Validate a cached sourcemap section header before use

[robobun]

Problem

• A cache hit reads the .pile sourcemap section with no hash check and no structural check, then hands the bytes to SavedSourceMap::put_mappings (src/jsc/RuntimeTranspilerCache.rs, Entry::load).
• Stack remapping later reads that blob as an InternalSourceMap. find walks the SyncEntry array and the window streams by the offsets in the blob header. A damaged header reads out of bounds.
• Repro: populate the cache, set the stored sourcemap header sync_count to a large value, run again. The cache-hit run crashes (SIGSEGV on release, ASAN abort on a debug build) while it remaps a stack.

Fix

• After the sourcemap section is read, validate it with InternalSourceMap::is_valid_blob before it is stored on the Entry.
• On failure Entry::load returns the new InvalidSourceMap error. The existing unlink guard in from_file_with_cache_file_path then removes the entry, so the next run transpiles and writes a correct one.
• Correct because is_valid_blob checks the header invariants that find trusts: total_len equals the section length, and stream_offset sits after the SyncEntry array and inside the section. The check is a fixed set of header comparisons, so a cache hit pays no cost proportional to the sourcemap size.
• Verified: test/cli/run/transpiler-cache.test.ts. The new test crashes on the unfixed build and passes with the fix. All 20 tests in the file pass.

Background

• The runtime transpiler cache stores the transpiled output, a sourcemap, and an ES module record per source file. Entry::load reads each section before the module runs.
• InternalSourceMap is Bun's in-process sourcemap format. The blob header holds total_len, sync_count, and stream_offset. SavedSourceMap stores the blob and find reads it during stack remapping.
• is_valid_blob already guards the same format for --compile embedded blobs. This change applies it to the disk cache, which an external process or a torn write can damage.

Notes

This does not restore the full per-section hashing from #39717, which was reverted in #40948 because hashing the whole sourcemap on every cache hit costs time proportional to its size. is_valid_blob is an O(1) header check, so it fits the hot path. It catches a corrupt outer header (the repro). It does not walk per-window SyncEntry.byte_offset values, so a corruption inside a window can still mislead find. That is the same limitation is_valid_blob has for --compile blobs.

The metadata header offsets used by the test: sourcemap_byte_offset at byte 54, sourcemap_byte_length at 62 (Metadata::encode: u32 version, two u8, then twelve u64).

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no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/cli/run/transpiler-cache.test.ts

[robobun]

^{Updated 8:34 PM PT - Sep 5th, 2026}

❌ @robobun, your commit 761009f has 1 failures in Build #110582 (All Failures):

• test/napi/node-napi-tests/test/js-native-api/test_object/do.test.ts - stalled in the parallel batch on 🐧 13 x64

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Walkthrough

The transpiler cache now validates sourcemap blobs before storing them. Invalid blobs return InvalidSourceMap. A regression test verifies cache rejection and regeneration after sourcemap corruption.

Changes

Sourcemap cache validation

Layer / File(s)	Summary
Validate sourcemaps during cache loading src/jsc/RuntimeTranspilerCache.rs, src/jsc/error.rs	Entry::load validates sourcemap blobs before assignment and returns the new Error::InvalidSourceMap variant for malformed data.
Verify corrupted cache regeneration test/cli/run/transpiler-cache.test.ts	The test corrupts cached sourcemap metadata and verifies rejection, regeneration, successful output, and normal process exit.

Suggested reviewers: jarred-sumner, dylan-conway

Merge Risk: ⚪ Minimal · up to 76100

Malformed cached sourcemaps are rejected and regenerated instead of being used for stack remapping. The covered corruption scenario preserves successful execution, with no current merge-blocking risk identified.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description clearly explains the problem, fix, validation approach, and test results. It does not use the template headings exactly, but it includes the required information.
Title check	✅ Passed	The title is concise, specific, and accurately describes the main change: validation of cached sourcemap section headers before use.
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.

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In `@test/cli/run/transpiler-cache.test.ts`:
- Line 650: Update the filler fixture construction near the `filler` constant to
use `Buffer.alloc(...).toString()` for the repeated content instead of
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repetition counts.
- Line 672: In the assertion group for the first transpiler-cache run, move the
`first.exitCode` assertion to after the `existsSync(cache_dir)` filesystem
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[claude]

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

LGTM — the read-path guard reuses the existing is_valid_blob helper and returns via the same crate::CrateError early-return pattern as the neighboring hash checks, so a corrupt entry falls through to the existing unlink-and-regenerate path. One inline nit on the test's use of String.prototype.repeat.

Extended reasoning...

Overview

The PR adds an O(1) header validation of the sourcemap section read from a .pile runtime-transpiler-cache entry before it is stored on the Entry and later handed to SavedSourceMap/InternalSourceMap::find. It introduces a new InvalidSourceMap variant on the bun_jsc crate's Error enum (plus its label() arm) and a regression test in test/cli/run/transpiler-cache.test.ts that corrupts the on-disk sourcemap header's sync_count and asserts the cache-hit run still completes cleanly.

Security risks

None material. The transpiler cache lives in the user's own cache directory; an attacker with write access there can already substitute arbitrary transpiled output, so this is robustness hardening (crash on torn/corrupt write) rather than a security boundary. The change strictly tightens validation of on-disk bytes before they drive pointer arithmetic — exactly what REVIEW.md's "treat all size/index/length arithmetic on external data as adversarial" rule asks for.

Level of scrutiny

Low-to-moderate. The Rust change is ~12 lines and mirrors the adjacent InvalidHash/MissingData early returns byte-for-byte in shape; it reuses the existing bun_sourcemap::is_valid_blob (already guarding --compile embedded blobs) rather than hand-rolling validation. I checked that the new early return does not introduce a new resource-leak path: there are already error returns after self.output_code is populated (e.g., the esm_record hash check at line 526), so cleanup semantics are unchanged. sourcemap is a Box<[u8]> and drops naturally on the error return. No on-disk format change, so no version bump is needed.

Other factors

The test asserts positive output ("OK" in stdout) before exitCode/signalCode, follows the file's existing spawnSync/temp_dir/env fixtures, and verifies the precondition (corrupted >= 1) so it cannot pass vacuously. The one confirmed finding is a test-performance nit ("x".repeat(120) vs Buffer.alloc), which does not affect correctness.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[robobun]

CI on 761009f: 180 of 181 jobs pass. The one failure is test/napi/node-napi-tests/test/js-native-api/test_object/do.test.ts on debian 13 x64, which also fails on main and is unrelated to this change. test/cli/run/transpiler-cache.test.ts passes on every lane. Ready for review.