# Safety
## Unsafe boundary
All compression and decompression logic is `#[forbid(unsafe_code)]`. The remaining
unsafe (~210 blocks across 14 internal modules) performs unchecked memory copies,
unaligned reads, SIMD intrinsics, and `Vec::set_len` calls whose bounds are proven
by safe-region margins computed in the algorithm code. Every unsafe block has a
`debug_assert!` guard. No `unsafe` is exposed in the public API.
Unsafe is confined to three categories of leaf modules:
1. **`primitives.rs`** in `bitstream/`, `huffman/`, `encode/`, `decode/`, `xxhash/`:
`#[inline(always)]` wrappers around `get_unchecked`, `read_unaligned`,
`set_len`, and `copy_nonoverlapping`.
2. **`simd/`** submodules (`scalar.rs`, `copy.rs`, `x86_64/*.rs`, `aarch64/*.rs`):
intrinsics and raw pointer arithmetic for wildcopy, copy-match, and
common-prefix-length.
3. **`simd_decode/`** SIMD sequence decoders (`x86_64/decode.rs`,
`aarch64/decode.rs`): fused FSE decode + sequence execution with inline
SIMD operations.
## Why Rust matters here
C zstd's decompression path has had memory safety bugs that Rust's type system
and bounds checking prevent by construction:
| [#50](https://github.com/facebook/zstd/issues/50) | OOB heap read in `ZSTD_copy8` (via `ZSTD_wildcopy` during `ZSTD_execSequence`): 8-byte read past buffer on malformed input (AFL) | Wildcopy uses slice operations bounded by destination length |
| [#49](https://github.com/facebook/zstd/issues/49) | OOB stack read in `HUF_readStats`: `rankVal` array accessed past bounds on malformed Huffman table (AFL) | Stack arrays are slices; indexing panics on OOB |
| [#22](https://github.com/facebook/zstd/issues/22) | Destination buffer bounds not checked during compress/decompress, heap overflow on undersized output | Output written via `&mut [u8]` slice; writes past end panic |
| [#4499](https://github.com/facebook/zstd/issues/4499) | Stack buffer overflow read in `FASTCOVER_hashPtrToIndex` training with many 1-byte samples | Slice indexing panics on OOB |
| [OSV-2020-405](https://osv.dev/vulnerability/OSV-2020-405) (HIGH) | Stack buffer overflow write in `ZSTD_decodeLiteralsBlock` (OSS-Fuzz #16445) | Slice write panics on OOB |
| [OSV-2020-654](https://osv.dev/vulnerability/OSV-2020-654) (MEDIUM) | Heap buffer overflow read 16 in `ZSTD_copy16` during `ZSTD_decompressSequences_bmi2` (OSS-Fuzz #17451) | Wildcopy bounded by slice length |
| [OSV-2020-429](https://osv.dev/vulnerability/OSV-2020-429) (MEDIUM) | Heap buffer overflow read 4 in `MEM_read32` during legacy v05 sequence decoding (OSS-Fuzz #14368) | `u32::from_le_bytes` requires a bounds-checked `[u8; 4]` |
| [OSV-2021-859](https://osv.dev/vulnerability/OSV-2021-859) (HIGH) | Heap buffer overflow write 1 in `FSE_writeNCount_generic` during Huffman weight compression (OSS-Fuzz #35209) | Slice write panics on OOB |
| [OSV-2022-110](https://osv.dev/vulnerability/OSV-2022-110) (HIGH) | Heap buffer overflow write 1 in `ZSTD_compressLiterals` (OSS-Fuzz #44239) | Slice write panics on OOB |
| [OSV-2022-96](https://osv.dev/vulnerability/OSV-2022-96) (HIGH) | Heap buffer overflow write 16 in sequence compression API (OSS-Fuzz #44122) | Slice write panics on OOB |
| [CVE-2024-11477](https://www.crowdfense.com/cve-2024-11477-7zip-zstd-buffer-overflow/) (CVSS 7.8, 7-Zip's zstd) | Integer underflow in decompression wraps array index, heap buffer overflow write | `usize` underflow panics in debug; slice indexing catches OOB in release |
| [PR #1803](https://github.com/facebook/zstd/pull/1803) | Memory over-read from wildcopy changes (PR #1756), required increasing `WILDCOPY_OVERLENGTH` to 16 (OSS-Fuzz) | Wildcopy in Rust uses slice operations bounded by destination length |
| [PR #2784](https://github.com/facebook/zstd/pull/2784) | Multiple Huffman decompression bugs: pointer underflow and NULL pointer issues (fuzzer) | No null references in Rust; pointer arithmetic replaced by slice indexing |
| [PR #1722](https://github.com/facebook/zstd/pull/1722) | Buffer overflow in legacy v0.3 decompression | Slice indexing panics on OOB |
| [PR #1590](https://github.com/facebook/zstd/pull/1590) | OOB read in `ZSTD_decompressBound` on malformed frame (fuzzer) | Slice indexing panics on OOB |