bun:ffi: add the full byteOffset in read.*, do not truncate it to int32

[robobun]

Problem

• Every bun:ffi read.* function converts byteOffset with to_int32() (src/runtime/ffi/FFIObject.rs:269). An offset of 2^31 or more saturates to i32::MAX, so read.u8(base, 2**32 + 1) reads base + 2^31 - 1 with no error. Code that indexes a region over 2 GiB from mmap hits this.
• A non-number offset reaches asInt32() on a non-int32 value. A debug build aborts with ASSERTION FAILED: isInt32(). A release build uses the low bits of the JSValue encoding: read.u8(p, undefined) reads p + 10.

Fix

• addr_from_args now handles byteOffset like toArrayBuffer() and toBuffer(): to_int64() with saturating add or subtract. undefined and null mean no offset. Another non-number throws Expected number for byteOffset. A non-finite number throws byteOffset must be a finite number.
• Correct because a JS number holds every integer up to 2^53 and to_int64() keeps all of them. The address is a usize, so the int32 step had no purpose.
• toArrayBuffer() and toBuffer() now throw the same non-finite message (was ptr must be a finite number.), and check it before the address math, so -Infinity no longer reports ptr cannot be zero.
• Verified: two new tests in test/js/bun/ffi/ffi.test.js (read edge cases) and three rows in ffi-error-messages.test.ts. Bun 1.4.1 fails them. Also ran all of test/js/bun/ffi/.

Background

• read.X(ptr, byteOffset) reads one value of type X at ptr + byteOffset. All twelve readers share reader::addr_from_args, which turns the two arguments into one usize.
• JSValue::to_int32() is not ECMAScript ToInt32. It truncates a double and saturates at the i32 bounds. For a non-number it calls asInt32(), which assumes an int32 payload.
• The test reaches a 4 GiB offset without a 4 GiB allocation: base = ptr(buf) + 8 - off, for off on both sides of the int32 range. Only the full offset lands back in the buffer.

Notes

• ptr(buf, null) has a separate hole in ptr_: it calls off.to_int64() on null and a debug build asserts in JSC__JSValue__toInt64. bun:ffi: name the received type in ptr() errors and throw them #40732 converts ptr_ to thrown errors and owns its messages, so the guard is requested there and this PR leaves ptr_ alone.
• CString goes through the same get_ptr_slice, but its C++ entry maps a falsy byteOffset (including NaN) to 0 first. Only Infinity and -Infinity reach the finite check from CString.
• The old DOMJIT fast path took an int32_t offset. headers.h still declares Reader__*__fastpath(..., int64_t, int32_t) but nothing defines or calls them. ZigGeneratedCode.cpp registers only the __slowpath wrappers.
• bun:ffi: handle negative byteOffset in read.* instead of panicking #32260 changed the same addr_from_args lines to fix the negative offset panic. Main already fixed that panic in bun:ffi: use the engine-native FFI when available #35246 (the test a negative byteOffset does not abort the process exists on main), so bun:ffi: handle negative byteOffset in read.* instead of panicking #32260 was closed as superseded. This PR covers the remaining to_int64 change.
• Repro from the report, on bun 1.4.1 (linux-x64): mmap 6 GiB anonymous, write 171 at base + 2**32 + 1, then read.u8(base, 2**32 + 1) returns 90 (a marker placed at base + 2**31 - 1). With this fix it returns 171.
• On 1.4.1, read.u8(p, undefined) returns the byte at p + 10 and read.u8(p, null) the byte at p + 2. The debug build aborts on the isInt32() assertion in JSCJSValue.h.
• Both new tests run the reads in a subprocess. On the unfixed build the truncated offset lands 2 GiB away from the buffer, and the process segfaults (exit 139).
• ptr argument: ArrayBuffer cells through an FTL-compiled call site and integer identities work for all possible values > int64_t time out at 5 s in the local debug ASAN run. Neither calls read.*. Their time goes to 300k ArrayBuffer allocations plus GC, and to 32k BigInt FFI calls. Both pass in CI.

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no test proof · iteration 1 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/ffi/ffi.test.js, test/js/bun/ffi/ffi-error-messages.test.ts

[robobun]

Status: ready for review.

Reproduced on bun 1.4.1 (linux-x64) with the mmap repro from the report. read.u8(base, 2**32 + 1) returned the byte at base + 2**31 - 1 instead of the byte at base + 2**32 + 1. read.u8(p, undefined) returned the byte at p + 10. A debug build aborted on ASSERTION FAILED: isInt32().

The two new tests in test/js/bun/ffi/ffi.test.js fail on 1.4.1 (the subprocess segfaults) and pass with this branch.

CI (build 107727): 181 of 182 jobs pass. test/js/bun/ffi/ passes on every lane. The one red lane is test/js/web/url/url.test.ts on darwin x64, which this change does not touch and which also fails on main. It is reported separately.

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Walkthrough

Changes

The FFI reader validates byteOffset values and applies signed 64-bit offsets with saturating address arithmetic. Regression tests cover large offsets, default values, invalid inputs, and corrected error messages across FFI read variants.

Changes

FFI byteOffset handling

Layer / File(s)	Summary
Validate and apply 64-bit offsets src/runtime/ffi/FFIObject.rs	reader::addr_from_args validates numeric offsets and uses signed 64-bit saturating address arithmetic. get_ptr_slice reports invalid byteOffset values.
Cover offset behavior test/js/bun/ffi/ffi.test.js, test/js/bun/ffi/ffi-error-messages.test.ts	Subprocess tests cover large offsets, default values, invalid inputs, and corrected error messages across FFI read variants.

Suggested reviewers: dylan-conway, jarred-sumner

Merge Risk: ⚪ Minimal · up to 26623

This change corrects large and invalid byte-offset handling in FFI reads and updates the related error-message expectation. No actionable merge-blocking risk remains beyond normal checks and review.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly states the main change: preserving the full byteOffset in bun:ffi read.* functions instead of truncating it to int32.
Description check	✅ Passed	The description explains the problem, fix, behavior changes, verification steps, test coverage, and known unrelated failures. It does not use the template headings exactly, but it provides the require…

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Explanation

The description explains the problem, fix, behavior changes, verification steps, test coverage, and known unrelated failures. It does not use the template headings exactly, but it provides the required information and is substantially complete.

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@test/js/bun/ffi/ffi.test.js`:
- Line 812: Replace the require("bun:ffi") calls in the large-offset and
invalid-offset subprocess sources with module-scope imports of ptr and read from
bun:ffi. Update both affected sites in test/js/bun/ffi/ffi.test.js: lines
812-812 and 855-855; both require direct replacement, with no other changes.

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[robobun]

^{Updated 9:25 AM PT - Aug 28th, 2026}

✅ @robobun, your commit d39b75e278dc7f1fa81d610d505bb8006e7c7a46 passed in Build #107769! 🎉

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[claude]

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