zlib: reach the z_stream only through a raw pointer while zlib-ng holds its back-pointer by robobun · Pull Request #46607 · oven-sh/bun

robobun · 2026-10-23T17:04:19Z

Fixes #46598

Behaviour change: none

Problem

zlib-ng stores a pointer to the z_stream in its internal state at init (vendor/zlib/deflate.c:299 s->strm = strm, inflate.c:247) and reads and writes the stream through it inside every later call (fill_window: s->strm->avail_in, deflate_stored.c: s->strm->next_out).
Rust held a &mut zStream_struct across those calls: fn step(strm: &mut zStream_struct, ..) in src/zlib/lib.rs, and &mut self methods over structs that held the z_stream inline (ZlibReaderArrayList, ZlibCompressorArrayList, node:zlib Context). A foreign access to a protected &mut is undefined behaviour under Stacked Borrows and Tree Borrows. BorrowSanitizer reports it on DeflateEncoder::step. Same bug as Tree borrows violation occurs when using zlib backend rust-lang/flate2-rs#392.

Fix

Add ZStream (src/zlib/stream.rs): a heap-pinned z_stream behind a NonNull. It never hands out a reference to the stream. Field reads and writes go through the raw pointer, deflate_init/inflate_init carry the zlib version and struct size, and every other call into zlib takes as_ptr(). bun_zlib no longer re-exports the raw z_stream.
Every stream owner holds a ZStream: DeflateEncoder, InflateDecoder, both ArrayList wrappers, node:zlib Context, and the Web CompressionStream coder. A &mut over the owner now covers the pointer field only, so the &mut self methods stay.
The ArrayList wrappers no longer return Box<Self>. They were boxed so the inline stream did not move. The stream is pinned on its own now.
Verified: a Miri model of the new shape passes under both aliasing models (the old shape fails both). test/js/node/zlib/*, test/js/web/streams/compression.test.ts, test/js/web/fetch/fetch-gzip.test.ts, fetch-compress.test.ts, and the three websocket-permessage-deflate* files pass on the debug build.
c_thunks_for_zone! (src/bun_alloc/c_thunks.rs) created two malloc zones per label, one in the alloc thunk and one in the free thunk. It now creates one. Only a macOS debug build without ASAN reaches that code.
Self-reviewed: 6 concerns raised, 6 addressed (the missed CompressionStreamCoder owner, the dead allocator thunks, a wrong helper name in a SAFETY comment, init helpers on ZStream, the z_stream re-export, the split malloc zone).

Background

z_stream is zlib's public stream struct. The caller sets next_in/avail_in/next_out/avail_out and calls deflate/inflate. zlib keeps a pointer to that struct in its private state and uses it in every call, so the struct must not move and Rust must not hold an exclusive borrow of it while zlib runs.
Tree Borrows is the aliasing model this repo checks against (scripts/rust-miri.ts). The old shape fails under it as well as under Stacked Borrows.
A &mut T parameter is noalias to LLVM. Release builds link Rust and vendor/zlib with cross-language ThinLTO, so once deflate() inlines into step() LLVM can reorder the C side's stores through s->strm against the Rust side's loads.
Considered the minimal patch (raw pointer in step only). It leaves the three inline-z_stream owners with the same shape and nothing stops a later method from forming a &mut again. One owning type puts the rule in the place every producer passes through.

Downsides

node:zlib handles pay one 112-byte heap allocation per handle at construction (the z_stream was inline in the handle before). The other owners allocate the same count as before: one per stream.
CompressionStream/DecompressionStream now give zlib-ng Bun's allocator thunks instead of a null zalloc (zlib-ng's own malloc). The internal state lands on the same heap as every other zlib stream in Bun.
Per call: no new allocation, syscall, or branch. Field access is the same single indirection the Box<zStream_struct> had. Binary size not measured: no release build of the merge base in this container, and bloaty is not installed.

Notes

No JS test can observe the aliasing. Miri cannot call into zlib-ng, and BorrowSanitizer needs its own toolchain. The reporter's Dockerfile in Aliasing bug in zlib crate #46598 runs cargo bsan test -p bun_zlib against a src/zlib/tests/repro.rs that calls DeflateEncoder::new(6, 15 + 16, 8, 0) then step(.., FlushValue::Finish).
Miri models: the old shape (Box<Stream> owner, step(&mut self.strm), C reads (*(*s).strm).avail_in) fails under Stacked Borrows ("not granting access to tag because that would remove [Unique] which is strongly protected") and under -Zmiri-tree-borrows ("the accessed tag is foreign to the protected tag"). The new shape (NonNull owner, &mut Context with the owner inline, step(&mut ZStream), &self getters) passes both.
ZlibCompressorArrayList used the untagged mi_malloc_items thunks. It now uses the same "zlib" heap-breakdown zone thunks as every other stream. The bytes go to the same allocator. Those untagged thunks had no other user, so bun_alloc::c_thunks drops them.
ZlibCompressorArrayList::init no longer points next_out at the list before deflateInit2_. read_all sets the output window from the list's spare capacity before each deflate call, and try_reserve_exact in init could move the list after that first pointer was taken.
total_in/total_out were set to the input and list lengths in the ArrayList inits. deflateInit2_/inflateInit2_ reset both to 0 (deflate.c:493, inflate.c:67), so the values were never observed.
test/js/web/fetch/fetch.stream.test.ts fails in this container on the tests after the timeout 100 case. The same tests fail the same way with the released bun, so it is the container, not this diff.

no test proof · iteration 0 · the description declares no behaviour change, so there is no failing test to prove; the existing suite in CI is the check

…ds its back-pointer zlib-ng stores the z_stream pointer in its internal state at init and reads and writes the stream through it inside every later call. Rust formed a `&mut zStream_struct` (or a `&mut self` over a struct that held one inline) and kept it alive across those calls. That foreign access to a protected borrow is undefined behaviour under Stacked Borrows and Tree Borrows. Add `ZStream`, a heap-pinned z_stream behind a NonNull. It never hands out a reference to the stream: field reads and writes go through the raw pointer, and the C calls take `as_ptr()`. `ZlibReaderArrayList`, `ZlibCompressorArrayList`, `DeflateEncoder`, `InflateDecoder`, and the node:zlib `Context` all hold a `ZStream` now. The ArrayList wrappers no longer need to be boxed, since the stream is pinned on its own.

coderabbitai · 2026-10-23T17:04:29Z

No actionable comments were generated in the recent review. 🎉

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Branch: robobun/3b69a136/zlib-stream-raw-pointer
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src/bun_alloc/c_thunks.rs

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Walkthrough

The change adds a ZStream wrapper and updates zlib callers to use its pointer and accessors. Array-list codec initializers now return unboxed values. The plain zlib allocator and free thunks are removed.

Changes

Zlib stream ownership migration

Layer / File(s)	Summary
ZStream ownership and accessors `src/zlib/stream.rs`, `src/zlib/lib.rs`, `src/bun_alloc/c_thunks.rs`	Adds `ZStream` allocation, initialization, pointer access, field accessors, and cleanup. Exports the wrapper and removes the plain zlib allocator and free thunks.
Array-list codecs `src/zlib/lib.rs`	The reader and compressor use `ZStream` for stream state and zlib calls. Their initializers return `Self` instead of `Box<Self>`.
Owned encoders and decoders `src/zlib/lib.rs`	The owned deflate encoder and inflate decoder use `ZStream` for initialization, buffer access, reset, stepping, and cleanup.
Node zlib context `src/runtime/node/zlib/NativeZlib.rs`	The Node zlib context uses `ZStream` across initialization, buffer handling, processing, and cleanup. Mode selection and error handling remain unchanged.
Web CompressionStream `src/runtime/webcore/CompressionStreamCoder.rs`	The deflate and inflate backends use `ZStream` for initialization, buffer handling, processing, and cleanup.

Possibly related issues

Aliasing bug in zlib crate: The changes replace raw stream access in zlib callers with the ZStream wrapper and its pointer/accessor methods, matching the issue’s described aliasing objective.

Suggested reviewers: dylan-conway, jarred-sumner

Priority: ➖ Normal

Merge Risk: ⚪ Minimal · up to 4c243

No actionable merge-blocking risk was established for this change.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title clearly and concisely describes the main change: accessing z_stream through a raw pointer while zlib-ng retains its back-pointer.
Description check		The description explains the problem, implementation, behavior impact, verification steps, known test limitations, and tradeoffs. It does not use the template headings exactly, but it provides the req…
Linked Issues check		Issue `#46598` requires that zlib not read the z_stream while Rust holds a live `&mut zStream_struct` across a zlib call. The reviewed head adds `ZStream` in `src/zlib/stream.rs`. It stores the `z_strea…
Out of Scope Changes check		The changes follow from the `#46598` fix or support it. Moving every stream owner to `ZStream` applies the same aliasing rule to each producer. Removing the `Box<Self>` return from the ArrayList wrapper…

Full details: Description check

Explanation

The description explains the problem, implementation, behavior impact, verification steps, known test limitations, and tradeoffs. It does not use the template headings exactly, but it provides the required information in equivalent sections.

Full details: Linked Issues check

Explanation

Issue #46598 requires that zlib not read the z_stream while Rust holds a live &mut zStream_struct across a zlib call. The reviewed head adds ZStream in src/zlib/stream.rs. It stores the z_stream in a heap allocation behind NonNull and never returns a reference to it. Field reads and writes (next_in, avail_in, avail_out, total_out, err_msg, set_input, set_output) go through as_ptr(). deflate_init and inflate_init pass as_ptr() to zlib with the linked version and struct size. The PR description states that DeflateEncoder, InflateDecoder, both ArrayList wrappers, the node:zlib Context, and the Web CompressionStream coder now own a ZStream, and that bun_zlib no longer re-exports the raw z_stream. The &mut borrow over each owner now covers only the pointer field. The author reports a Miri model that passes under Stacked Borrows and Tree Borrows, and reports passing zlib, compression, fetch-gzip, and websocket-permessage-deflate test suites on a debug build. This assessment did not rerun Miri, BorrowSanitizer, or the test suites. The owner-level call sites in src/zlib/lib.rs and NativeZlib.rs were assessed from the change summary and the description, not from a line-by-line read. The coding requirement is met by the reviewed code shape.

Full details: Out of Scope Changes check

Explanation

The changes follow from the #46598 fix or support it. Moving every stream owner to ZStream applies the same aliasing rule to each producer. Removing the Box<Self> return from the ArrayList wrappers follows from pinning the stream separately. Removing the untagged mi_malloc_items and mi_free_opaque thunks in src/bun_alloc/c_thunks.rs follows from moving ZlibCompressorArrayList onto the zone-tagged zlib thunks. The diff for c_thunks.rs confirms those removals. The macro change in c_thunks_for_zone! (one zone() accessor shared by the alloc and free thunks instead of two get_zone! expansions) is a separate bug fix. It is disclosed in the PR description. It also touches the zone thunks that ZStream::new now installs on every zlib stream (ZlibAllocator uses c_thunks_for_zone!("zlib")), so the alloc and free paths must agree for the new owners. The description also discloses the total_in/total_out initialization change, the ZlibCompressorArrayList::init next_out change, and the Web CompressionStream allocator change. These are tied to the owner migration. No unrelated change is demonstrated.

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robobun · 2026-10-23T17:04:55Z

^{Updated 11:50 AM PT - Oct 23rd, 2026}

✅ @robobun, your commit 4c243d981509d168c95493feb49e013bca884528 passed in Build #131399! 🎉

🧪 To try this PR locally:

bunx bun-pr 46607

That installs a local version of the PR into your bun-46607 executable, so you can run:

bun-46607 --bun

claude

Beyond the inline findings, I also checked the behavior deltas in the ArrayList constructors: dropping the pre-init next_out in ZlibCompressorArrayList::init is safe because read_all installs the output window from spare capacity before every deflate (and the old pointer would have been stale after try_reserve_exact anyway), the removed total_in/total_out seeds are overwritten by deflateInit2_/inflateInit2_, and returning Self instead of Box<Self> is fine since zlib-ng's back-pointer targets the separately heap-allocated z_stream, not the owner.

Extended reasoning...

The change moves every zlib stream owner in src/zlib/lib.rs and src/runtime/node/zlib/NativeZlib.rs onto a new heap-pinned ZStream wrapper in src/zlib/stream.rs so no &mut to the z_stream is live across FFI calls; it touches no auth, crypto, or input-parsing surface beyond the existing decompression paths. The refactor side effects on the ArrayList constructors (pointer seeding, total counters, Box removal) were traced and are behavior-preserving.

Findings marked 🟡 are optional suggestions and need no follow-up push.

… allocator thunks CompressionStreamCoder still held Box<z_stream>. It now holds ZStream like every other owner. The untagged zlib allocator thunks in bun_alloc lost their last user, so remove them.

claude

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

The follow-up commit addresses the CompressionStream owner, the untagged thunks, and the Drop SAFETY comment from the earlier round. Beyond the inline note, I also checked that the three callers of the ArrayList init functions (BunObject.rs, StandaloneModuleGraph.rs, create_command.rs) bind the result without a Box annotation, that ZlibCompressorArrayList::read_all sets the output window before every deflate so dropping the init-time next_out changes nothing, and that NativeZlib::drop calls close() before Context's ZStream frees the z_stream.

Extended reasoning...

The second push migrated src/runtime/webcore/CompressionStreamCoder.rs onto ZStream, deleted mi_malloc_items/mi_free_opaque from src/bun_alloc/c_thunks.rs, and corrected the SAFETY comment in src/zlib/stream.rs. This run's remaining concerns were the changed init return types, the removed init-time next_out, and drop ordering for node:zlib handles; each checked out from the code. The change is an FFI aliasing refactor across five files with no automated test, so a human look is still appropriate.

Still open from earlier reviews (2):

Unresolved: 2 minor or pre-existing.

ZStream::deflate_init and ZStream::inflate_init carry the zlib version and struct size, so no owner names zStream_struct any more. bun_zlib no longer re-exports z_stream. Shorter comments in stream.rs.

claude

The CompressionStreamCoder owner and the Drop SAFETY wording from the earlier pass are now in line with the rest of the migration. Beyond the inline finding, I also checked the init-failure paths that lost their explicit drop(zlib_reader) in src/zlib/lib.rs (try_reserve_exact failure and the non-Ok init codes): both ArrayList wrappers still run their owner Drop, which calls deflateEnd/inflateEnd before ZStream frees the cell, so no zlib-ng state leaks there. The four external callers of ZlibReaderArrayList::init/ZlibCompressorArrayList::init only use method calls, so the Box<Self> to Self return change is source-compatible.

Extended reasoning...

The diff adds a heap-pinned ZStream wrapper and rewires five zlib owners (ArrayList wrappers, DeflateEncoder/InflateDecoder, node:zlib Context, Web CompressionStream coder) to reach the z_stream only through a raw pointer, deleting the untagged allocator thunks and the z_stream re-export. It touches unsafe FFI and allocator pairing but no auth, injection or data-exposure surface. A human look is still worthwhile because the change is a cross-cutting unsafe refactor with no committed test and one confirmed (pre-existing) allocator-zone finding posted inline.

c_thunks_for_zone! expanded get_zone! once in malloc_size and once in free. Each expansion is its own static, so on a macOS debug build without ASAN the free thunk used a different zone than the alloc thunk.

coderabbitai

Actionable comments posted: 1

🪄 Fix comments: Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @src/zlib/stream.rs:
- Around line 67-68: Add an automated aliasing regression test for
DeflateEncoder that initializes the retained zlib back-pointer and exercises it
through DeflateEncoder::step. Ensure the test detects the pre-fix borrowing
behavior without changing unrelated code.

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Branch: robobun/3b69a136/zlib-stream-raw-pointer
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📒 Files selected for processing: 5

src/bun_alloc/c_thunks.rs
src/runtime/node/zlib/NativeZlib.rs
src/runtime/webcore/CompressionStreamCoder.rs
src/zlib/lib.rs
src/zlib/stream.rs

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claude

I re-reviewed the latest push (4c243d9) and found no new bugs; the points raised in earlier rounds are now addressed in the code, but because this introduces a cross-cutting unsafe/FFI ownership type used by node:zlib, CompressionStream, and Bun.gzipSync/gunzipSync, a human look is still worthwhile.

What was reviewed:

c_thunks_for_zone! now resolves one zone() per label, so alloc and free share a single malloc zone; get_zone! is otherwise unused.
Every z_stream owner (both ArrayList wrappers, DeflateEncoder/InflateDecoder, node:zlib Context, CompressionStreamCoder) reaches the stream only via ZStream::as_ptr()/accessors; no leftover field access, &raw mut, or z_stream re-export consumer remains in src/.
Drop ordering checked: deflateEnd/inflateEnd runs in each owner's Drop (or NativeZlib::drop -> close()) before the ZStream field frees the struct; init-failure paths hit *End on a null internal_state, which is a defined no-op.
Callers of the Result<Box<Self>> -> Result<Self> change (BunObject.rs, create_command.rs, StandaloneModuleGraph.rs) use the value by method call only; mi_malloc_items/mi_free_opaque have no remaining users. The diff still ships no automated test.

Extended reasoning...

The change adds src/zlib/stream.rs (a heap-pinned z_stream behind NonNull with accessor methods) and migrates all five zlib stream owners to it, deletes two allocator thunks, and fixes the zone thunk macro to create one malloc zone per label. It touches no auth, crypto, or injection surface, but it is unsafe/FFI memory-ownership code on paths reachable from user input (node:zlib, CompressionStream, Bun.gzipSync) and silently changes the allocator CompressionStream and node:zlib hand to zlib-ng. The bug hunt ran dry and my own read confirmed drop ordering, caller migration, and dead-code removal, which argues against silence now that the last commit addressed the previously raised zone-split finding. Deciding against approve: the PR introduces a new cross-cutting abstraction, the diff carries no automated test, and several review threads were resolved only by the author.

github-actions Bot added the claude label Oct 23, 2026

claude Bot reviewed Oct 23, 2026

View reviewed changes

Comment thread src/zlib/stream.rs Outdated

Comment thread src/zlib/stream.rs Outdated

Comment thread src/zlib/lib.rs

Comment thread src/zlib/lib.rs

Comment thread src/runtime/node/zlib/NativeZlib.rs Outdated

zlib: move the CompressionStream coder onto ZStream and drop the dead…

eecf281

… allocator thunks CompressionStreamCoder still held Box<z_stream>. It now holds ZStream like every other owner. The untagged zlib allocator thunks in bun_alloc lost their last user, so remove them.