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Fix buffer.transcode() abort when the output needs 2 GiB or more - #42875
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…when an allocation fails buffer.transcode() sized its result and its UTF-16 scratch copies with WTF::Vector::grow(). A WTF::Vector holds less than 2^31 bytes, so a result of 2 GiB or more aborted the process, also inside try/catch. Each path now measures its output and converts straight into an uninitialized Buffer, which removes the Vector limit and one copy of the result. The scratch copies use tryGrow() and throw RangeError: Out of memory when they fail.
…each allocation The narrow encoder sized its result with simdutf::count_utf16le and then wrote one byte per unit that is not a trail surrogate. simdutf selects its implementation at run time, and its stub for an unsupported CPU returns 0, so the writes could pass the end of the result. The count now applies the same test as the writes. Tests: the narrow encoder at the simdutf block sizes, each UTF-16 copy under the debug allocation cap, and the ascii source at the size limit. The 2 GiB test no longer sets its own timeout.
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Status Reproduction, on a release build of main ( bun -e 'try { require("node:buffer").transcode(new Uint8Array(2 ** 30), "latin1", "ucs2") } catch (e) { console.log("caught") }'The process aborts with
The fix is in this PR (#42875). Update. 6c9b2ed removes a slowdown that an earlier revision of this PR added to CI. Green on |
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WalkthroughTranscoding now uses fallible decoding, direct ChangesTranscoding allocation and error handling
Priority: ➖ Normal Merge Risk: 🔵 Low · up to Large surrogate-containing Latin-1 transcodes can unnecessarily fail with out-of-memory when the valid fallback output would fit. Size the fallback output before allocating the fast-path buffer. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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The automated review found no actionable items, so this push stays as it is. The PR has no open review threads. CI is still running. |
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Updated 12:03 AM PT - Sep 16th, 2026
❌ @robobun, your commit 2cc3e81 has 1 failures in 🧪 To try this PR locally: bunx bun-pr 42875That installs a local version of the PR into your bun-42875 --bun |
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…ment two review points The test that reserves 2 GiB of untouched memory skips on a host with less than 4 GiB, which can refuse the reservation. Comments: the count check in the utf8 to ucs2 path does not bound the writes, and a ucs2 source of one byte fails as it does in Node.
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Re-reviewed the follow-up push (3fcf6e4) and found no new bugs; the os.totalmem() gate on the 2 GiB reservation test resolves the CI-host concern raised earlier. Since this rewrites native size arithmetic and the write paths of buffer.transcode(), a human look is still worthwhile before merging.
What was reviewed:
- The three direct paths and the pivot paths now each size their output once and write into
createUninitializedBuffer; every allocation is followed byRETURN_IF_EXCEPTION, andtranscodeEncodeNarrowreturns nullptr with the exception pending so the caller's check is sufficient. - The narrow encoder's hand-rolled code-point count matches the substitution loop's skip condition (
U16_IS_TRAIL), so the bulk latin1 fast path only runs when no surrogates exist and the index writes cannot exceed the allocation. - The ucs2 odd-byte handling pre-sizes the vector by one and assigns
last(), equivalent to the priorappend.
Extended reasoning...
Overview
The PR rewrites jsBufferTranscode in src/jsc/modules/NodeBufferModule.cpp so that every encoding path allocates the result Buffer up front via WebCore::createUninitializedBuffer and converts directly into it, replacing the infallible WTF::Vector::grow() calls (which CRASH() past the 2^31-byte Vector limit or on allocation failure) with tryGrow() plus throwOutOfMemoryError. The test file gains a describe("transcode allocation limits") block with a runCases subprocess helper and a code-point-count pinning test for the narrow encoder. The latest commit (3fcf6e4) adds an os.totalmem() skip gate to the 2 GiB reservation test and adds comments on the utf8-to-ucs2 count check and the one-byte ucs2 source case, which were the three items raised in the prior review.
Security risks
The code operates on user-supplied byte lengths, so the relevant risks are size arithmetic and out-of-bounds writes. length * 2 and expected * 2 are computed in size_t on 64-bit targets and bounded by JSC's typed-array limits before any write, and createUninitializedBuffer throws rather than returning a short buffer. The narrow encoder's writes are bounded by a count computed in the same function with the same predicate as the write loop, so they cannot depend on a simdutf return value. The utf8-to-ucs2 path still converts from a source that can be a SharedArrayBuffer view mutated between the length pass and the convert pass; that write bound is pre-existing on main and was already noted inline, and the new comment acknowledges it rather than fixing it, so it remains a reason for a human maintainer to weigh whether it should ride along.
Level of scrutiny
Moderate to high. This is native C++ memory-handling code in a Node-compat API reachable from arbitrary user input, and the diff changes how every path sizes and fills its result. The logic is small and readable, and each path was traced against the previous implementation for behavior preservation (result sizes, error codes, odd-byte handling), but the combination of adversarial length arithmetic, a known unbounded-write path left in place, and the CI cost of the multi-GiB tests means a human should confirm the trade-offs the author documented (Node throws U_ILLEGAL_ARGUMENT_ERROR where Bun now throws RangeError: Out of memory).
Other factors
The new tests spawn subprocesses with bunExe()/bunEnv, drain pipes concurrently, use test.each-style case tables, and gate the memory-heavy cases on os.totalmem() and isDebug; the debug-only test relies on BUN_JSC_maxSingleAllocationSize, which fails the intermediate Vector allocations but not the typed-array storage, so result-allocation failure is covered by only one case (latin1 to ucs2 past the Buffer limit). The author's description states the full buffer.test.js passes and a differential run against the old implementation was done; that is a claim, not something verified here, but the diff is consistent with it. The exit reason was dry_streak, and no third-party objections are outstanding in the timeline.
The narrow encoder counts and writes with one shared predicate, so the code states what the longer comment explained.
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Replies to the automated reviews, for the two pushes after the first review:
The output is unchanged: the 9,520-input differential run against the old implementation still matches, and the transcode tests pass. |
… count The narrow encoder counted its output bytes with a scalar loop before the simdutf bulk conversion, so an in-range latin1 result paid for a pass that only the substitution path needs. A 64 KiB ucs2 to latin1 call took 14.4 us, against 5.3 us on main. The bulk conversion now runs first, into a Buffer of one byte per unit. The count runs only when that conversion fails. The substitution path reuses the Buffer unless the source has surrogate pairs, which need a shorter result. The same call takes 4.4 us.
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A measurement of this branch before the merge found one slower path. For 6c9b2ed runs the bulk conversion first, as main does, and counts only when that conversion fails. The same call now takes 4.4 us, and The output is unchanged: a 14,000-input differential run of a release build and a debug ASAN build matches main, and |
…scode-large-output
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Actionable comments posted: 1
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Inline comments:
In `@src/jsc/modules/NodeBufferModule.cpp`:
- Line 114: Update transcodeEncodeNarrow to compute its output length before
allocating the result, excluding trailing surrogate units that do not produce
separate bytes. Use that length for the Latin-1 bulk-allocation eligibility and
the substitution-path allocation so a smaller fallback result can still be
allocated when the larger buffer fails.
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This review covers commit 6c9b2ed, which is no longer the latest commit on this pull request; later commits are not covered by it.
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Problem
buffer.transcode()aborts the process when its output needs 2^31 bytes or more:panic(main thread): abort() called, exit code 134, also insidetry/catch. Example:transcode(new Uint8Array(2 ** 30), "latin1", "ucs2"). Node v26.3.0 returns 2,147,483,648 bytes.jsBufferTranscode(src/jsc/modules/NodeBufferModule.cpp:186) sizes its result and its UTF-16 scratch copies withWTF::Vector::grow().grow()callsCRASH()past the Vector limit or on a failed allocation (wtf/Vector.h:228).Fix
RangeError: Out of memorywhen it fails or the result passes the Buffer limit of 2^32 bytes.tryGrow()and throw the same error.U_INVALID_CHAR_FOUND.test/js/node/buffer.test.js(three new tests exit 134 on main),test-icu-transcode.js, and a 14,000-input differential run against main. Timings are in Notes. Self-reviewed: 8 concerns raised, 7 addressed. Not addressed: a shared memory gate for huge-allocation tests.Background
WTF::Vectorholds at most(UINT_MAX >> 1) / sizeof(T)elements: 2^31 - 1 bytes, or 2^30 - 1 UTF-16 units.grow()aborts past that.tryGrow()returns false.transcodehas three direct paths, for example latin1 to ucs2. Every other pair decodes the source to a UTF-16 copy, then encodes it.WebCore::createUninitializedBufferallocates a Buffer with no zero fill and throwsRangeError: Out of memoryon failure. Throw instead of aborting when an ERR_* error message passes the string length limit #42202 uses that error for a result that cannot exist.Notes
Where Bun still throws and Node returns a value. The scratch copies stay
WTF::Vectors, so they keep the Vector limit:U_ILLEGAL_ARGUMENT_ERROR(for example latin1 to utf8 at 2^30 bytes, ucs2 to ucs2 at 2^30 bytes). A comment in the code records this difference with a link to Node's source.Node v26.3.0 on this machine:
Verified by hand on the debug ASAN build, not in the tests (each one touches 3 to 7 GiB):
The narrow encoder. A latin1 or ascii target writes one byte per code point, and a surrogate pair becomes one
?. For a latin1 target the simdutf bulk conversion runs first, into a Buffer of one byte per unit, as on main. When it fails, and for an ascii target, the substitution path counts the units that are not a trail surrogate, and its loop skips trail surrogates: the count and the writes share one predicate (writesByte). The substitution path reuses the Buffer of the bulk attempt unless the source has surrogate pairs, which need a shorter result.The first revision took that count from
simdutf::count_utf16le. The self-review found that this made the bounds of the writes depend on a simdutf return value. simdutf selects its implementation at run time, and every function of itsunsupportedstub returns 0 (SIMDUTF_FORCE_IMPLEMENTATION=nonsenseselects the stub). The result was then 0 bytes long and the loop wrote past it. The count is now a plain loop in the same function. The bulk attempt writes at most one byte per unit into a Buffer of that size, and the stub reports an error and writes nothing. With the stub forced, the debug ASAN build reports no error for the narrow paths. #41374 and #30642 own the stub itself. This PR does not change what the other paths return when the stub is active: old and new both return whatever the stub left in the buffer.Timings. Release builds with ThinLTO of three trees on one base (
367d939d9): main, this PR before6c9b2ed2c8, and this PR now. One pinned core, 5 alternating runs for each binary, 15 rounds in each run, minimum ns per call:"before" counted the output bytes with a scalar loop ahead of the bulk conversion, so an in-range latin1 result was 2.7 times slower than main at 64 KiB. A measurement before the merge found this. The bulk conversion now runs first, and the in-range result is 18 to 20 % faster than main. A source with a surrogate pair pays for the failed bulk attempt and for a second allocation: 6 to 8 % over "before", and 45 % under main. The substitution paths are faster than main. They write through an index, and main called
append()for each byte.The trailing odd byte of a ucs2 source was an
append()after thegrow(), so it reallocated the whole copy. The copy is now sized once.Tests.
transcode to latin1 and ascii writes one byte for each code point: 330 comparisons in process, at lengths on both sides of the simdutf block sizes and of the 1000 bytes above which a typed array is allocated with malloc. It passes on main too. It pins the output of the rewritten encoder.throws when the UTF-16 copy of the source cannot be allocated(debug builds):BUN_JSC_maxSingleAllocationSizefails each of the five scratch allocations with a source of 3 or 10 MiB. On main the infallible allocation asserts.throws past the size limit of a Buffer or a Vector: the child reserves 2 GiB and never writes to it. It takes 0.4 s in a debug ASAN build and its RSS stays at the baseline. It skips below 4 GiB of total memory, because a small host can refuse the reservation.returns a result of 2 GiB: the child writes the whole result (2.4 GB RSS, 2.4 s in a debug ASAN build). It skips below 10 GiB of total memory, the gate its neighbor uses.Other checks. The full
buffer.test.jspasses (683 pass, 1 skip that is also skipped on main). The transcode matrix and the new cases pass underBUN_JSC_validateExceptionChecks=1. The differential run compares a release and a debug ASAN build of this branch with a release build of main. It covers all 16 encoding pairs with random bytes, ASCII, UTF-8 text of every width, UTF-16 with and without lone surrogates, Latin-1 range UTF-16 with and without one unit above U+00FF at a random place, odd lengths, lengths around the SIMD block sizes, and views at an oddbyteOffset.Not changed here. The same differential run against Node shows an older difference in the substitution paths: ICU drops default-ignorable code points such as U+00AD, U+200B and U+FEFF when the target cannot encode them, and Bun writes
?. For exampletranscode(Buffer.from([0x41, 0xad, 0x42]), "latin1", "ascii")is[65, 66]in Node and[65, 63, 66]in Bun. This PR keeps that output as it was.Related open PRs. #42300 changes the ascii fix-up loop in
transcodeDecodeToUtf16. #41574 copies aSharedArrayBuffersource before the conversion. Neither touches thegrow()calls. Expect a small textual conflict with each.no test proof · iteration 5 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/node/buffer.test.js