add fuzzer, fix memcopy overlap

Author: Pascal Seitz

fuzz/.gitignore

@@ -0,0 +1,4 @@
+
+target
+corpus
+artifacts

fuzz/Cargo.toml

@@ -0,0 +1,26 @@
+
+[package]
+name = "lz4_flex-fuzz"
+version = "0.0.0"
+authors = ["Automatically generated"]
+publish = false
+edition = "2018"
+
+[package.metadata]
+cargo-fuzz = true
+
+[dependencies]
+libfuzzer-sys = "0.3"
+
+[dependencies.lz4_flex]
+path = ".."
+
+# Prevent this from interfering with workspaces
+[workspace]
+members = ["."]
+
+[[bin]]
+name = "fuzz_target_1"
+path = "fuzz_targets/fuzz_target_1.rs"
+test = false
+doc = false

fuzz/fuzz_targets/fuzz_target_1.rs

@@ -0,0 +1,11 @@
+#![no_main]
+use libfuzzer_sys::fuzz_target;
+
+use lz4_flex::block::decompress::decompress_size_prepended;
+use lz4_flex::block::compress::compress_prepend_size;
+fuzz_target!(|data: &[u8]| {
+    // fuzzed code goes here
+    let compressed = compress_prepend_size(data);
+    let decompressed = decompress_size_prepended(&compressed).unwrap();
+    assert_eq!(data, decompressed);
+});

src/block/decompress.rs

@@ -136,13 +136,16 @@ fn is_safe_distance(input_pos: usize, in_len: usize) -> bool {
     input_pos < in_len
 }
 
+/// We copy 24 byte blocks, because aligned copies are faster
+const BLOCK_COPY_SIZE: usize = 24;
+
 #[inline]
 fn block_copy_from_src(source: *const u8, dst_ptr: *mut u8, num_items: usize) {
-    debug_assert!(num_items <= 24);
+    debug_assert!(num_items <= BLOCK_COPY_SIZE);
     unsafe {
         let dst_ptr_end = dst_ptr.add(num_items);
         if (dst_ptr as usize) < dst_ptr_end as usize {
-            std::ptr::copy_nonoverlapping(source, dst_ptr, 24);
+            std::ptr::copy_nonoverlapping(source, dst_ptr, BLOCK_COPY_SIZE);
         }
     }
 }
@@ -209,8 +212,9 @@ pub fn decompress_into(input: &[u8], output: &mut Vec<u8>) -> Result<(), Error>
             let start_ptr = unsafe { output_ptr.sub(offset as usize) };
 
             let match_length = (4 + (token & 0xF)) as usize;
-            // Write the duplicate segment to the output buffer.
-            if (output_ptr as usize) < unsafe { start_ptr.add(match_length) } as usize {
+            // Write the duplicate segment to the output buffer from the output buffer
+            // The blocks can overlap, make sure they are at least BLOCK_COPY_SIZE apart
+            if (output_ptr as usize) < unsafe { start_ptr.add(match_length).add(BLOCK_COPY_SIZE) } as usize {
                 duplicate_overlapping(&mut output_ptr, start_ptr, match_length);
             } else {
                 unsafe {