getrandom/src/lib.rs

// Copyright 2019 Developers of the Rand project.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! Interface to the operating system's random number generator.
//!
//! # Supported targets
//!
//! | Target            | Target Triple      | Implementation
//! | ----------------- | ------------------ | --------------
//! | Linux, Android    | `*‑linux‑*`        | [`getrandom`][1] system call if available, otherwise [`/dev/urandom`][2] after successfully polling `/dev/random`
//! | Windows           | `*‑windows‑*`      | [`BCryptGenRandom`]
//! | macOS             | `*‑apple‑darwin`   | [`getentropy`][3] if available, otherwise [`/dev/random`][4] (identical to `/dev/urandom`)
//! | iOS               | `*‑apple‑ios`      | [`SecRandomCopyBytes`]
//! | FreeBSD           | `*‑freebsd`        | [`getrandom`][5] if available, otherwise [`kern.arandom`][6]
//! | OpenBSD           | `*‑openbsd`        | [`getentropy`][7]
//! | NetBSD            | `*‑netbsd`         | [`kern.arandom`][8]
//! | Dragonfly BSD     | `*‑dragonfly`      | [`getrandom`][9] if available, otherwise [`/dev/random`][10]
//! | Solaris, illumos  | `*‑solaris`, `*‑illumos` | [`getrandom`][11] if available, otherwise [`/dev/random`][12]
//! | Fuchsia OS        | `*‑fuchsia`        | [`cprng_draw`]
//! | Redox             | `*‑redox`          | `/dev/urandom`
//! | Haiku             | `*‑haiku`          | `/dev/random` (identical to `/dev/urandom`)
//! | Hermit            | `x86_64-*-hermit`  | [`RDRAND`]
//! | SGX               | `x86_64‑*‑sgx`     | [`RDRAND`]
//! | VxWorks           | `*‑wrs‑vxworks‑*`  | `randABytes` after checking entropy pool initialization with `randSecure`
//! | ESP-IDF           | `*‑espidf`         | [`esp_fill_random`]
//! | Emscripten        | `*‑emscripten`     | `/dev/random` (identical to `/dev/urandom`)
//! | WASI              | `wasm32‑wasi`      | [`random_get`]
//! | Web Browser       | `wasm32‑*‑unknown` | [`Crypto.getRandomValues`], see [WebAssembly support]
//! | Node.js           | `wasm32‑*‑unknown` | [`crypto.randomBytes`], see [WebAssembly support]
//! | SOLID             | `*-kmc-solid_*`    | `SOLID_RNG_SampleRandomBytes`
//!
//! There is no blanket implementation on `unix` targets that reads from
//! `/dev/urandom`. This ensures all supported targets are using the recommended
//! interface and respect maximum buffer sizes.
//!
//! Pull Requests that add support for new targets to `getrandom` are always welcome.
//!
//! ## Unsupported targets
//!
//! By default, `getrandom` will not compile on unsupported targets, but certain
//! features allow a user to select a "fallback" implementation if no supported
//! implementation exists.
//!
//! All of the below mechanisms only affect unsupported
//! targets. Supported targets will _always_ use their supported implementations.
//! This prevents a crate from overriding a secure source of randomness
//! (either accidentally or intentionally).
//!
//! ### RDRAND on x86
//!
//! *If the `rdrand` Cargo feature is enabled*, `getrandom` will fallback to using
//! the [`RDRAND`] instruction to get randomness on `no_std` `x86`/`x86_64`
//! targets. This feature has no effect on other CPU architectures.
//!
//! ### WebAssembly support
//!
//! This crate fully supports the
//! [`wasm32-wasi`](https://github.com/CraneStation/wasi) and
//! [`wasm32-unknown-emscripten`](https://www.hellorust.com/setup/emscripten/)
//! targets. However, the `wasm32-unknown-unknown` target (i.e. the target used
//! by `wasm-pack`) is not automatically
//! supported since, from the target name alone, we cannot deduce which
//! JavaScript interface is in use (or if JavaScript is available at all).
//!
//! Instead, *if the `js` Cargo feature is enabled*, this crate will assume
//! that you are building for an environment containing JavaScript, and will
//! call the appropriate methods. Both web browser (main window and Web Workers)
//! and Node.js environments are supported, invoking the methods
//! [described above](#supported-targets) using the
//! [wasm-bindgen](https://github.com/rust-lang/rust-bindgen) toolchain.
//!
//! This feature has no effect on targets other than `wasm32-unknown-unknown`.
//!
//! ### Custom implementations
//!
//! The [`register_custom_getrandom!`] macro allows a user to mark their own
//! function as the backing implementation for [`getrandom`]. See the macro's
//! documentation for more information about writing and registering your own
//! custom implementations.
//!
//! Note that registering a custom implementation only has an effect on targets
//! that would otherwise not compile. Any supported targets (including those
//! using `rdrand` and `js` Cargo features) continue using their normal
//! implementations even if a function is registered.
//!
//! ### Indirect Dependencies
//!
//! If `getrandom` is not a direct dependency of your crate, you can still
//! enable any of the above fallback behaviors by enabling the relevant
//! feature in your root crate's `Cargo.toml`:
//! ```toml
//! [dependencies]
//! getrandom = { version = "0.2", features = ["js"] }
//! ```
//!
//! ## Early boot
//!
//! Sometimes, early in the boot process, the OS has not collected enough
//! entropy to securely seed its RNG. This is especially common on virtual
//! machines, where standard "random" events are hard to come by.
//!
//! Some operating system interfaces always block until the RNG is securely
//! seeded. This can take anywhere from a few seconds to more than a minute.
//! A few (Linux, NetBSD and Solaris) offer a choice between blocking and
//! getting an error; in these cases, we always choose to block.
//!
//! On Linux (when the `getrandom` system call is not available), reading from
//! `/dev/urandom` never blocks, even when the OS hasn't collected enough
//! entropy yet. To avoid returning low-entropy bytes, we first poll
//! `/dev/random` and only switch to `/dev/urandom` once this has succeeded.
//!
//! ## Error handling
//!
//! We always choose failure over returning insecure "random" bytes. In general,
//! on supported platforms, failure is highly unlikely, though not impossible.
//! If an error does occur, then it is likely that it will occur on every call to
//! `getrandom`, hence after the first successful call one can be reasonably
//! confident that no errors will occur.
//!
//! [1]: http://man7.org/linux/man-pages/man2/getrandom.2.html
//! [2]: http://man7.org/linux/man-pages/man4/urandom.4.html
//! [3]: https://www.unix.com/man-page/mojave/2/getentropy/
//! [4]: https://www.unix.com/man-page/mojave/4/random/
//! [5]: https://www.freebsd.org/cgi/man.cgi?query=getrandom&manpath=FreeBSD+12.0-stable
//! [6]: https://www.freebsd.org/cgi/man.cgi?query=random&sektion=4
//! [7]: https://man.openbsd.org/getentropy.2
//! [8]: https://man.netbsd.org/sysctl.7
//! [9]: https://leaf.dragonflybsd.org/cgi/web-man?command=getrandom
//! [10]: https://leaf.dragonflybsd.org/cgi/web-man?command=random&section=4
//! [11]: https://docs.oracle.com/cd/E88353_01/html/E37841/getrandom-2.html
//! [12]: https://docs.oracle.com/cd/E86824_01/html/E54777/random-7d.html
//!
//! [`BCryptGenRandom`]: https://docs.microsoft.com/en-us/windows/win32/api/bcrypt/nf-bcrypt-bcryptgenrandom
//! [`Crypto.getRandomValues`]: https://www.w3.org/TR/WebCryptoAPI/#Crypto-method-getRandomValues
//! [`RDRAND`]: https://software.intel.com/en-us/articles/intel-digital-random-number-generator-drng-software-implementation-guide
//! [`SecRandomCopyBytes`]: https://developer.apple.com/documentation/security/1399291-secrandomcopybytes?language=objc
//! [`cprng_draw`]: https://fuchsia.dev/fuchsia-src/zircon/syscalls/cprng_draw
//! [`crypto.randomBytes`]: https://nodejs.org/api/crypto.html#crypto_crypto_randombytes_size_callback
//! [`esp_fill_random`]: https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/system/random.html#_CPPv415esp_fill_randomPv6size_t
//! [`random_get`]: https://github.com/WebAssembly/WASI/blob/main/phases/snapshot/docs.md#-random_getbuf-pointeru8-buf_len-size---errno
//! [WebAssembly support]: #webassembly-support

#![doc(
    html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk.png",
    html_favicon_url = "https://www.rust-lang.org/favicon.ico",
    html_root_url = "https://docs.rs/getrandom/0.2.5"
)]
#![no_std]
#![warn(rust_2018_idioms, unused_lifetimes, missing_docs)]
#![cfg_attr(docsrs, feature(doc_cfg))]

#[macro_use]
extern crate cfg_if;

mod error;
mod util;
// To prevent a breaking change when targets are added, we always export the
// register_custom_getrandom macro, so old Custom RNG crates continue to build.
#[cfg(feature = "custom")]
mod custom;
#[cfg(feature = "std")]
mod error_impls;

pub use crate::error::Error;

// System-specific implementations.
//
// These should all provide getrandom_inner with the same signature as getrandom.
cfg_if! {
    if #[cfg(any(target_os = "emscripten", target_os = "haiku",
                 target_os = "redox"))] {
        mod util_libc;
        #[path = "use_file.rs"] mod imp;
    } else if #[cfg(any(target_os = "android", target_os = "linux"))] {
        mod util_libc;
        mod use_file;
        #[path = "linux_android.rs"] mod imp;
    } else if #[cfg(any(target_os = "illumos", target_os = "solaris"))] {
        mod util_libc;
        mod use_file;
        #[path = "solaris_illumos.rs"] mod imp;
    } else if #[cfg(any(target_os = "freebsd", target_os = "netbsd"))] {
        mod util_libc;
        #[path = "bsd_arandom.rs"] mod imp;
    } else if #[cfg(target_os = "dragonfly")] {
        mod util_libc;
        mod use_file;
        #[path = "dragonfly.rs"] mod imp;
    } else if #[cfg(target_os = "fuchsia")] {
        #[path = "fuchsia.rs"] mod imp;
    } else if #[cfg(target_os = "ios")] {
        #[path = "ios.rs"] mod imp;
    } else if #[cfg(target_os = "macos")] {
        mod util_libc;
        mod use_file;
        #[path = "macos.rs"] mod imp;
    } else if #[cfg(target_os = "openbsd")] {
        mod util_libc;
        #[path = "openbsd.rs"] mod imp;
    } else if #[cfg(target_os = "wasi")] {
        #[path = "wasi.rs"] mod imp;
    } else if #[cfg(all(target_arch = "x86_64", target_os = "hermit"))] {
        #[path = "rdrand.rs"] mod imp;
    } else if #[cfg(target_os = "vxworks")] {
        mod util_libc;
        #[path = "vxworks.rs"] mod imp;
    } else if #[cfg(target_os = "solid_asp3")] {
        #[path = "solid.rs"] mod imp;
    } else if #[cfg(target_os = "espidf")] {
        #[path = "espidf.rs"] mod imp;
    } else if #[cfg(windows)] {
        #[path = "windows.rs"] mod imp;
    } else if #[cfg(all(target_arch = "x86_64", target_env = "sgx"))] {
        #[path = "rdrand.rs"] mod imp;
    } else if #[cfg(all(feature = "rdrand",
                        any(target_arch = "x86_64", target_arch = "x86")))] {
        #[path = "rdrand.rs"] mod imp;
    } else if #[cfg(all(feature = "js",
                        target_arch = "wasm32", target_os = "unknown"))] {
        #[path = "js.rs"] mod imp;
    } else if #[cfg(feature = "custom")] {
        use custom as imp;
    } else if #[cfg(all(target_arch = "wasm32", target_os = "unknown"))] {
        compile_error!("the wasm32-unknown-unknown target is not supported by \
                        default, you may need to enable the \"js\" feature. \
                        For more information see: \
                        https://docs.rs/getrandom/#webassembly-support");
    } else {
        compile_error!("target is not supported, for more information see: \
                        https://docs.rs/getrandom/#unsupported-targets");
    }
}

/// Fill `dest` with random bytes from the system's preferred random number
/// source.
///
/// This function returns an error on any failure, including partial reads. We
/// make no guarantees regarding the contents of `dest` on error. If `dest` is
/// empty, `getrandom` immediately returns success, making no calls to the
/// underlying operating system.
///
/// Blocking is possible, at least during early boot; see module documentation.
///
/// In general, `getrandom` will be fast enough for interactive usage, though
/// significantly slower than a user-space CSPRNG; for the latter consider
/// [`rand::thread_rng`](https://docs.rs/rand/*/rand/fn.thread_rng.html).
pub fn getrandom(dest: &mut [u8]) -> Result<(), Error> {
    if dest.is_empty() {
        return Ok(());
    }
    imp::getrandom_inner(dest)
}