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Shortened the three comments the review flagged (b3bb2a7). No code change. |
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The two comments are one line each now (2ce10c7). The code is unchanged. |
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Findings marked 🟡 are optional suggestions and need no follow-up push.
Additional findings (outside the current diff — GitHub can't attach inline comments there):
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🔴
src/jsc/bindings/sliceAnsi.cpp— Users whose slice overflows the builder inside the speculative ellipsis zone still get a crash (debug/ASAN assert, release null-buffer use) instead of the promised RangeError. With RecordOverflow (sliceAnsi.cpp:882) an overflowed builder drops its buffer and sets length to UINT_MAX, yet sliceAnsi.cpp:1322 calls result.shrink(specZoneMark) on it before the hasOverflowed() check at 1348. Fix: never touchresultafter it overflows: return std::nullopt at walkDone when result.hasOverflowed() before the shrink, with the same guard in enterSpecZone; a guard only inside enterSpecZone leaves this path open because the zone is already entered when the overflow happens. [also at: src/jsc/bindings/sliceAnsi.cpp:1006 - Debug builds can still hit an assertion abort on an oversized slice instead of the newRangeError, when the builder overflows before the walk reaches columnend.]Extended reasoning...
This is the sibling of the enterSpecZone/result.length() finding but a different site and ordering: there the overflow happens before the zone is entered; here it happens after, so a fix that only guards enterSpecZone does not cover it. WTF::StringBuilder::shrink asserts !hasOverflowed() in debug; in release it proceeds with m_buffer == nullptr and a null m_string, then the following appends at 1343-1347 memcpy m_length bytes from a null span and toString at 1350 reads a null buffer. Trace: input s = "\x1b[31m" + "a"×(K-1) + U+200B×M + "a" + "b" with K+M+6 = 2^31-1 (about 4 GiB of 16-bit input), call Bun.sliceAnsi(s, 1, K, "…" + U+200B×10). start=1 so needStartEllipsis, start becomes 2 (973-976); end is finite and cutEndKnown is false so end becomes K-1, specEnd = K (980-988). The walk emits open code (5) + ellipsis (11) + K-3 letters. The U+200B run sits at column K-1 >= end, so enterSpecZone runs (1082-1083) and each zero-width cluster is appended in the zone (1084) while position stays below…
Verification: normal (debug/ASAN abort is the same abort the base already has on this input; what is new is that release builds now call StringBuilder::shrink on an overflowed RecordOverflow builder, violating WTF's
ASSERT(!hasOverflowed())precondition instead of hitting the base's clean CRASH()). Trigger: the result crosses String::MaxLength (or a builder allocation fails) AFTERenterSpecZone()and… -
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src/jsc/bindings/sliceAnsi.cpp— Users slicing a string that carries one very long SGR sequence with a colon or more than 32 parameters still abort the process on allocation failure instead of receiving the RangeError this PR promises. sliceAnsi.cpp:303-306 copies the whole opaque sequence character by character into a default CRASH-policy StringBuilder on every parse, the same per-parse copy shape the PR removed for hyperlinks at :575 and :593. Fix: store the opaque SGR open code as a span of the input (as parseHyperlink now does) or build it with OverflowPolicy::RecordOverflow and propagate nullopt, so every builder inside sliceAnsi shares the same catchable failure path.Extended reasoning...
Input: "\x1b[" + "1:" repeated ~500 M times + "m" + "abc" (an SGR with colon params, accepted by parseCsi at :443-460 as canonical SGR), sliced with start 0. applySgrToState reaches the hasColon branch at :296; the loop at :304-305 appends one UChar per code unit to openSb with the default policy. The builder grows by doubling; each growth that fails allocation calls didOverflow() which is CRASH() under the default policy, so the process aborts with the same 'panic(main thread): abort() called' the issue describes. Even when allocation succeeds, the copy doubles the memory held for the sequence on every parse of that sequence (SgrStyleState keeps it as a String, then emitOpenCodes appends it again into result). parseHyperlink had the identical shape and the PR replaced it with a std::span view for exactly the memory reason; this sibling site was left on the CRASH policy. Dismissed as pre-existing because the bytes stay Latin1, but the PR's stated class is 'abort inside WTF from sliceAnsi', and the remaining abort site is in the same function family. Population: adversarial or…
Verification: pre-existing. Trigger: an input whose single SGR sequence (colon params, or >32 params which sets overflow and is folded into hasColon at src/jsc/bindings/sliceAnsi.cpp:295) is hundreds of MB to ~2 GiB long, and the copy's allocation fails (memory limit / no overcommit). Mechanism verified: applySgrToState at :303-306 still does `StringBuilder openSb; for (...)… | pre-existing (the base branch…
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On the three findings:
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I re-reviewed after 9da75a4 and found no new bugs; the canUpconvert gate in emitSliceStreaming addresses the reserve regression I raised on the previous push. Since this is native overflow handling in a WTF StringBuilder path with a large-memory test, a human look is still worthwhile.
What was reviewed:
- The new reserve logic: the full
input.size()reserve for all-Latin1 results, the capped reserve only when a 16-bit character can arrive, and thehasOverflowed()/exceedsStringLimitcheck beforetoString(). - The hyperlink
std::spanviews: every consumer (activeHyperlinkCode,pendingHl, theflushPendingreopen) lives insideemitSliceStreaming, and the input span outlives the call, so no dangling view. - The ASCII fast path: length is pre-checked and
tryMakeStringnull is routed tonullopt; the movedexceedsStringLimitkeeps the same predicate for stream consumers via theusing.
Extended reasoning...
Overview
The PR changes Bun.sliceAnsi to return a catchable RangeError: Out of memory instead of crashing inside WTF when the result would exceed the string length limit. It adds src/jsc/bindings/StringSizeLimit.h (hoisting exceedsStringLimit from WebStreamsInternals.h and adding cappedStringBuilderReserve), switches the streaming builder in sliceAnsi.cpp to OverflowPolicy::RecordOverflow, pre-checks and uses tryMakeString on the ASCII fast path, replaces per-hyperlink StringBuilder copies with std::span views of the input, documents the throw in bun.d.ts, and adds tests under a 64 KiB synthetic limit plus two memory-gated 2^30 code-unit cases.
Security risks
None specific to this change. The input is a JS string already bounded by String::MaxLength; the new arithmetic (content.length() + ellipsisCount * ellipsis.length()) is in size_t and cannot overflow. The change strictly reduces crash surface (a user-triggered abort becomes a catchable error).
Level of scrutiny
Moderate. It is native C++ in a string-building path where the failure mode is a process abort or an over-allocation, and the cap logic depends on non-obvious WTF StringBuilder growth behavior (the 8-bit to 16-bit doubling). The follow-up commit 9da75a4 correctly narrows the capped reserve to the cases where up-conversion is possible (sizeof(Char) == 2 || !ellipsis.is8Bit()), which was the regression I flagged previously. I also traced the std::span hyperlink views through parseHyperlink, tryParseAnsi, computeTotalWidth, activeHyperlinkCode, and pendingHl: all are consumed inside emitSliceStreaming while the input span is alive. A separate pre-existing limitation for 8-bit results past ~1 GiB that later up-convert remains as noted inline on the prior push and is not introduced here.
Other factors
The test block covers at-limit and one-past-limit boundaries for SGR, wide characters, ellipsis in three positions, and both hyperlink forms, and asserts the exact error class and message with a combined {stdout, stderr, exitCode} object; the large-memory cases assert signalCode === null. The 2^30 tests are skipped below 8 GiB, which limits CI exposure but means those paths are only exercised on large runners. No outstanding third-party objections are visible in the timeline, but given the WTF-internal assumptions and the memory-gated tests, I am not confident enough to say no human needs to look.
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Thanks, nothing further to change from the re-review. One note for the human reviewer: the red |
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Updated 10:36 AM PT - Sep 16th, 2026
✅ @robobun, your commit 3a3553357ce3057e7a858f955dd58c66dea888d5 passed in 🧪 To try this PR locally: bunx bun-pr 42941That installs a local version of the PR into your bun-42941 --bun |
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why is this stacked on another branch? |
…tring length limit The output StringBuilder records overflow instead of crashing, and the binding throws an out-of-memory RangeError when the result is too long. The ASCII fast path checks the length and uses tryMakeString. The reserve of the output builder is capped at half of the longest 16-bit string when the input or the ellipsis is 16-bit, so that the doubled capacity after the first 16-bit character stays a valid length. A hyperlink sequence is now a view of the input instead of a copy in a StringBuilder. The string length predicate moves from WebStreamsInternals.h into a shared StringSizeLimit.h next to the reserve cap.
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It was stacked because #42929 (open) changes the same two functions in That is not worth a PR that targets another branch. The PR now targets main and is self-contained (3a35533). |
Walkthrough
ChangessliceAnsi string-limit handling
Suggested reviewers: Priority: ➖ Normal Severity of issue fixed: Medium Merge Risk: 🔵 Low · up to An oversized ellipsis can bypass the documented out-of-memory error path. Add the shared checked construction before merging. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Actionable comments posted: 2
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Inline comments:
In `@src/jsc/bindings/sliceAnsi.cpp`:
- Around line 1344-1345: Update emitSliceStreaming and the printable-ASCII fast
path so every bare-ellipsis return uses one shared allocation-aware helper that
checks ellipsis.length() against the string limit and returns std::nullopt when
oversized; preserve normal ellipsis output otherwise. Add a regression case
covering an oversized bare ellipsis with the synthetic limit enabled and verify
it propagates as the existing RangeError.
In `@test/js/bun/util/sliceAnsi.test.ts`:
- Line 1697: Replace the unrestricted repetitive string construction in the
affected Bun.sliceAnsi test expressions with Buffer.alloc(...).toString(), using
UTF-16LE conversion for the 16-bit character case. Preserve the existing counts,
fill values, and test behavior while avoiding string.repeat for these large
inputs.
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I re-reviewed the latest push and found no new bugs; since packages/bun-types/bun.d.ts is a CODEOWNER path and the C++ change reworks the builder's overflow handling, a human look is still worthwhile.
What was reviewed:
- The reserve cap is now gated on
canUpconvert(16-bit input or 16-bit ellipsis), so all-Latin1 slices keep the fullinput.size()reserve — this addresses the regrowth concern from my earlier inline comment. - Hyperlink
std::spanviews (activeHyperlinkCode,pendingHl) point into the inputStringViewthat the host function holds for the whole call, so none outlives its backing memory. - Test boundaries: traced the SGR/ellipsis/hyperlink at-limit inputs to a result length of exactly 65536 against the
length > limitpredicate, and confirmedBUN_FEATURE_FLAG_SYNTHETIC_MEMORY_LIMITis read insrc/jsc/VirtualMachine.rs. - The
node:url/node:utilcommits in the diff range are already-merged main commits (#42269, #42283), not part of this PR's change.
Extended reasoning...
Overview
The PR proper is a single commit touching src/jsc/bindings/sliceAnsi.cpp, a new src/jsc/bindings/StringSizeLimit.h, src/jsc/bindings/webcore/streams/WebStreamsInternals.h (now a using of the hoisted exceedsStringLimit), a @ throws JSDoc note in packages/bun-types/bun.d.ts, and new tests in test/js/bun/util/sliceAnsi.test.ts. The other two commits in the diff range (bec98aae, a8e4e904) carry merged PR numbers and are base-branch commits, not PR content. The native change switches the output StringBuilder to OverflowPolicy::RecordOverflow, caps the reserve only when the result can be upconverted to 16-bit, pre-checks the ASCII fast path's result length and uses tryMakeString, and returns std::optional<WTF::String> so the binding throws RangeError: Out of memory instead of aborting. Hyperlink sequences become std::span views of the input rather than per-hyperlink copies.
Security risks
No injection, auth, or data-exposure surface. The memory-safety question is the new std::span<const Char> views held in activeHyperlinkCode and the pendingHl tuples; they reference the input span, which is backed by the StringView the host function holds until return, so they cannot dangle. result.append(span) copies into the builder. The overflow path checks hasOverflowed() before toString(), which is required since toString() asserts on an overflowed builder. For the non-upconvert case, reserveCapacity(input.size()) is bounded by the JSString's length, which already fits String::MaxLength.
Level of scrutiny
Medium. The change is focused and the failure-mode reasoning (8-bit builder doubling on first 16-bit char, max16BitLength / 2 cap with static_asserts) is sound, and the latest push fixed the one regression I raised earlier by gating the cap on canUpconvert. However *.d.ts is a CODEOWNER path and a human maintainer commented after the last round; I cannot see that text, so I cannot confirm every human concern is settled. The bug hunt exited on a dry streak with no findings. The prior pre-existing note about an 8-bit result past ~1 GiB that later upconverts remains a WTF-level limitation the PR does not claim to fix.
Other factors
The synthetic-limit test computes exact result lengths for SGR, ellipsis (start/end/both), ESC and C1 hyperlinks, and wide characters, hitting the at-limit / one-past boundary on each, and BUN_FEATURE_FLAG_SYNTHETIC_MEMORY_LIMIT is read in VirtualMachine.rs. The 2^30-code-unit real-scale tests are memory-gated with skipIf and assert signalCode === null. The node:url and node:util changes were checked anyway: ERR_INVALID_URL attaches input natively in ErrorCode.cpp, so dropping the $putByIdDirect wrapper loses nothing, and the createAbortedListener factory avoids the user-replaceable Function.prototype.bind lookup.
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On the two CodeRabbit threads: the bare-ellipsis returns copy a JS string that already fits in a string, so they cannot pass the real limit, and the test uses |
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Problem
Bun.sliceAnsiaborts the process on five large inputs:panic(main thread): abort() calledfromCRASH()inWTF::StringBuilder::didOverflow, or frommakeString. A script cannot catch it. Fixes Bun.sliceAnsi still aborts the process in StringBuilder and makeString when the input is near the string length limit #42931.sliceAnsi.cpp:876). The first 16-bit character doubles that reserve, and with an input of 2^30 code units the doubled capacity is longer than the longest 16-bit string, so the builder fails for a result of two characters. The per-hyperlink builder inparseHyperlink(sliceAnsi.cpp:572) fails the same way for a 16-bit URI past 2^30.String::MaxLength. The slice reopens the active styles, closes them at the end, and adds the ellipsis, so the output is longer than the input.Fix
sliceAnsiImplreturnsstd::optional, wherenulloptmeans out of memory, and the binding throwsRangeError: Out of memory. The output builder usesOverflowPolicy::RecordOverflow, and the ASCII fast path checks the result length and usestryMakeString. The merged stream consumers throw the same error for a string past the limit.std::spanof the input instead of a copy in aStringBuilder. The input outlives the call, and the copy was per hyperlink and per character.src/jsc/bindings/StringSizeLimit.hholds the reserve cap andexceedsStringLimit, which moves out ofWebStreamsInternals.h(ausingkeeps its callers as they are). Throw ERR_STRING_TOO_LONG from Bun.wrapAnsi instead of aborting when the output passes the string length limit #42225 can use the cap forBun.wrapAnsi.test/js/bun/util/sliceAnsi.test.ts, new block "a result past the string length limit" (fails on main, 3 tests). Also the rest of that file,sliceAnsi.npm.test.ts,stripANSI.test.ts,streams-string-limit.test.ts, and the five inputs from the issue on a debug ASAN build (table in Notes).Background
WTF::StringBuilderkeeps an 8-bit buffer until the first character above U+00FF, then allocates a 16-bit buffer of twice the current capacity. A 16-bitStringImplholds at most 2147483635 code units, 12 fewer thanString::MaxLength. StringBuilder: cap a doubled capacity at the longest string of the buffer's character type WebKit#631 fixes that cap in WTF. This PR does not depend on it.CRASH(). WithRecordOverflow, the builder sets a flag, later appends are no-ops, andhasOverflowed()reports it.toString()on an overflowed builder asserts, so the check comes first.Bun__stringSyntheticAllocationLimitis a process-wide cap on string lengths thatBUN_FEATURE_FLAG_SYNTHETIC_MEMORY_LIMITlowers, so a test reaches the limit with 64 KiB strings.Notes
The five inputs from the issue, on a debug ASAN build of this branch (Linux x64, 32 GB). Before, all five exit 134.
Bun.sliceAnsi("\u3042".repeat(2 ** 30), 0, 4)"ああ"Bun.sliceAnsi("\x1b[31m" + "a".repeat(2 ** 30), 0, 4, "\u2026")Bun.sliceAnsi("\x1b]8;;" + "\u3042".repeat(2 ** 30) + "\x07x", 0, 1)Bun.sliceAnsi("\x1b[31m" + "a".repeat(2 ** 31 - 6), 1)RangeError: Out of memoryBun.sliceAnsi("a".repeat(2 ** 31 - 2), 1, undefined, "\u2026" + "\u200b".repeat(10))RangeError: Out of memoryThe first two rows are the real-scale tests in the PR, behind
skipIf(memory < 8 GiB). The peak is the 2 GiB input: the capped 8-bit reserve (1 GiB) and the upconverted 16-bit buffer (4 GiB) are allocated and not written. Rows 3 to 5 are too slow and too large for CI under ASAN, so the synthetic-limit test covers those code paths at 64 KiB instead.Error choice. An earlier draft threw
ERR_STRING_TOO_LONG, as #42225 does. Every mergedhasOverflowed()site (NodeVM.cpp,FormatStackTraceForJS.cpp,BunStreamConsumers.cpp) throwsRangeError: Out of memory, and so does #42929 for the same function, so this PR does too. A builder cannot tell a length pastMaxLengthfrom a failed allocation.Overlap with #42929 (open). It makes the two
Vectors inemitSliceStreamingthrow the same error, through the samestd::optionalreturn. The two PRs touch the same signatures and the samependingHlline. Whichever lands second needs a small rebase, andnulloptmeans the same thing in both.With the reserve capped, an input of more than 2^30 code units grows the builder once more than before. That is one extra reallocation on a 1 GiB string.
The test builds its inputs with
repeat().Buffer.toString()checks the synthetic limit and throws for the 70000-character inputs.Suites run:
test/js/bun/util/sliceAnsi.test.ts(170 pass),sliceAnsi.npm.test.ts(48 pass),stripANSI.test.ts(298 pass),test/js/web/streams/streams-string-limit.test.ts(8 pass).test/integration/bun-types/bun-types.test.tshas 10 failures that are the same on main without this change.