Enable support for espidf (esp32 family of microcontrollers) (#245)

src/espidf.rs

@@ -0,0 +1,26 @@
+// Copyright 2021 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.
+
+//! Implementation for ESP-IDF
+use crate::Error;
+use core::ffi::c_void;
+
+extern "C" {
+    fn esp_fill_random(buf: *mut c_void, len: usize) -> u32;
+}
+
+pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    // Not that NOT enabling WiFi, BT, or the voltage noise entropy source (via `bootloader_random_enable`)
+    // will cause ESP-IDF to return pseudo-random numbers based on the voltage noise entropy, after the initial boot process:
+    // https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/system/random.html
+    //
+    // However tracking if some of these entropy sources is enabled is way too difficult to implement here
+    unsafe { esp_fill_random(dest.as_mut_ptr().cast(), dest.len()) };
+
+    Ok(())
+}

src/lib.rs

@@ -27,6 +27,7 @@
 //! | Hermit            | `x86_64-*-hermit`  | [`RDRAND`][18]
 //! | SGX               | `x86_64‑*‑sgx`     | [RDRAND][18]
 //! | VxWorks           | `*‑wrs‑vxworks‑*`  | `randABytes` after checking entropy pool initialization with `randSecure`
+//! | ESP-IDF           | `*‑espidf`         | [`esp_fill_random()`][23]
 //! | Emscripten        | `*‑emscripten`     | `/dev/random` (identical to `/dev/urandom`)
 //! | WASI              | `wasm32‑wasi`      | [`random_get`][17]
 //! | Web Browser       | `wasm32‑*‑unknown` | [`Crypto.getRandomValues()`][14], see [WebAssembly support][16]
@@ -142,6 +143,7 @@
 //! [20]: https://www.unix.com/man-page/mojave/4/random/
 //! [21]: https://www.freebsd.org/cgi/man.cgi?query=getrandom&manpath=FreeBSD+12.0-stable
 //! [22]: https://leaf.dragonflybsd.org/cgi/web-man?command=getrandom
+//! [23]: https://docs.espressif.com/projects/esp-idf/en/release-v4.1/api-reference/system/system.html?highlight=esp_fill_random#_CPPv415esp_fill_randomPv6size_t
 
 #![doc(
     html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk.png",
@@ -209,6 +211,8 @@ cfg_if! {
         #[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"))] {