Add getrandom_uninit_slice(dest: &mut [MaybeUninit<u8>]) -> .... (#291

)

* Add `getrandom_uninit(dest: &mut [MaybeUninit<u8>]) -> ...`.

Add a public API for filling an `&mut [MaybeUninit<u8>]`. This will primarily
serve as the building block for more typeful APIs for constructing random
arrays.

Increase the MSRV to 1.36, as `MaybeUninit` was added in that release.

Fixes #226.

* Revert testing changes

Signed-off-by: Joe Richey <joerichey@google.com>

* Allow rdrand tests to work with new implementation

Signed-off-by: Joe Richey <joerichey@google.com>

* Add Additional benchmarks and buffer size

Signed-off-by: Joe Richey <joerichey@google.com>

* Use pointer casts instead of transmute

Signed-off-by: Joe Richey <joerichey@google.com>

* Avoid initializing the buffer in `getrandom_uninit` benchmarks.

* Benchmarks: Consume the result in `black_box`.

Signed-off-by: Joe Richey <joerichey@google.com>
Co-authored-by: Joe Richey <joerichey@google.com>

.github/workflows/tests.yml

@@ -44,7 +44,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, windows-latest]
-        toolchain: [nightly, beta, stable, 1.34]
+        toolchain: [nightly, beta, stable, 1.36]
         # Only Test macOS on stable to reduce macOS CI jobs
         include:
           - os: macos-latest

CHANGELOG.md

@@ -4,6 +4,15 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+### Added
+- `getrandom_uninit` [#291]
+
+### Breaking Changes
+- Update MSRV to 1.36 [#291]
+
+[#291]: https://github.com/rust-random/getrandom/pull/291
+
 ## [0.2.8] - 2022-10-20
 ### Changed
 - The [Web Cryptography API] will now be preferred on `wasm32-unknown-unknown`

README.md

@@ -52,7 +52,7 @@ crate features, WASM support and Custom RNGs see the
 
 ## Minimum Supported Rust Version
 
-This crate requires Rust 1.34.0 or later.
+This crate requires Rust 1.36.0 or later.
 
 # License
 

benches/mod.rs

@@ -1,94 +1,64 @@
 #![feature(test)]
-extern crate test;
-
-use std::{
-    alloc::{alloc_zeroed, dealloc, Layout},
-    ptr::NonNull,
-};
-
-// AlignedBuffer is like a Box<[u8; N]> except that it is always N-byte aligned
-struct AlignedBuffer<const N: usize>(NonNull<[u8; N]>);
+#![feature(maybe_uninit_as_bytes)]
 
-impl<const N: usize> AlignedBuffer<N> {
-    fn layout() -> Layout {
-        Layout::from_size_align(N, N).unwrap()
-    }
-
-    fn new() -> Self {
-        let p = unsafe { alloc_zeroed(Self::layout()) } as *mut [u8; N];
-        Self(NonNull::new(p).unwrap())
-    }
-
-    fn buf(&mut self) -> &mut [u8; N] {
-        unsafe { self.0.as_mut() }
-    }
-}
+extern crate test;
 
-impl<const N: usize> Drop for AlignedBuffer<N> {
-    fn drop(&mut self) {
-        unsafe { dealloc(self.0.as_ptr() as *mut u8, Self::layout()) }
-    }
-}
+use std::mem::MaybeUninit;
 
 // Used to benchmark the throughput of getrandom in an optimal scenario.
 // The buffer is hot, and does not require initialization.
 #[inline(always)]
-fn bench<const N: usize>(b: &mut test::Bencher) {
-    let mut ab = AlignedBuffer::<N>::new();
-    let buf = ab.buf();
+fn bench_getrandom<const N: usize>(b: &mut test::Bencher) {
+    b.bytes = N as u64;
     b.iter(|| {
+        let mut buf = [0u8; N];
         getrandom::getrandom(&mut buf[..]).unwrap();
-        test::black_box(&buf);
+        test::black_box(buf);
     });
-    b.bytes = N as u64;
 }
 
 // Used to benchmark the throughput of getrandom is a slightly less optimal
 // scenario. The buffer is still hot, but requires initialization.
 #[inline(always)]
-fn bench_with_init<const N: usize>(b: &mut test::Bencher) {
-    let mut ab = AlignedBuffer::<N>::new();
-    let buf = ab.buf();
+fn bench_getrandom_uninit<const N: usize>(b: &mut test::Bencher) {
+    b.bytes = N as u64;
     b.iter(|| {
-        for byte in buf.iter_mut() {
-            *byte = 0;
-        }
-        getrandom::getrandom(&mut buf[..]).unwrap();
-        test::black_box(&buf);
+        let mut buf: MaybeUninit<[u8; N]> = MaybeUninit::uninit();
+        let _ = getrandom::getrandom_uninit(buf.as_bytes_mut()).unwrap();
+        let buf: [u8; N] = unsafe { buf.assume_init() };
+        test::black_box(buf)
     });
-    b.bytes = N as u64;
 }
 
-// 32 bytes (256-bit) is the seed sized used for rand::thread_rng
-const SEED: usize = 32;
-// Common size of a page, 4 KiB
-const PAGE: usize = 4096;
-// Large buffer to get asymptotic performance, 2 MiB
-const LARGE: usize = 1 << 21;
+macro_rules! bench {
+    ( $name:ident, $size:expr ) => {
+        pub mod $name {
+            #[bench]
+            pub fn bench_getrandom(b: &mut test::Bencher) {
+                super::bench_getrandom::<{ $size }>(b);
+            }
 
-#[bench]
-fn bench_seed(b: &mut test::Bencher) {
-    bench::<SEED>(b);
-}
-#[bench]
-fn bench_seed_init(b: &mut test::Bencher) {
-    bench_with_init::<SEED>(b);
+            #[bench]
+            pub fn bench_getrandom_uninit(b: &mut test::Bencher) {
+                super::bench_getrandom_uninit::<{ $size }>(b);
+            }
+        }
+    };
 }
 
-#[bench]
-fn bench_page(b: &mut test::Bencher) {
-    bench::<PAGE>(b);
-}
-#[bench]
-fn bench_page_init(b: &mut test::Bencher) {
-    bench_with_init::<PAGE>(b);
-}
+// 16 bytes (128 bits) is the size of an 128-bit AES key/nonce.
+bench!(aes128, 128 / 8);
 
-#[bench]
-fn bench_large(b: &mut test::Bencher) {
-    bench::<LARGE>(b);
-}
-#[bench]
-fn bench_large_init(b: &mut test::Bencher) {
-    bench_with_init::<LARGE>(b);
-}
+// 32 bytes (256 bits) is the seed sized used for rand::thread_rng
+// and the `random` value in a ClientHello/ServerHello for TLS.
+// This is also the size of a 256-bit AES/HMAC/P-256/Curve25519 key
+// and/or nonce.
+bench!(p256, 256 / 8);
+
+// A P-384/HMAC-384 key and/or nonce.
+bench!(p384, 384 / 8);
+
+// Initializing larger buffers is not the primary use case of this library, as
+// this should normally be done by a userspace CSPRNG. However, we have a test
+// here to see the effects of a lower (amortized) syscall overhead.
+bench!(page, 4096);

src/3ds.rs

@@ -9,8 +9,9 @@
 //! Implementation for Nintendo 3DS
 use crate::util_libc::sys_fill_exact;
 use crate::Error;
+use core::mem::MaybeUninit;
 
-pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     sys_fill_exact(dest, |buf| unsafe {
         libc::getrandom(buf.as_mut_ptr() as *mut libc::c_void, buf.len(), 0)
     })

src/bsd_arandom.rs

@@ -8,9 +8,9 @@
 
 //! Implementation for FreeBSD and NetBSD
 use crate::{util_libc::sys_fill_exact, Error};
-use core::ptr;
+use core::{mem::MaybeUninit, ptr};
 
-fn kern_arnd(buf: &mut [u8]) -> libc::ssize_t {
+fn kern_arnd(buf: &mut [MaybeUninit<u8>]) -> libc::ssize_t {
     static MIB: [libc::c_int; 2] = [libc::CTL_KERN, libc::KERN_ARND];
     let mut len = buf.len();
     let ret = unsafe {
@@ -30,7 +30,7 @@ fn kern_arnd(buf: &mut [u8]) -> libc::ssize_t {
     }
 }
 
-pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     // getrandom(2) was introduced in FreeBSD 12.0 and NetBSD 10.0
     #[cfg(target_os = "freebsd")]
     {
@@ -41,7 +41,9 @@ pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
 
         if let Some(fptr) = GETRANDOM.ptr() {
             let func: GetRandomFn = unsafe { core::mem::transmute(fptr) };
-            return sys_fill_exact(dest, |buf| unsafe { func(buf.as_mut_ptr(), buf.len(), 0) });
+            return sys_fill_exact(dest, |buf| unsafe {
+                func(buf.as_mut_ptr() as *mut u8, buf.len(), 0)
+            });
         }
     }
     // Both FreeBSD and NetBSD will only return up to 256 bytes at a time, and

src/custom.rs

@@ -7,8 +7,8 @@
 // except according to those terms.
 
 //! An implementation which calls out to an externally defined function.
-use crate::Error;
-use core::num::NonZeroU32;
+use crate::{util::uninit_slice_fill_zero, Error};
+use core::{mem::MaybeUninit, num::NonZeroU32};
 
 /// Register a function to be invoked by `getrandom` on unsupported targets.
 ///
@@ -90,10 +90,16 @@ macro_rules! register_custom_getrandom {
 }
 
 #[allow(dead_code)]
-pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     extern "C" {
         fn __getrandom_custom(dest: *mut u8, len: usize) -> u32;
     }
+    // Previously we always passed a valid, initialized slice to
+    // `__getrandom_custom`. Ensure `dest` has been initialized for backward
+    // compatibility with implementations that rely on that (e.g. Rust
+    // implementations that construct a `&mut [u8]` slice from `dest` and
+    // `len`).
+    let dest = uninit_slice_fill_zero(dest);
     let ret = unsafe { __getrandom_custom(dest.as_mut_ptr(), dest.len()) };
     match NonZeroU32::new(ret) {
         None => Ok(()),

src/dragonfly.rs

@@ -12,8 +12,9 @@ use crate::{
     util_libc::{sys_fill_exact, Weak},
     Error,
 };
+use std::mem::MaybeUninit;
 
-pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     static GETRANDOM: Weak = unsafe { Weak::new("getrandom\0") };
     type GetRandomFn = unsafe extern "C" fn(*mut u8, libc::size_t, libc::c_uint) -> libc::ssize_t;
 

src/espidf.rs

@@ -8,13 +8,13 @@
 
 //! Implementation for ESP-IDF
 use crate::Error;
-use core::ffi::c_void;
+use core::{ffi::c_void, mem::MaybeUninit};
 
 extern "C" {
     fn esp_fill_random(buf: *mut c_void, len: usize) -> u32;
 }
 
-pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+pub fn getrandom_inner(dest: &mut [MaybeUninit<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

src/fuchsia.rs

@@ -8,13 +8,14 @@
 
 //! Implementation for Fuchsia Zircon
 use crate::Error;
+use core::mem::MaybeUninit;
 
 #[link(name = "zircon")]
 extern "C" {
     fn zx_cprng_draw(buffer: *mut u8, length: usize);
 }
 
-pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
-    unsafe { zx_cprng_draw(dest.as_mut_ptr(), dest.len()) }
+pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
+    unsafe { zx_cprng_draw(dest.as_mut_ptr() as *mut u8, dest.len()) }
     Ok(())
 }

src/ios.rs

@@ -8,16 +8,16 @@
 
 //! Implementation for iOS
 use crate::Error;
-use core::{ffi::c_void, ptr::null};
+use core::{ffi::c_void, mem::MaybeUninit, ptr::null};
 
 #[link(name = "Security", kind = "framework")]
 extern "C" {
     fn SecRandomCopyBytes(rnd: *const c_void, count: usize, bytes: *mut u8) -> i32;
 }
 
-pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     // Apple's documentation guarantees kSecRandomDefault is a synonym for NULL.
-    let ret = unsafe { SecRandomCopyBytes(null(), dest.len(), dest.as_mut_ptr()) };
+    let ret = unsafe { SecRandomCopyBytes(null(), dest.len(), dest.as_mut_ptr() as *mut u8) };
     // errSecSuccess (from SecBase.h) is always zero.
     if ret != 0 {
         Err(Error::IOS_SEC_RANDOM)

src/js.rs

@@ -5,10 +5,10 @@
 // <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.
-use crate::Error;
+use crate::{util::uninit_slice_fill_zero, Error};
 
 extern crate std;
-use std::thread_local;
+use std::{mem::MaybeUninit, thread_local};
 
 use js_sys::{global, Function, Uint8Array};
 use wasm_bindgen::{prelude::wasm_bindgen, JsCast, JsValue};
@@ -28,12 +28,16 @@ thread_local!(
     static RNG_SOURCE: Result<RngSource, Error> = getrandom_init();
 );
 
-pub(crate) fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+pub(crate) fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     RNG_SOURCE.with(|result| {
         let source = result.as_ref().map_err(|&e| e)?;
 
         match source {
             RngSource::Node(n) => {
+                // XXX(perf): `random_fill_sync` requires a `&mut [u8]` so we
+                // have to ensure the memory in `dest` is initialized.
+                let dest = uninit_slice_fill_zero(dest);
+
                 if n.random_fill_sync(dest).is_err() {
                     return Err(Error::NODE_RANDOM_FILL_SYNC);
                 }
@@ -49,7 +53,9 @@ pub(crate) fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
                     if crypto.get_random_values(&sub_buf).is_err() {
                         return Err(Error::WEB_GET_RANDOM_VALUES);
                     }
-                    sub_buf.copy_to(chunk);
+
+                    // SAFETY: `sub_buf`'s length is the same length as `chunk`
+                    unsafe { sub_buf.raw_copy_to_ptr(chunk.as_mut_ptr() as *mut u8) };
                 }
             }
         };