ExternError, R::Value)> = panic::catch_unwind(panic::AssertUnwindSafe(|| { init_backtraces_once(); match callback() { Ok(v) => (ExternError::default(), v.into_ffi_value()), Err(e) => (e.into(), R::ffi_default()), } })); match res { Ok((err, o)) => { *out_error = err; o } Err(e) => { log::error!("Caught a panic calling rust code: {:?}", e); if abort_on_panic { std::process::abort(); } *out_error = e.into(); R::ffi_default() } } } /// This module exists just to expose a variant of [`call_with_result`] and [`call_with_output`] /// that aborts, instead of unwinding, on panic. pub mod abort_on_panic { use super::*; /// Same as the root `call_with_result`, but aborts on panic instead of unwinding. See the /// `call_with_result` documentation for more. pub fn call_with_result<R, E, F>(out_error: &mut ExternError, callback: F) -> R::Value where F: FnOnce() -> Result<R, E>, E: Into<ExternError> + std::fmt::Debug, R: IntoFfi, { super::call_with_result_impl(out_error, callback, true) } /// Same as the root `call_with_output`, but aborts on panic instead of unwinding. As a result, /// it doesn't require a [`ExternError`] out argument. See the `call_with_output` documentation /// for more info. pub fn call_with_output<R, F>(callback: F) -> R::Value where F: FnOnce() -> R, R: IntoFfi, { let mut dummy = ExternError::success(); super::call_with_result_impl( &mut dummy, || -> Result<_, ExternError> { Ok(callback()) }, true, ) } } #[cfg(feature = "log_backtraces")] fn init_backtraces_once() { use std::sync::{Once, ONCE_INIT}; static INIT_BACKTRACES: Once = ONCE_INIT; INIT_BACKTRACES.call_once(move || { // Turn on backtraces for failure, if it's still listening. std::env::set_var("RUST_BACKTRACE", "1"); // Turn on a panic hook which logs both backtraces and the panic // "Location" (file/line). We do both in case we've been stripped, // ). std::panic::set_hook(Box::new(move |panic_info| { let (file, line) = if let Some(loc) = panic_info.location() { (loc.file(), loc.line()) } else { // Apparently this won't happen but rust has reserved the // ability to start returning None from location in some cases // in the future. ("<unknown>", 0) }; log::error!("### Rust `panic!` hit at file '{}', line {}", file, line); // We could use failure for failure::Backtrace (and we enable RUST_BACKTRACE // to opt-in to backtraces on failure errors if possible), however: // - we don't already have a failure dependency (one is likely inevitable, // and all our clients do, so this doesn't matter) // - `failure` only checks the RUST_BACKTRACE variable once, and we could have errors // before this. So we just use the backtrace crate directly. log::error!(" Complete stack trace:\n{:?}", backtrace::Backtrace::new()); })); }); } #[cfg(not(feature = "log_backtraces"))] fn init_backtraces_once() {} /// ByteBuffer is a struct that represents an array of bytes to be sent over the FFI boundaries. /// There are several cases when you might want to use this, but the primary one for us /// is for returning protobuf-encoded data to Swift and Java. The type is currently rather /// limited (implementing almost no functionality), however in the future it may be /// more expanded. /// /// ## Caveats /// /// Note that the order of the fields is `len` (an i64) then `data` (a `*mut u8`), getting /// this wrong on the other side of the FFI will cause memory corruption and crashes. /// `i64` is used for the length instead of `u64` and `usize` because JNA has interop /// issues with both these types. /// /// ByteBuffer does not implement Drop. This is intentional. Memory passed into it will /// be leaked if it is not explicitly destroyed by calling [`ByteBuffer::destroy`]. This /// is because in the future, we may allow it's use for passing data into Rust code. /// ByteBuffer assuming ownership of the data would make this a problem. /// /// Note that alling `destroy` manually is not typically needed or recommended, /// and instead you should use [`define_bytebuffer_destructor!`]. /// /// ## Layout/fields /// /// This struct's field are not `pub` (mostly so that we can soundly implement `Send`, but also so /// that we can verify rust users are constructing them appropriately), the fields, their types, and /// their order are *very much* a part of the public API of this type. Consumers on the other side /// of the FFI will need to know its layout. /// /// If this were a C struct, it would look like /// /// ```c,no_run /// struct ByteBuffer { /// int64_t len; /// uint8_t *data; // note: nullable /// }; /// ``` /// /// In rust, there are two fields, in this order: `len: i64`, and `data: *mut u8`. /// /// ### Description of fields /// /// `data` is a pointer to an array of `len` bytes. Not that data can be a null pointer and therefore /// should be checked. /// /// The bytes array is allocated on the heap and must be freed on it as well. Critically, if there /// are multiple rust packages using being used in the same application, it *must be freed on the /// same heap that allocated it*, or you will corrupt both heaps. /// /// Typically, this object is managed on the other side of the FFI (on the "FFI consumer"), which /// means you must expose a function to release the resources of `data` which can be done easily /// using the [`define_bytebuffer_destructor!`] macro provided by this crate. #[repr(C)] pub struct ByteBuffer { len: i64, data: *mut u8, } impl From<Vec<u8>> for ByteBuffer { #[inline] fn from(bytes: Vec<u8>) -> Self { Self::from_vec(bytes) } } impl ByteBuffer { /// Creates a `ByteBuffer` instance from a `Vec` instance. /// /// The contents of the vector will not be dropped. Instead, `destroy` must /// be called later to reclaim this memory or it will be leaked. /// /// ## Caveats /// /// This will panic if the buffer length (`usize`) cannot fit into a `i64`. #[inline] pub fn from_vec(bytes: Vec<u8>) -> Self { let mut buf = bytes.into_boxed_slice(); let data = buf.as_mut_ptr(); let len = buf.len(); assert!( len == (len as i64) as usize, "buffer length cannot fit into a i64." ); std::mem::forget(buf); Self { data, len: len as i64, } } /// Reclaim memory stored in this ByteBuffer. /// /// You typically should not call this manually, and instead expose a /// function that does so via [`define_bytebuffer_destructor!`]. /// /// ## Caveats /// /// This is safe so long as the buffer is empty, or the data was allocated /// by Rust code, e.g. this is a ByteBuffer created by /// `ByteBuffer::from_vec` or `Default::default`. /// /// If the ByteBuffer were passed into Rust (which you shouldn't do, since /// theres no way to see the data in Rust currently), then calling `destroy` /// is fundamentally broken. #[inline] pub fn destroy(self) { if !self.data.is_null() { unsafe { drop(Vec::from_raw_parts( self.data, self.len as usize, self.len as usize, )); } } } } impl Default for ByteBuffer { #[inline] fn default() -> Self { Self { len: 0 as i64, data: std::ptr::null_mut(), } } }