fix: fix UB in getValueBigintWords

examples/js_dsl/mod.test.ts

@@ -94,6 +94,24 @@ describe("primitive types", () => {
 		expect(value).toEqual(2 ** 63);
 	});
 
+	describe("getValueBigintWords", () => {
+		it("reads words with null sign_bit (unsigned only)", () => {
+			expect(mod.bigIntFirstWord(0n)).toEqual(0);
+			expect(mod.bigIntFirstWord(1n)).toEqual(1);
+			expect(mod.bigIntFirstWord(0xdeadbeefn)).toEqual(0xdeadbeef);
+		});
+
+		it("reads correct sign (non-null sign_bit path)", () => {
+			expect(mod.bigIntSign(0n)).toEqual(0);
+			expect(mod.bigIntSign(1n)).toEqual(0);
+			expect(mod.bigIntSign(0xffffffffffffffffn)).toEqual(0);
+
+			expect(mod.bigIntSign(-1n)).toEqual(1);
+			expect(mod.bigIntSign(-0xffffffffffffffffn)).toEqual(1);
+		});
+
+	});
+
 	it("tomorrow adds one day", () => {
 		const now = new Date("2025-01-01T00:00:00Z");
 		const result = mod.tomorrow(now);

examples/js_dsl/mod.zig

@@ -100,6 +100,24 @@ pub fn doubleBigInt(n: BigInt) !BigInt {
     return BigInt.from(val * 2);
 }
 
+/// Read a BigInt's first u64 word via `getValueBigintWords` passing `null` for `sign_bit`.
+///
+/// Throws if word_count > 1.
+pub fn bigIntFirstWord(n: BigInt) !Number {
+    var words: [1]u64 = .{0};
+    const got = try n.toValue().getValueBigintWords(null, &words);
+    if (got.len > 1) return error.BigIntTooLarge;
+    return Number.from(words[0]);
+}
+
+/// Read a BigInt's sign as 0/1 via `getValueBigintWords` passing a non-null `sign_bit`.
+pub fn bigIntSign(n: BigInt) !Number {
+    var sign: u1 = 0;
+    var words: [1]u64 = .{0};
+    _ = try n.toValue().getValueBigintWords(&sign, &words);
+    return Number.from(@as(u32, sign));
+}
+
 /// Add one day (86400000ms) to a Date.
 pub fn tomorrow(d: Date) Date {
     const ts = d.assertTimestamp();

src/Value.zig

@@ -216,12 +216,25 @@ pub fn getValueBigintUint64(self: Value, lossless: ?*bool) NapiError!u64 {
     return bigint;
 }
 
-/// https://nodejs.org/api/n-api.html#napi_get_value_bigint_words
-pub fn getValueBigintWords(self: Value, sign_bit: *u1, words: []u64) NapiError![]u64 {
+/// Reads a JS `BigInt` into `words`.
+///
+/// Pass `null` for `sign_bit` to skip getting the sign result.
+///
+/// In Ethereum's context, this is useful for big integers defined in the spec to be
+/// unsigned.
+///
+/// NOTE: napi's C entry takes `int*` (4-byte aligned). Casting a u1 to int* is UB, since that
+/// is 4-bytes aligned. We use a local `c_int` for the napi call and narrow back to `u1` for the caller.
+///
+/// Source: https://nodejs.org/api/n-api.html#napi_get_value_bigint_words
+pub fn getValueBigintWords(self: Value, sign_bit: ?*u1, words: []u64) NapiError![]u64 {
     var word_count: usize = words.len;
+    var raw_sign: c_int = 0;
     try status.check(
-        c.napi_get_value_bigint_words(self.env, self.value, @alignCast(@ptrCast(sign_bit)), &word_count, @ptrCast(words)),
+        c.napi_get_value_bigint_words(self.env, self.value, &raw_sign, &word_count, words.ptr),
     );
+    // napi guarantees raw_sign ∈ {0, 1}
+    if (sign_bit) |s| s.* = @intCast(raw_sign);
     return words[0..word_count];
 }