rust: fix 64-bit scalar wire-format alignment on 32-bit targets in Push::alignment (#9465 )

In the FlatBuffers binary wire format specification, every scalar field is aligned to its own byte size (`1`, `2`, `4`, or `8` bytes), regardless of the host CPU architecture. Both the `flatc` schema compiler (when computing table `max_align` and struct field offsets) and the C++ `FlatBufferBuilder::Align` / `PushElement` implementation align 64-bit scalars (`u64`, `i64`, `f64`) to 8 bytes on all architectures. However, in the Rust `flatbuffers` crate (`rust/flatbuffers/src/push.rs`), the `Push` trait provides a default implementation: ```rust fn alignment() -> PushAlignment { PushAlignment::new(align_of::<Self::Output>()) } ``` And `impl_push_for_endian_scalar!` did not override `Push::alignment()`, inheriting `core::mem::align_of::<T>()` for all primitive scalar types (`bool`, `u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`, `i64`, `f32`, `f64`). On 64-bit targets (`x86_64`, `aarch64`) and 32-bit ARM (`armv7`), `align_of::<u64>() == 8`, so `align_of::<T>()` happens to equal `size_of::<T>()`. On 32-bit x86 (`i686`, System V i386 ABI), however, the host ABI alignment of 64-bit scalars (`u64`, `i64`, `f64`) is **4 bytes** (`align_of::<u64>() == 4`), while their FlatBuffer wire-format size and required wire alignment is **8 bytes** (`size_of::<u64>() == 8`). As a consequence, on 32-bit x86 (`i686`): 1. `FlatBufferBuilder::push::<u64 / i64 / f64>` only aligns the builder's cursor and tracks `min_align` to 4 bytes instead of 8 bytes when writing 64-bit table fields or vectors of 64-bit scalars. 2. When a table containing a 64-bit scalar field (`uint64`, `int64`, `double`) is serialized on `i686`, the field can be placed at a buffer offset that is 4-byte aligned but not 8-byte aligned. 3. When the resulting FlatBuffer is verified by C++ `flatbuffers::Verifier` (which checks `VerifyField<uint64_t>` with `sizeof(uint64_t) == 8` alignment when `FLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE` / alignment checks are active) or transferred to a 64-bit reader, verification fails or unaligned 64-bit reads occur. Fix `impl_push_for_endian_scalar!` to explicitly implement `Push::alignment()` using `size_of::<$ty>()` so that all endian scalar types always use their wire-format byte size as their FlatBuffer alignment across all target architectures.
2026-11-12 15:06:21 +00:00 · 2026-11-06 15:15:33 -08:00
3 changed files with 103 additions and 84 deletions
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-      brace-expansion: 2.1.7
+      brace-expansion: 2.0.2

  ms@2.1.3: {}

@@ -1267,7 +1261,7 @@ snapshots:

  path-key@3.1.1: {}

-  picomatch@2.3.2: {}
+  picomatch@2.3.1: {}

  prelude-ls@1.2.1: {}

@@ -1309,12 +1303,12 @@ snapshots:
    dependencies:
      prelude-ls: 1.2.1

-  typescript-eslint@8.34.1(eslint@9.29.0(supports-color@7.2.0))(supports-color@7.2.0)(typescript@5.8.3):
+  typescript-eslint@8.34.1(eslint@9.29.0)(typescript@5.8.3):
    dependencies:
-      '@typescript-eslint/eslint-plugin': 8.34.1(@typescript-eslint/parser@8.34.1(eslint@9.29.0(supports-color@7.2.0))(supports-color@7.2.0)(typescript@5.8.3))(eslint@9.29.0(supports-color@7.2.0))(supports-color@7.2.0)(typescript@5.8.3)
-      '@typescript-eslint/parser': 8.34.1(eslint@9.29.0(supports-color@7.2.0))(supports-color@7.2.0)(typescript@5.8.3)
-      '@typescript-eslint/utils': 8.34.1(eslint@9.29.0(supports-color@7.2.0))(supports-color@7.2.0)(typescript@5.8.3)
-      eslint: 9.29.0(supports-color@7.2.0)
+      '@typescript-eslint/eslint-plugin': 8.34.1(@typescript-eslint/parser@8.34.1(eslint@9.29.0)(typescript@5.8.3))(eslint@9.29.0)(typescript@5.8.3)
+      '@typescript-eslint/parser': 8.34.1(eslint@9.29.0)(typescript@5.8.3)
+      '@typescript-eslint/utils': 8.34.1(eslint@9.29.0)(typescript@5.8.3)
+      eslint: 9.29.0
      typescript: 5.8.3
    transitivePeerDependencies:
      - supports-color
@@ -83,6 +83,11 @@ macro_rules! impl_push_for_endian_scalar {
            unsafe fn push(&self, dst: &mut [u8], _written_len: usize) {
                emplace_scalar::<$ty>(dst, *self);
            }
+
+            #[inline]
+            fn alignment() -> PushAlignment {
+                PushAlignment::new(size_of::<$ty>())
+            }
        }
    };
 }
@@ -98,3 +103,23 @@ impl_push_for_endian_scalar!(u64);
 impl_push_for_endian_scalar!(i64);
 impl_push_for_endian_scalar!(f32);
 impl_push_for_endian_scalar!(f64);
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn endian_scalar_push_alignment_matches_wire_size() {
+        assert_eq!(<bool as Push>::alignment().value(), 1);
+        assert_eq!(<u8 as Push>::alignment().value(), 1);
+        assert_eq!(<i8 as Push>::alignment().value(), 1);
+        assert_eq!(<u16 as Push>::alignment().value(), 2);
+        assert_eq!(<i16 as Push>::alignment().value(), 2);
+        assert_eq!(<u32 as Push>::alignment().value(), 4);
+        assert_eq!(<i32 as Push>::alignment().value(), 4);
+        assert_eq!(<f32 as Push>::alignment().value(), 4);
+        assert_eq!(<u64 as Push>::alignment().value(), 8);
+        assert_eq!(<i64 as Push>::alignment().value(), 8);
+        assert_eq!(<f64 as Push>::alignment().value(), 8);
+    }
+}
@@ -3,6 +3,6 @@
  "type": "module",
  "private": true,
  "devDependencies": {
-    "lodash": "4.18.1"
+    "lodash": "4.17.23"
  }
 }