Add support for Custom RNGS

Author: josephlr

Cargo.toml

@@ -30,5 +30,7 @@ wasi = "0.7"
 
 [features]
 std = []
+# Feature to enable custom RNG implementations
+custom = []
 # Unstable feature to support being a libstd dependency
 rustc-dep-of-std = ["compiler_builtins", "core"]

src/custom.rs

@@ -0,0 +1,18 @@
+// Copyright 2018 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.
+
+//! An implementation which calls out to an externally defined function.
+use crate::Error;
+
+pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    extern "Rust" {
+        #[allow(improper_ctypes)] // See rust-lang/rust#64593
+        fn __getrandom_custom(dest: &mut [u8]) -> Result<(), Error>;
+    }
+    unsafe { __getrandom_custom(dest) }
+}

src/lib.rs

@@ -230,6 +230,8 @@ cfg_if! {
                   target_env = "sgx",
               )))] {
         #[path = "rdrand.rs"] mod imp;
+    } else if #[cfg(feature = "custom")] {
+        #[path = "custom.rs"] mod imp;
     } else {
         compile_error!("\
             target is not supported, for more information see: \
@@ -238,6 +240,23 @@ cfg_if! {
     }
 }
 
+/// Reister a function to be invoked by `getrandom` on custom targets. This
+/// function will only be invoked on targets not supported by `getrandom`. This
+/// prevents crate dependancies from either inadvertantly or maliciously
+/// overriding the secure RNG implementations in `getrandom`.
+///
+/// *This API requires the following crate features to be activated: `custom`*
+#[macro_export]
+#[cfg(feature = "custom")]
+macro_rules! register_custom_getrandom {
+    ($path:path) => {
+        #[no_mangle]
+        fn __getrandom_custom(dest: &mut [u8]) -> Result<(), Error> {
+            $path(dest)
+        }
+    };
+}
+
 /// Fill `dest` with random bytes from the system's preferred random number
 /// source.
 ///