feat: Added v0.3.3 backend for zkvm

src/backends.rs

@@ -7,8 +7,14 @@
 //! The function MUST NOT ever write uninitialized bytes into `dest`,
 //! regardless of what value it returns.
 
+mod zkvm;
+
 cfg_if! {
-    if #[cfg(getrandom_backend = "custom")] {
+    if #[cfg(target_arch = "riscv32")] {
+        // Auto-detect SP1 zkVM target
+        pub use zkvm::*;
+    }
+    else if #[cfg(getrandom_backend = "custom")] {
         mod custom;
         pub use custom::*;
     } else if #[cfg(getrandom_backend = "linux_getrandom")] {

src/backends/zkvm.rs

@@ -0,0 +1,41 @@
+// 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.
+
+//! zkVM implementation using custom backend
+use crate::Error;
+use core::mem::MaybeUninit;
+
+// Main function for getrandom 0.3.3 API - note the MaybeUninit
+#[allow(dead_code)]
+pub fn fill_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
+    extern "Rust" { // Call SP1's version
+        fn __getrandom_v03_custom(dest: *mut u8, len: usize) -> Result<(), Error>;
+    }
+
+    unsafe {
+        __getrandom_v03_custom(dest.as_mut_ptr() as *mut u8, dest.len())
+    }
+}
+
+#[allow(dead_code)]
+pub fn inner_u32() -> Result<u32, Error> {
+    let mut buf = [MaybeUninit::<u8>::uninit(); 4];
+    fill_inner(&mut buf)?;
+    // Safe because fill_inner initialized the bytes
+    let buf: [u8; 4] = unsafe { core::mem::transmute(buf) };
+    Ok(u32::from_ne_bytes(buf))
+}
+
+#[allow(dead_code)]
+pub fn inner_u64() -> Result<u64, Error> {
+    let mut buf = [MaybeUninit::<u8>::uninit(); 8];
+    fill_inner(&mut buf)?;
+    // Safe because fill_inner initialized the bytes
+    let buf: [u8; 8] = unsafe { core::mem::transmute(buf) };
+    Ok(u64::from_ne_bytes(buf))
+}