orderfile: stop a traced process's children from truncating its trace

[robobun]

Follow-up to #33302, per this review comment.

The bug

pagetrace.c's constructor opened BUN_PAGETRACE_OUT with O_CREAT|O_TRUNC and ftruncated it before reading /proc/self/maps to find out whether this process is even the binary under trace:

int fd = open(out, O_CREAT | O_TRUNC | O_RDWR, 0644);   // ← truncates
...
record = mmap(NULL, bytes, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
...
fclose(maps);
if (!region_count) return;                              // ← too late

LD_PRELOAD and both BUN_PAGETRACE_* variables are inherited across exec, so any dynamically linked child a workload spawns — a lifecycle script, /bin/sh, another bun — re-ran the constructor, zeroed the MAP_SHARED trace its parent was still writing into, and only then bailed. The pages lost are the earliest-touched ones, which is to say the hottest.

The fix

Find the regions first and leave the file alone when there are none. Then drop LD_PRELOAD and the two BUN_PAGETRACE_* variables from the environment once loaded, so a child that does map the same binary (another bun) never arms itself either — the region check alone can't catch that one. ptyrun already hands the preload down explicitly via PTYRUN_PRELOAD rather than relying on inheritance, so nothing depended on it; its comment is updated to say so.

Impact today

None, which is why it was still there. No workload currently spawns, and the order file comes out byte-for-byte the same work:

                      before fix            after fix
bun -e                2108 pages  +21636    2108 pages  +21636
bun hello.ts          2168 pages    +631    2168 pages    +631
bun server.js         2611 pages   +4809    2611 pages   +4809
bun test              3057 pages   +5174    3057 pages   +5174
bun install            630 pages   +1440     630 pages   +1440
bun install (cached)   540 pages     +83     540 pages     +83
bun cli.js (pipe)     3075 pages   +2024    3075 pages   +2024
bun cli.js (tty)      3348 pages   +2003    3369 pages   +2137   (pty jitter)

But bun install is one package.json away from running a lifecycle script, and the failure is silent: a truncated trace still produces an order file, just one missing the hot core.

Test

test/js/bun/perf/pagetrace-fixture.c reads one byte from each of 32 pages of its own .rodata, execs a dynamically linked child, then reads one more. Under the tracer each read is a recorded fault, so the count is the signal:

before:  Expected: >= 33   Received: 1
after:   9 pass, 1 skip, 0 fail

The stride is 64 KB so the page count is the same on kernels with 16 KB or 64 KB pages. Skipped where there's no C compiler, alongside the existing ptyrun test.

[robobun]

^{Updated 9:35 PM PT - Jul 4th, 2026}

@robobun, your commit 6d171fa is building: #68433

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro

Run ID: dc3d7ed9-cd7b-43ed-93c0-46f5f1de7d9a

📥 Commits

Reviewing files that changed from the base of the PR and between bb49700 and 6d171fa.

📒 Files selected for processing (4)

• scripts/orderfile/pagetrace.c
• scripts/orderfile/ptyrun.c
• test/js/bun/perf/linker-order.test.ts
• test/js/bun/perf/pagetrace-fixture.c

---

Walkthrough

The pagetrace.c constructor now reads and unsets environment variables (LD_PRELOAD, BUN_PAGETRACE_BIN/OUT) before scanning /proc/self/maps, deferring output-file creation/mmap until after region discovery. ptyrun.c's comment is updated accordingly. A new test suite and C fixture validate page-tracing behavior across fork/exec.

Changes

Pagetrace init reordering and tests

Layer / File(s)	Summary
Pagetrace init env handling and deferred output setup scripts/orderfile/pagetrace.c	Environment variables are read/unset earlier, and output-file creation/mmap is deferred until after region scanning.
ptyrun.c preload comment update scripts/orderfile/ptyrun.c	Comment updated to reflect that PTYRUN_PRELOAD is passed down as the child's LD_PRELOAD, with the tracer removing itself once loaded.
Shared compile helper and page tracer test suite test/js/bun/perf/linker-order.test.ts, test/js/bun/perf/pagetrace-fixture.c	A shared compile() helper replaces a duplicated local function; a new Linux-only test builds tracer/fixture/child binaries, runs them, and validates the trace output header; a new C fixture performs deterministic page touches and fork/execl.

Sequence Diagram(s)

sequenceDiagram
  participant Test as linker-order.test.ts
  participant Fixture as pagetrace-fixture
  participant Tracer as pagetrace.c (LD_PRELOAD)
  participant Child

  Test->>Fixture: spawn with LD_PRELOAD=tracer, BUN_PAGETRACE_BIN/OUT
  Fixture->>Fixture: touch .rodata pages (STRIDE reads)
  Fixture->>Child: fork + execl(child)
  Child-->>Fixture: exit
  Fixture->>Fixture: final touched-page read, print sum
  Test->>Test: read trace output, parse header, assert results

Loading

Estimated code review effort: Medium

Suggested labels: testing, linux, orderfile

Suggested reviewers: none identified from provided context

Poem
A rabbit traced each page it found,
Env vars hushed without a sound,
Fork, then exec, the child runs free,
A header counts what pages be,
Hop, hop — the trace is safe and sound. 🐇

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main fix: preventing traced children from truncating the parent trace.
Description check	✅ Passed	The description explains the bug, the fix, impact, and verification, covering the template’s required content.
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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[claude]

LGTM — focused build-tooling fix with a solid regression test.

Extended reasoning...

Overview

This PR reorders scripts/orderfile/pagetrace.c's constructor so it checks /proc/self/maps for the traced binary's regions before opening/truncating the output file, and additionally unsetenvs LD_PRELOAD + the two BUN_PAGETRACE_* vars once loaded so exec'd children never re-arm the tracer. The env values are correctly copied to stack buffers before unsetenv (which may invalidate getenv returns). ptyrun.c gets a comment-only update. A new C fixture and test in test/js/bun/perf/linker-order.test.ts verify a child exec no longer wipes the parent's trace.

Security risks

None. This is a Linux-only, build-time LD_PRELOAD shim used exclusively by scripts/orderfile/generate.ts during release builds to produce the linker symbol-ordering file. It is not shipped in the bun binary and has no user-facing surface.

Level of scrutiny

Low-to-medium. The changed code is developer tooling under scripts/, not runtime code — worst case is a suboptimal .text layout, which the existing verifyOrderFileApplied() CI check would still catch. The core change is a mechanical reordering of two existing blocks (region discovery ↔ file creation) plus three unsetenv calls, and the PR description confirms the generated order file is byte-identical for all current workloads.

Other factors

• Follow-up to a specific review comment on #33302; scope is exactly what was asked for.
• The regression test is well-constructed: compiles the real tracer, uses a fixture that touches 32 distinct 64 KB-strided .rodata pages, fork+execs a dynamically linked child, then touches one more — asserting ≥33 recorded faults. PR shows it fails (count=1) before the fix and passes after. Skipped on non-Linux / no compiler, uses tempDir/bunEnv/await using, drains pipes concurrently.
• The compile() helper was hoisted unchanged from the existing pty runner describe block to share it — no behavior change there.
• Minor: the test asserts stderr: "" on the fixture, which CLAUDE.md discourages for bun-debug spawns, but this is a plain cc -O1 C binary with no sanitizers, so it's fine here.