fix: initialize scratch buffer (#752)

Previously, the code took a reference to uninitialized memory, which is
undefined behavior. Allocating zeroed memory is relatively cheap, so the
scratch buffer allocates zeroed memory to initialize the memory.

Co-authored-by: Andrew Gazelka <andrew.gazelka@gmail.com>

crates/hyperion/src/lib.rs

@@ -383,26 +383,21 @@ impl HyperionCore {
 /// A scratch buffer for intermediate operations. This will return an empty [`Vec`] when calling [`Scratch::obtain`].
 #[derive(Debug)]
 pub struct Scratch<A: Allocator = std::alloc::Global> {
-    inner: Vec<u8, A>,
+    inner: Box<[u8], A>,
 }
 
 impl Default for Scratch<std::alloc::Global> {
     fn default() -> Self {
-        let inner = Vec::with_capacity(MAX_PACKET_SIZE);
-        Self { inner }
+        std::alloc::Global.into()
     }
 }
 
 /// Nice for getting a buffer that can be used for intermediate work
-///
-/// # Safety
-/// - every single time [`ScratchBuffer::obtain`] is called, the buffer will be cleared before returning
-/// - the buffer has capacity of at least `MAX_PACKET_SIZE`
-pub unsafe trait ScratchBuffer: sealed::Sealed + Debug {
+pub trait ScratchBuffer: sealed::Sealed + Debug {
     /// The type of the allocator the [`Vec`] uses.
     type Allocator: Allocator;
-    /// Obtains a buffer that can be used for intermediate work.
-    fn obtain(&mut self) -> &mut Vec<u8, Self::Allocator>;
+    /// Obtains a buffer that can be used for intermediate work. The contents are unspecified.
+    fn obtain(&mut self) -> &mut [u8];
 }
 
 mod sealed {
@@ -411,20 +406,23 @@ mod sealed {
 
 impl<A: Allocator + Debug> sealed::Sealed for Scratch<A> {}
 
-unsafe impl<A: Allocator + Debug> ScratchBuffer for Scratch<A> {
+impl<A: Allocator + Debug> ScratchBuffer for Scratch<A> {
     type Allocator = A;
 
-    fn obtain(&mut self) -> &mut Vec<u8, Self::Allocator> {
-        self.inner.clear();
+    fn obtain(&mut self) -> &mut [u8] {
         &mut self.inner
     }
 }
 
 impl<A: Allocator> From<A> for Scratch<A> {
     fn from(allocator: A) -> Self {
-        Self {
-            inner: Vec::with_capacity_in(MAX_PACKET_SIZE, allocator),
-        }
+        // A zeroed slice is allocated to avoid reading from uninitialized memory, which is UB.
+        // Allocating zeroed memory is usually very cheap, so there are minimal performance
+        // penalties from this.
+        let inner = Box::new_zeroed_slice_in(MAX_PACKET_SIZE, allocator);
+        // SAFETY: The box was initialized to zero, and u8 can be represented by zero
+        let inner = unsafe { inner.assume_init() };
+        Self { inner }
     }
 }
 

crates/hyperion/src/net/encoder/mod.rs

@@ -9,7 +9,6 @@
 use std::{
     fmt::Debug,
     io::{Cursor, Write},
-    mem::MaybeUninit,
 };
 
 use anyhow::ensure;
@@ -129,37 +128,27 @@ impl PacketEncoder {
         if data_len > threshold {
             let scratch = scratch.obtain();
 
-            debug_assert!(scratch.is_empty());
-
             let data_slice = &mut slice
                 [usize::try_from(data_write_start)?..usize::try_from(end_data_position_exclusive)?];
 
-            {
-                // todo: I think this kinda safe maybe??? ... lol. well I know at least scratch is always large enough
-                let written = {
-                    let scratch = scratch.spare_capacity_mut();
-                    let scratch = unsafe { MaybeUninit::slice_assume_init_mut(scratch) };
-
-                    let len = data_slice.len();
-                    let span = tracing::trace_span!("zlib_compress", bytes = len);
-                    let _enter = span.enter();
-                    compressor.zlib_compress(data_slice, scratch)?
-                };
+            let written = {
+                let len = data_slice.len();
+                let span = tracing::trace_span!("zlib_compress", bytes = len);
+                let _enter = span.enter();
+                compressor.zlib_compress(data_slice, scratch).unwrap()
+            };
 
-                unsafe {
-                    scratch.set_len(scratch.len() + written);
-                }
-            }
+            let compressed = &scratch[..written];
 
             let data_len = VarInt(data_len as u32 as i32);
 
-            let packet_len = data_len.written_size() + scratch.len();
+            let packet_len = data_len.written_size() + compressed.len();
             let packet_len = VarInt(packet_len as u32 as i32);
 
             let mut write = Cursor::new(&mut slice[..]);
             packet_len.encode(&mut write)?;
             data_len.encode(&mut write)?;
-            write.write_all(scratch)?;
+            write.write_all(compressed)?;
 
             let len = write.position();