zkvm: add guest support for getrandom (risc0#445)

This provides a zkvm with the getrandom functionality builtin to the guest
crate. This allows the guest to use the getrandom crate without any
customization.

risc0/zkvm/Cargo.toml

@@ -23,6 +23,7 @@ protobuf-src = { version = "1.1", optional = true }
 anyhow = { version = "1.0", default-features = false }
 bytemuck = "1.12"
 cfg-if = "1.0"
+getrandom = { version = "0.2", features = ["custom"] }
 hex = { version = "0.4.3", default-features = false, features = ["alloc"] }
 risc0-circuit-rv32im = { workspace = true }
 risc0-core = { workspace = true }

risc0/zkvm/methods/guest/Cargo.toml

@@ -19,6 +19,7 @@ risc0-zkvm = { path = "../..", default-features = false }
 risc0-zkvm-methods = { path = "..", default-features = false }
 risc0-zkvm-platform = { path = "../../platform" }
 serde = { version = "1.0", default-features = false, features = ["derive"] }
+getrandom = "0.2"
 
 [profile.release]
 lto = true

risc0/zkvm/methods/guest/src/bin/multi_test.rs

@@ -21,11 +21,12 @@ extern crate alloc;
 use alloc::vec;
 use core::arch::asm;
 
+use getrandom::getrandom;
 use risc0_zeroio::deserialize::Deserialize;
 use risc0_zkp::core::sha::{testutil::test_sha_impl, Digest, Sha256};
 use risc0_zkvm::guest::{env, memory_barrier, sha};
 use risc0_zkvm_methods::multi_test::{MultiTestSpec, MultiTestSpecRef, SYS_MULTI_TEST};
-use risc0_zkvm_platform::syscall::{nr::SYS_INITIAL_INPUT, sys_rand, sys_read};
+use risc0_zkvm_platform::syscall::{nr::SYS_INITIAL_INPUT, sys_read};
 
 risc0_zkvm::entry!(main);
 
@@ -117,16 +118,11 @@ pub fn main() {
             }
         }
         MultiTestSpecRef::DoRandom(_) => {
-            // TODO: replace this code with getrandom after merging code to getrandom crate
-            let mut buf = [0u32; 6];
-            unsafe {
-                sys_rand(buf.as_mut_ptr(), buf.len());
-            }
-            let mut result_buf = vec![0u8; 5];
-            result_buf.clone_from_slice(&bytemuck::cast_slice(buf.as_slice())[..5]);
-            assert_ne!(result_buf, vec![0u8; result_buf.len()]);
-
-            env::commit_slice(&result_buf);
+            // Test random number generation in the zkvm
+            let mut rand_buf = [0u8; 7];
+            getrandom(rand_buf.as_mut_slice()).expect("random number generation failed");
+            env::commit_slice(&rand_buf);
+            assert_ne!(rand_buf, vec![0u8; rand_buf.len()].as_slice());
         }
         MultiTestSpecRef::SysRead(sysread) => {
             let mut orig = sysread.orig().to_vec();

risc0/zkvm/src/guest/mod.rs

@@ -58,10 +58,34 @@ pub mod sha;
 
 use core::{arch::asm, mem, ptr};
 
-use risc0_zkvm_platform::{syscall::nr::SYS_PANIC, syscall::sys_panic};
+use getrandom::register_custom_getrandom;
+use getrandom::Error;
+use risc0_zkvm_platform::WORD_SIZE;
+use risc0_zkvm_platform::{
+    syscall::nr::SYS_PANIC,
+    syscall::{sys_panic, sys_rand},
+};
 
 pub use crate::entry;
 
+/// This is a getrandom handler for the zkvm. It's intended to hook into a
+/// getrandom crate or a depdent of the getrandom crate used by the guest code.
+pub fn zkvm_getrandom(dest: &mut [u8]) -> Result<(), Error> {
+    if dest.is_empty() {
+        return Ok(());
+    }
+
+    let words = (dest.len() + WORD_SIZE - 1) / WORD_SIZE;
+    let mut buf = ::alloc::vec![0u32; words];
+    unsafe {
+        sys_rand(buf.as_mut_ptr(), words);
+    }
+    dest.clone_from_slice(&bytemuck::cast_slice(buf.as_slice())[..dest.len()]);
+    Ok(())
+}
+
+register_custom_getrandom!(zkvm_getrandom);
+
 #[cfg(target_os = "zkvm")]
 core::arch::global_asm!(include_str!("memset.s"));
 #[cfg(target_os = "zkvm")]