Fix unsoundness in <BlockRng64 as RngCore>::next_u32, with less unsafe code

CHANGELOG.md

@@ -8,9 +8,16 @@ A [separate changelog is kept for rand_core](rand_core/CHANGELOG.md).
 
 You may also find the [Upgrade Guide](https://rust-random.github.io/book/update.html) useful.
 
-## [0.8.5] - unreleased
+## [0.8.5] - 2021-08-20
 ### Fixes
--  Fix build on non-32/64-bit architectures (#1144)
+- Fix build on non-32/64-bit architectures (#1144)
+
+### Platform support
+- Remove special cases for emscripten (#1142)
+
+### Documentation
+- Added docs about rand's use of const generics (#1150)
+- Better random chars example (#1157)
 
 ## [0.8.4] - 2021-06-15
 ### Additions

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "rand"
-version = "0.8.4"
+version = "0.8.5"
 authors = ["The Rand Project Developers", "The Rust Project Developers"]
 license = "MIT OR Apache-2.0"
 readme = "README.md"

rand_core/CHANGELOG.md

@@ -4,6 +4,10 @@ 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).
 
+## [0.6.4] - 2021-08-20
+### Fixed
+- Fix unsoundness in `<BlockRng64 as RngCore>::next_u32` (#1160)
+
 ## [0.6.3] - 2021-06-15
 ### Changed
 - Improved bound for `serde` impls on `BlockRng` (#1130)

rand_core/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "rand_core"
-version = "0.6.3"
+version = "0.6.4"
 authors = ["The Rand Project Developers", "The Rust Project Developers"]
 license = "MIT OR Apache-2.0"
 readme = "README.md"

rand_core/src/block.rs

@@ -352,27 +352,21 @@ where
 {
     #[inline]
     fn next_u32(&mut self) -> u32 {
-        let mut index = self.index * 2 - self.half_used as usize;
-        if index >= self.results.as_ref().len() * 2 {
+        let mut index = self.index - self.half_used as usize;
+        if index >= self.results.as_ref().len() {
             self.core.generate(&mut self.results);
             self.index = 0;
+            index = 0;
             // `self.half_used` is by definition `false`
             self.half_used = false;
-            index = 0;
         }
 
+        let shift = 32 * (self.half_used as usize);
+
         self.half_used = !self.half_used;
         self.index += self.half_used as usize;
 
-        // Index as if this is a u32 slice.
-        unsafe {
-            let results = &*(self.results.as_ref() as *const [u64] as *const [u32]);
-            if cfg!(target_endian = "little") {
-                *results.get_unchecked(index)
-            } else {
-                *results.get_unchecked(index ^ 1)
-            }
-        }
+        (self.results.as_ref()[index] >> shift) as u32
     }
 
     #[inline]
@@ -435,3 +429,109 @@ impl<R: BlockRngCore + SeedableRng> SeedableRng for BlockRng64<R> {
 }
 
 impl<R: BlockRngCore + CryptoRng> CryptoRng for BlockRng<R> {}
+
+#[cfg(test)]
+mod test {
+    use crate::{SeedableRng, RngCore};
+    use crate::block::{BlockRng, BlockRng64, BlockRngCore};
+
+    #[derive(Debug, Clone)]
+    struct DummyRng {
+        counter: u32,
+    }
+
+    impl BlockRngCore for DummyRng {
+        type Item = u32;
+
+        type Results = [u32; 16];
+
+        fn generate(&mut self, results: &mut Self::Results) {
+            for r in results {
+                *r = self.counter;
+                self.counter += 1;
+            }
+        }
+    }
+
+    impl SeedableRng for DummyRng {
+        type Seed = [u8; 4];
+
+        fn from_seed(seed: Self::Seed) -> Self {
+            DummyRng { counter: u32::from_le_bytes(seed) }
+        }
+    }
+
+    #[test]
+    fn blockrng_next_u32_vs_next_u64() {
+        let mut rng1 = BlockRng::<DummyRng>::from_seed(Default::default());
+        let mut rng2 = rng1.clone();
+        let mut rng3 = rng1.clone();
+
+        let mut a = [0; 16];
+        (&mut a[..4]).copy_from_slice(&rng1.next_u32().to_le_bytes());
+        (&mut a[4..12]).copy_from_slice(&rng1.next_u64().to_le_bytes());
+        (&mut a[12..]).copy_from_slice(&rng1.next_u32().to_le_bytes());
+
+        let mut b = [0; 16];
+        (&mut b[..4]).copy_from_slice(&rng2.next_u32().to_le_bytes());
+        (&mut b[4..8]).copy_from_slice(&rng2.next_u32().to_le_bytes());
+        (&mut b[8..]).copy_from_slice(&rng2.next_u64().to_le_bytes());
+        assert_eq!(a, b);
+
+        let mut c = [0; 16];
+        (&mut c[..8]).copy_from_slice(&rng3.next_u64().to_le_bytes());
+        (&mut c[8..12]).copy_from_slice(&rng3.next_u32().to_le_bytes());
+        (&mut c[12..]).copy_from_slice(&rng3.next_u32().to_le_bytes());
+        assert_eq!(a, c);
+    }
+
+    #[derive(Debug, Clone)]
+    struct DummyRng64 {
+        counter: u64,
+    }
+
+    impl BlockRngCore for DummyRng64 {
+        type Item = u64;
+
+        type Results = [u64; 8];
+
+        fn generate(&mut self, results: &mut Self::Results) {
+            for r in results {
+                *r = self.counter;
+                self.counter += 1;
+            }
+        }
+    }
+
+    impl SeedableRng for DummyRng64 {
+        type Seed = [u8; 8];
+
+        fn from_seed(seed: Self::Seed) -> Self {
+            DummyRng64 { counter: u64::from_le_bytes(seed) }
+        }
+    }
+
+    #[test]
+    fn blockrng64_next_u32_vs_next_u64() {
+        let mut rng1 = BlockRng64::<DummyRng64>::from_seed(Default::default());
+        let mut rng2 = rng1.clone();
+        let mut rng3 = rng1.clone();
+
+        let mut a = [0; 16];
+        (&mut a[..4]).copy_from_slice(&rng1.next_u32().to_le_bytes());
+        (&mut a[4..12]).copy_from_slice(&rng1.next_u64().to_le_bytes());
+        (&mut a[12..]).copy_from_slice(&rng1.next_u32().to_le_bytes());
+
+        let mut b = [0; 16];
+        (&mut b[..4]).copy_from_slice(&rng2.next_u32().to_le_bytes());
+        (&mut b[4..8]).copy_from_slice(&rng2.next_u32().to_le_bytes());
+        (&mut b[8..]).copy_from_slice(&rng2.next_u64().to_le_bytes());
+        assert_ne!(a, b);
+
+        let mut c = [0; 16];
+        (&mut c[..8]).copy_from_slice(&rng3.next_u64().to_le_bytes());
+        (&mut c[8..12]).copy_from_slice(&rng3.next_u32().to_le_bytes());
+        (&mut c[12..]).copy_from_slice(&rng3.next_u32().to_le_bytes());
+        assert_eq!(b, c);
+    }
+}