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shell: set every pipeline child up before starting any - #40788
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When a pipeline's per-command dup of the cwd fd failed at command i > 0, commands 0..i were already running. The pipeline finished with exit 1 and its parent freed it, which freed those commands too, but not the subtrees they own (a Subshell's Script, a Cmd's Expansion). The orphaned subtree later reported to the freed node: panic "child_done on freed" or a read of a freed slot. Pipeline::setup_commands now creates the pipes, dupes the env and inits every child before any child starts, as the Zig interpreter did. A failed pipe or dup leaves only children that never ran, so Pipeline::deinit frees them cleanly. write_failing_error keeps the state EnqueuingWriteErr only while it is inside IOWriter::enqueue. A completion dispatched re-entrantly from under that call (a synchronous uv write failure on Windows) now only records Done and lets the caller's trampoline, which has the pipeline on its pipeline_stack, remove and free it.
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Included review availability: Your plan provides up to 10 included reviews per hour; 8 remain after this review. WalkthroughChangesThe pipeline now initializes runnable children and pipe wiring before starting children. Startup advances by runnable-child index. Setup failures close unclaimed descriptors and deinitialize unstarted children. Stderr handling matches output variants directly. A POSIX EMFILE stress test covers subshell-headed pipelines. Suggested reviewers: Merge Risk: 🔵 Low · up to The change initializes all pipeline children before any start, preventing setup failures from leaving running descendants that can later access freed pipeline state. The remaining concern is that the low-file-descriptor regression test may be sensitive to unrelated stderr output, so the PR is mergeable with owner awareness or a follow-up to make that assertion robust. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
Full details: Description checkExplanation The description explains the problem, fix, rationale, and verification details. It provides the required information from the template, although it uses Problem and Fix headings instead of the exact template headings. Comment |
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Status: ready for review. CI on the current head ( The self-review trimmed the PR to the setup-before-start change. The Pipeline-local guard for the Windows re-entrant IOWriter error path was dropped, see the Notes in the PR body. Reproduced on main (69c6138, debug build) with: where The new test in |
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I reviewed this and didn't find any bugs. The setup/start separation and the EnqueuingWriteErr re-entrancy guard both trace correctly through the trampoline, but this is a UAF fix that reworks the pipeline state machine's teardown ordering and adds a Windows-only nested-trampoline path with no test coverage — a human look at the state-machine invariants is worthwhile.
What was reviewed:
- Traced all three
write_failing_errorcompletion paths (POSIX sync yield, Windows nested trampoline, async) againstYield::run/drain_pipelines— each lands onNext(this)on a trampoline that either hasthison itspipeline_stackor has an empty one, so the freed-while-on-stack case is closed. - Checked the
.expect("set by setup_commands")and bothunreachable!arms are true invariants:setup_commandsalways setscmdsbefore returningNone, and only started slots (idx < current) can becomeResult. - Verified fd ownership on the dup-failure path:
child_iodrop closespipes[cmd_idx-1][0]/pipes[cmd_idx][1], the twoclosefdloops cover the unclaimed remainder, and inited children incmdsclose their ends viadeinit— no leak, no double-close. - Ruled out a third test output state (setup succeeds but the subshell's inner run hits EMFILE):
Subshell::nextreuses the env Pipeline already duped, so no extra fd is consumed after setup.
Extended reasoning...
Overview
This PR restructures Pipeline::next_starting in the Bun shell interpreter to init every pipeline child (create pipes, dupe envs, allocate Cmd/Subshell/If/CondExpr nodes) before starting any of them. Previously setup and start were interleaved one child per trampoline re-entry, so an EMFILE on child i's env dup would tear down the pipeline while children 0..i were already running — and a running Subshell owns a Script subtree that Pipeline::deinit does not reach, leaving an orphan that later reports to a freed node. The pipes field is dropped (fds are owned solely by per-child IOReader/IOWriter Arcs). A second change adds PipelineState::EnqueuingWriteErr so a completion dispatched re-entrantly from under IOWriter::enqueue (Windows uv error path) only records Done and lets the caller's trampoline — which still has the pipeline on its pipeline_stack — emit Next(this). A new test in bunshell.test.ts sweeps (echo hi) | cat | ... under ulimit -n 32.
Security risks
None in the injection/auth/data-exposure sense. The change is memory-safety-relevant: it fixes a use-after-free and reorders fd/env teardown. I traced fd ownership on both failure paths (pipe creation and per-command dup) and found each fd closed exactly once — either by the closefd loops over unclaimed ends, by drop(child_io), or by deinit_node on the inited-but-unstarted children now stashed in cmds before write_failing_error runs.
Level of scrutiny
High. This is a state-machine refactor in a component where the failure mode is UAF, and the re-entrancy fix depends on the precise interaction between two Yield::run trampolines with separate pipeline_stacks. I traced each write_failing_error return path against Yield.rs and each holds: the outer trampoline always sees Next(this) with is_done() == true and removes the entry before next() reports to the parent; the nested trampoline (Windows) sees Yield::done() and drains an empty stack. The .expect and unreachable! sites are provable invariants (setup always populates cmds before returning None; only started children can transition to Result). However, the Windows nested-trampoline path is explicitly untested per the PR notes, and the correctness of drain_pipelines popping a WaitingWriteErr pipeline (neither is_starting_cmds nor is_done) on the async path is load-bearing — a human familiar with the trampoline should confirm.
Other factors
The new test mirrors the existing sibling test's structure exactly and is gated skipIf(isWindows) via the enclosing describe. The await Bun.sleep(0) is one event-loop turn to make the orphaned write's EPIPE fire before the next iteration reuses the arena slot — deterministic ordering, not a timed race wait, and commented as such. I confirmed the test's clean two-state [ok..., emfile...] assertion is sound after the fix by checking Subshell::next does not dupe again (it reuses the env Pipeline provided), so no additional fd is consumed once setup_commands succeeds. No CODEOWNERS cover these paths. The exit reason was dry_streak.
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Two notes on the points the review flags for a human look. The pop of a The nested trampoline path only exists on Windows. |
…r path The synchronous uv write failure that calls IOWriter::on_error from under enqueue is a Windows-only IOWriter contract violation shared by every enqueue caller. A guard in one state node is the wrong layer for it, so Pipeline keeps the single WaitingWriteErr state it shares with Cmd and CondExpr.
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Inline comments:
In `@test/js/bun/shell/bunshell.test.ts`:
- Line 673: Relax the outer process stderr assertion in the low-FD-limit test:
replace the exact empty-string check on proc.stderr with an assertion that only
verifies no uncaught-error output, while preserving the exact inner EMFILE
stderr assertion at the existing inner-process check.
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src/runtime/shell/states/Pipeline.rstest/js/bun/shell/bunshell.test.ts
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Problem
dupfails at commandi > 0crashes later. Repro:(echo hi) | cat | cat | ...underulimit -n 28. The pipeline printsbun: Too many open filesand exits 1, then the process panics withinternal error: entered unreachable code: child_done on freed Node#3(interpreter.rs:794) on the next event loop turn. With a different slot layout it reads a freed node instead (ASAN SEGV inStmt::next).Pipeline::next_starting(src/runtime/shell/states/Pipeline.rs). It inits and starts one child per call, so at commandithe commands0..ialready run.write_failing_errorfinishes the pipeline, the parent frees it, andPipeline::deinitfrees those children. It does not reach the subtrees they own: a Subshell's Script, a Cmd's Expansion. The orphaned subtree still writes into the pipe. When the pipe closes under it, it reports to the freed node.Fix
Pipeline::setup_commandscreates the pipes, dupes the env and inits every child before any child starts. A failed pipe or dup leaves only children that never ran, soPipeline::deinitfrees them cleanly and their IOReader/IOWriter drops close the pipe ends. This is the order the Zig interpreter used (setupCommandsbeforestart).pipesfield goes away: the fd numbers only matter inside setup.deinitruns from the parent'schild_done, after the child finished.test/js/bun/shell/bunshell.test.ts, new testreports EMFILE from the per-command env dup when the head is a subshell(panics on the released bun). Alsobunshell.test.ts,pipeline_stack.test.ts,yield.test.ts,assignments-in-pipeline.test.ts,epipe.test.ts,throw.test.ts,exec.test.ts,shell-hang.test.ts,shell-write-fault.test.ts,shell-pipe-read-fault.test.tsand the rest oftest/js/bun/shell/.Background
Script,Stmt,Pipeline,Cmd,Subshell, ...). Each step returns aYield.Yield::runis the trampoline that drives it. A node reports completion withinterp.child_done(parent, this, exit_code). The parent then frees the child withdeinit_node.deinitfrees what the node owns, not its running children.Subshell::deinitfrees the duped env only.Cmd::deinittears down a builtin or subprocess, but a Cmd still expanding its arguments has anExpansionchild node instead.IOWriteris the shared async writer for an fd.enqueuequeues bytes for aChildPtrand later calls that child'son_io_writer_chunk. The orphaned echo in the repro holds its own clone of the pipe's IOWriter, which is why its chunk outlives the pipeline.Notes
echo hi | cat | ...with the builtincat, where every child is a plain Cmd. Those have no subtree, and their IOWriter dies with the node, so the pending chunk never fires. A Subshell head has a Script subtree that holds its own clone of the IO, so the chunk outlives the pipeline.BUN_DEBUG_SHELL=1,ulimit -n 28,(echo a) | cat x6): the subshell (Node#3) starts, its echo enqueuesa\non the socketpair writer, children 1..5 start, the dup for child 6 fails,Pipeline Node#2 deinit,Subshell Node#3 deinit. On the next tick the writer gets EPIPE (the read end closed with child 1),Cmd Node#6 execDone exit=65504,Stmt Node#5 childDone,Script::finishcallschild_done(Node#3): panic.n = 2..16underulimit -n 32and awaitsBun.sleep(0)after each pipeline. That is one event loop turn, not a timed wait: the orphaned echo's EPIPE completion is delivered then, while the old interpreter's slot is still free, so the unfixed build panics deterministically instead of dispatching into a reused slot. The window where the dup fails ati > 0spans about four values ofnfor any fd budget, so the sweep hits it regardless of the base fd count of the machine.write_failing_errornow clones the stderrArc<IOWriter>out of the node beforeenqueueinstead of holding a borrow of the node across the call. Behavior is unchanged.WindowsBufferedWriter::writecan callIOWriter::on_errorfrom underenqueuewhen uv rejects the write synchronously (start_with_current_pipereturnsOkunconditionally). That runs the completion on a nested trampoline while the caller's trampoline still has the pipeline on itspipeline_stack. On POSIX the same failure is returned as aYieldthroughon_sync_error. An earlier revision of this PR carried a Pipeline-local guard for it (anEnqueuingWriteErrstate). The self-review found that everyenqueuecaller has the same exposure and that the contractIOWriter::writedocuments (failures are returned, never dispatched) is what Windows breaks, so the guard was dropped in favor of a fix at the IOWriter layer. The open shell refactor shell: remove unsafe from the interpreter, state nodes, IOWriter/IOReader and Builtin #40228 defers to the same io-layer follow-up. Windows behavior is unchanged from main.pidfd_openfails with EMFILE blocks inwait4. A sweep with(sleep 0.3; echo a) | cat | ...(the realcat) hangs at some budgets on main and on this branch alike.lsandrmtests undertest/js/bun/shell/commands/fail in this container for environment reasons (root user for the permission denied cases, no registry for thenode_modulescases). They do not involve this change.cargo check -p bun_runtime --target x86_64-pc-windows-msvcandcargo clippy -p bun_runtime --no-depsare clean.no test proof · iteration 3 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/shell/bunshell.test.ts