103 lines
3.9 KiB
Rust
103 lines
3.9 KiB
Rust
// Copyright 2018 Developers of the Rand project.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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//! An implementation which calls out to an externally defined function.
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use crate::Error;
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use core::num::NonZeroU32;
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/// Register a function to be invoked by `getrandom` on unsupported targets.
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///
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/// ## Writing a custom `getrandom` implementation
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///
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/// The function to register must have the same signature as
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/// [`getrandom::getrandom`](crate::getrandom). The function can be defined
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/// wherever you want, either in root crate or a dependent crate.
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///
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/// For example, if we wanted a `failure-getrandom` crate containing an
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/// implementation that always fails, we would first depend on `getrandom`
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/// (for the [`Error`] type) in `failure-getrandom/Cargo.toml`:
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/// ```toml
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/// [dependencies]
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/// getrandom = "0.2"
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/// ```
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/// Note that the crate containing this function does **not** need to enable the
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/// `"custom"` Cargo feature.
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///
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/// Next, in `failure-getrandom/src/lib.rs`, we define our function:
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/// ```rust
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/// use core::num::NonZeroU32;
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/// use getrandom::Error;
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///
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/// // Some application-specific error code
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/// const MY_CUSTOM_ERROR_CODE: u32 = Error::CUSTOM_START + 42;
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/// pub fn always_fail(buf: &mut [u8]) -> Result<(), Error> {
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/// let code = NonZeroU32::new(MY_CUSTOM_ERROR_CODE).unwrap();
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/// Err(Error::from(code))
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/// }
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/// ```
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///
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/// ## Registering a custom `getrandom` implementation
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///
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/// Functions can only be registered in the root binary crate. Attempting to
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/// register a function in a non-root crate will result in a linker error.
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/// This is similar to
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/// [`#[panic_handler]`](https://doc.rust-lang.org/nomicon/panic-handler.html) or
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/// [`#[global_allocator]`](https://doc.rust-lang.org/edition-guide/rust-2018/platform-and-target-support/global-allocators.html),
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/// where helper crates define handlers/allocators but only the binary crate
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/// actually _uses_ the functionality.
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///
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/// To register the function, we first depend on `failure-getrandom` _and_
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/// `getrandom` in `Cargo.toml`:
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/// ```toml
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/// [dependencies]
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/// failure-getrandom = "0.1"
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/// getrandom = { version = "0.2", features = ["custom"] }
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/// ```
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///
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/// Then, we register the function in `src/main.rs`:
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/// ```rust
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/// # mod failure_getrandom { pub fn always_fail(_: &mut [u8]) -> Result<(), getrandom::Error> { unimplemented!() } }
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/// use failure_getrandom::always_fail;
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/// use getrandom::register_custom_getrandom;
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///
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/// register_custom_getrandom!(always_fail);
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/// ```
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///
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/// Now any user of `getrandom` (direct or indirect) on this target will use the
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/// registered function. As noted in the
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/// [top-level documentation](index.html#custom-implementations) this
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/// registration only has an effect on unsupported targets.
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#[macro_export]
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#[cfg_attr(docsrs, doc(cfg(feature = "custom")))]
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macro_rules! register_custom_getrandom {
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($path:path) => {
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// We use an extern "C" function to get the guarantees of a stable ABI.
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#[no_mangle]
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extern "C" fn __getrandom_custom(dest: *mut u8, len: usize) -> u32 {
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let f: fn(&mut [u8]) -> Result<(), $crate::Error> = $path;
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let slice = unsafe { ::core::slice::from_raw_parts_mut(dest, len) };
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match f(slice) {
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Ok(()) => 0,
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Err(e) => e.code().get(),
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}
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}
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};
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}
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#[allow(dead_code)]
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pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
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extern "C" {
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fn __getrandom_custom(dest: *mut u8, len: usize) -> u32;
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}
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let ret = unsafe { __getrandom_custom(dest.as_mut_ptr(), dest.len()) };
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match NonZeroU32::new(ret) {
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None => Ok(()),
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Some(code) => Err(Error::from(code)),
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}
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}
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