fix: OrderedDict.get() crashes on wasi by jiwahn · Pull Request #8896 · RustPython/RustPython

jiwahn · 2026-09-29T02:32:44Z

Closes #xxxx

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Summary

Problem: On wasi, OrderedDict.get() crashes with an out of bounds memory access
Cause: OrderedDict uses by_hash: HashMap which requires 8 byte align
Fix: Wrap by_hash in Box<HashMap<...>> to bring align back down to 4 bytes

Assisted-by: Claude Code:claude-sonnet-5

Below is the reproducer of this issue

╰─$ cd ~/RustPython                                                                                                                
BIN=bin_path
wasmer run --volume "$(pwd)" "$BIN" -- -c "
from collections import OrderedDict
o = OrderedDict()
print(o.get('x', 0))
o['x'] = 1
print(o.get('x', 0))
"
RuntimeError: out of bounds memory access
    at <unnamed> (<module>[8872]:0x4f516a)
    at <unnamed> (<module>[8870]:0x4f4ea5)
    at <unnamed> (<module>[12320]:0x697621)
    at <unnamed> (<module>[12321]:0x697727)
    at <unnamed> (<module>[8362]:0x4c57c2)
    at <unnamed> (<module>[8976]:0x4fd738)
    at <unnamed> (<module>[9499]:0x56dd74)
    at <unnamed> (<module>[9436]:0x529a49)
    at <unnamed> (<module>[8975]:0x4fd506)
    at <unnamed> (<module>[3174]:0x1e64f5)
    at <unnamed> (<module>[3141]:0x1e29e7)
    at <unnamed> (<module>[3121]:0x1deb15)
    at <unnamed> (<module>[83]:0xea20)
    at <unnamed> (<module>[34]:0xac6c)
    at <unnamed> (<module>[33]:0xa0aa)
    at <unnamed> (<module>[86]:0x113ec)
    at <unnamed> (<module>[32]:0xa086)

Summary by CodeRabbit

Bug Fixes
- Reported memory sizes for ordered dictionaries now account more accurately for their allocated storage. This improves the accuracy of size information without changing how ordered dictionaries behave or how their contents are accessed.

coderabbitai · 2026-09-29T02:33:02Z

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crates/vm/src/stdlib/_collections/ordered_dict.rs

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📝 Walkthrough

Walkthrough

ODictLinks.by_hash now stores its hash map in a Box. PyOrderedDict::__sizeof__ uses the boxed map type in its size calculation.

Changes

Ordered dictionary storage

Layer / File(s)	Summary
Box and account for the hash map `crates/vm/src/stdlib/_collections/ordered_dict.rs`	`ODictLinks.by_hash` changes to a boxed hash map. `PyOrderedDict::__sizeof__` uses the boxed map type in its size calculation.

Priority: ➖ Normal

Estimated code review effort: 1 (Trivial) | ~2 minutes

Change: Bug fix

Suggested reviewers: youknowone

Merge Risk: ⚪ Minimal · up to 03d09

The reported WASI alignment fix and memory accounting are complete; no merge-blocking risk remains.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 03d09

The change is confined to ordered-dictionary storage and memory-size reporting. It does not add an entrypoint or change who can access the dictionary. No new security issue was established, but the WASI fix has not been verified by a runtime result here.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

inferred — Python code can still exercise the existing OrderedDict operations; the changed private storage does not add an observed caller, privilege, or cross-service path.

Trust Boundaries and Controls

observed — The hash index remains behind the existing ordered-dictionary methods and link mutex; neither is removed by the two-line change.

Resilience and Maintainability Implications

inferred — The reported WASI failure is an availability concern for code exercising OrderedDict. Source-level alignment reasoning supports the fix, while its runtime effect and any broader exposure remain unverified.

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Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files.
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.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies the main change: fixing an `OrderedDict.get()` crash on WASI. This matches the PR objectives and the boxed `HashMap` alignment fix.

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Inline comments:
Review comments at @crates/vm/src/stdlib/_collections/ordered_dict.rs:
- Line 51: Update PyOrderedDict::__sizeof__ to include the separately allocated
HashMap header held by ODictLinks::by_hash, in addition to the existing
ODictLinks and node storage size calculations.

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crates/vm/src/stdlib/_collections/ordered_dict.rs

Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 9 remain after this review.

- Problem: On wasi, OrderedDict.get() crashes with an out of bounds memory access - Cause: OrderedDict uses by_hash: HashMap which requires 8 byte align - Fix: Wrap by_hash in Box<HashMap<...>> to bring align back down to 4 bytes Assisted-by: Claude Code:claude-sonnet-5 Signed-off-by: Jiwoo Ahn <[email protected]>

youknowone · 2026-09-29T02:51:55Z

        nodes: Vec<Option<ODictNode>>,
        free: Vec<usize>,
-        by_hash: HashMap<PyHash, Vec<usize>>,
+        by_hash: Box<HashMap<PyHash, Vec<usize>>>,


what's happening in wasi? since both Box and HashMap are indirection, this will cause double indirection, which is not very preferred. I'd like to understand how and why this broken in wasi

In wasi, Py<T> has 28 bytes header size and when the T is OrderedDict, the payload requires 8 byte alignment (but the base PyDict has 4 byte align). So accessing an OrderedDict through its PyDict base results to using the wrong payload addrress

Thanks, we have to adjust alignment rather than adding a Box.

Opened #8905 as a follow-up to #7663 and the layout analysis here. It adds compile-time checks for native subclass payload offsets and object alignment, and aligns PyDict and _IOBase to eight bytes so the affected hierarchies satisfy them without adding a Box.

The original #7663 inherited-getter regression is included. The OrderedDict.get() reproducer passes on wasm32-wasip1 under Wasmtime.

AI assistance: Codex (GPT-6).

It seems that #8905 fixes this so imma close this one

JMLX42 · 2026-09-29T09:28:19Z

We hit the same trap in a wasm32-wasip2 component embedding RustPython at d31ccae (requests reaches it through resolve_proxies on every redirect). Some measurements that answer why it shows only on wasm32:

OrderedDict.get is inherited from dict, so PyDict::get runs on a &Py<PyDict> cast from the OrderedDict object (the derive gives PyOrderedDict the PyDict payload type id). Py<T> places payload: T after the header at an offset that depends on align_of::<T>(). ODictLinks::by_hash's HashMap carries a RandomState of two u64, so it raises PyOrderedDict's alignment to 8 on wasm32, where PyDict's is 4:

Target	`align_of` PyDict	`align_of` PyOrderedDict	payload offset in `Py<PyDict>`	payload offset in `Py<PyOrderedDict>`
x86_64-unknown-linux-gnu	8	8	48	48
wasm32-wasip2	4	8	28	32

On 64-bit targets the offsets match, so the bug is invisible there. On wasm32 PyDict::get reads 4 bytes early and traps (out of bounds memory access). setdefault and pop, which OrderedDict overrides, take &Py<PyOrderedDict> and work.

Reproducer (x86_64 prints 1; wasm32-wasip2 release under wasmtime traps):

from collections import OrderedDict
print(OrderedDict({"x": 1}).get("x"))

The same class of defect applies to any #[pyclass(base = X)] whose payload has a larger alignment than X on some target. A compile-time check in the derive, next to the existing offset_of!(..) == 0 check, would catch it everywhere, e.g. asserting that the payload offset of Py<Sub> equals that of Py<Base> (payload_offset::<T>() in crates/vm/src/object/core.rs computes it). We have not built that check; it is a suggestion.

The measurements above were run locally on both targets.

🤖 Written by Claude Code (2.1.283) using model claude-opus-5-5

github-actions Bot added the z-ca-2026 Tag to track Contribution Academy 2026 label Sep 29, 2026

github-actions Bot assigned jiwahn Sep 29, 2026

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Comment thread crates/vm/src/stdlib/_collections/ordered_dict.rs

jiwahn force-pushed the wasi-ordereddict-align branch from 4041b30 to 03d090e Compare September 29, 2026 02:48

youknowone reviewed Sep 29, 2026

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jiwahn marked this pull request as draft September 29, 2026 14:17

1ndahous3 mentioned this pull request Sep 29, 2026

Validate native subclass object layouts #8905

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jiwahn closed this Sep 29, 2026

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fix: OrderedDict.get() crashes on wasi - #8896

fix: OrderedDict.get() crashes on wasi#8896
jiwahn wants to merge 1 commit into
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jiwahn:wasi-ordereddict-align

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