fix: drop MDB_WRITEMAP to avoid LMDB freelist assertion crash by fzipi · Pull Request #3639 · owasp-modsecurity/ModSecurity

fzipi · 2026-09-20T10:38:18Z

Summary

The macOS (with lmdb) CI job fails deterministically on test/test-cases/regression/action-expirevar.json and test/test-cases/regression/issue-1831.json with:

mdb.c:3900: Assertion 'x <= dl[0].mid' failed in mdb_freelist_save()

This reproduces with a minimal standalone LMDB program with no ModSecurity code involved: a get/del/put/commit cycle on a key, interleaved with a read-only get and a delete-only commit, crashes mdb_freelist_save() when the environment is opened with MDB_WRITEMAP.

The failing assertion is in the mt_loose_pgs/dirty-list reconciliation loop inside mdb_freelist_save(), which only executes if (txn->mt_flags & MDB_TXN_WRITEMAP). This guard is still present at the current tip of LMDB's own mdb.RE/1.0 branch, so it is not fixed by upgrading LMDB.

Apple Silicon macOS uses a 16KB page size (sysctl hw.pagesize), versus 4KB on the Linux CI runners. The Linux with lmdb jobs use liblmdb-dev 0.9.24 from Ubuntu and pass; the macOS job installs whatever Homebrew's lmdb formula currently resolves to (1.0.2 at the time of this PR). The same C++ code path is exercised on both platforms; only the macOS job fails, consistently, across multiple runs and multiple unrelated commits on v3/master.

Change

src/collection/backend/lmdb.cc: mdb_env_open() no longer passes MDB_WRITEMAP, only MDB_NOSUBDIR.

Test plan

Reproduced the crash with a standalone LMDB C program (no ModSecurity involved), confirmed it disappears when MDB_WRITEMAP is removed from mdb_env_open, using the same environment (LMDB 1.0.2 via Homebrew, arm64 macOS).
Built v3/master plus this one-line change locally with ./configure --with-lmdb --enable-assertions=yes and make.
make check: 5043 total, 5025 pass, 18 skip, 0 fail (previously 3 failures, all in the two files above, both LMDB-collection related).
Ran action-expirevar.json and issue-1831.json individually, all sub-tests pass.

Summary by CodeRabbit

Bug Fixes
- Updated database access behavior to open LMDB files without write-map semantics, improving compatibility with environments where memory-mapped writes are unavailable.

The macOS (with lmdb) CI job crashed deterministically in action-expirevar.json and issue-1831.json with: mdb.c:3900: Assertion 'x <= dl[0].mid' failed in mdb_freelist_save() Reproduced with a minimal standalone LMDB program (no ModSecurity code involved): a get/del/put/commit cycle on the same key, interleaved with a read-only get and a delete-only commit, crashes LMDB's mdb_freelist_save() when the environment is opened with MDB_WRITEMAP. The failing assertion is in the mt_loose_pgs/dirty-list reconciliation loop, which only runs "if (txn->mt_flags & MDB_TXN_WRITEMAP)" - this is still present in LMDB's current upstream 1.0 branch, i.e. not something fixed by upgrading. Apple Silicon's 16KB page size makes it far easier to hit than Linux's 4KB pages, which is why only the macOS lmdb job failed while Linux (same code, older liblmdb) passed. Opening the environment without MDB_WRITEMAP avoids the buggy code path entirely; the same repro runs cleanly, and the full test suite now passes (previously 3 failures, all lmdb-collection related). Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

coderabbitai · 2026-09-20T10:38:31Z

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⚙️ Run configuration

Configuration used: defaults

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Plan: Advanced

Run ID: d13ca6b9-80fb-418b-bda2-8c43a9a003b3

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Reviewing files that changed from the base of the PR and between 7ea9fef and 3ac2809.

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src/collection/backend/lmdb.cc

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📝 Walkthrough

Walkthrough

The LMDB environment now opens the shared collection database with MDB_NOSUBDIR only. The MDB_WRITEMAP flag was removed from the mdb_env_open call.

Changes

LMDB open configuration

Layer / File(s)	Summary
Disable write-map mode `src/collection/backend/lmdb.cc`	The `MDBEnvProvider` constructor removes `MDB_WRITEMAP` from the `mdb_env_open` flags and keeps `MDB_NOSUBDIR` with mode `0664`.

Priority: ➖ Normal

Estimated code review effort: 1 (Trivial) | ~2 minutes

Change: Bug fix

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: removing MDB_WRITEMAP to prevent the LMDB freelist assertion crash.

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sonarqubecloud · 2026-09-20T10:43:07Z

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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Copilot

Copilot review overview

🟢 Approval recommended

The reported regression tests pass and the macOS crash is resolved.

Review effort: Lite
Findings: None

What changed in this PR

Removes LMDB write-map mode to prevent deterministic freelist assertion crashes on Apple Silicon macOS.

Changes:

Opens LMDB with MDB_NOSUBDIR only.
Preserves existing LMDB persistence behavior.

File	Description
`src/collection/backend/lmdb.cc`	Removes `MDB_WRITEMAP` from LMDB environment initialization.

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airween

LGTM, thank you!

airween · 2026-09-20T13:24:24Z

Thank you, @fzipi!

fzipi changed the title ~~Drop MDB_WRITEMAP to avoid LMDB freelist assertion crash~~ Sep 20, 2026

airween requested a lite review from Copilot September 20, 2026 13:19

Copilot started reviewing on behalf of airween September 20, 2026 13:19

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airween approved these changes Sep 20, 2026

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airween merged commit 2dada4c into owasp-modsecurity:v3/master Sep 20, 2026
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fzipi mentioned this pull request Oct 12, 2026

ip collection does not seem to work properly with latest ModSecurity #3394

Open

This was referenced Oct 27, 2026

CPTAddElement memory leak fix #3628

Open

add return value to acmp_build_binary_tree and check it result #3626

Open

fix(collections): Honour SecDataDir for LMDB, fix literal setvar writ… #3597

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fix: drop MDB_WRITEMAP to avoid LMDB freelist assertion crash - #3639

fix: drop MDB_WRITEMAP to avoid LMDB freelist assertion crash#3639
airween merged 1 commit into
owasp-modsecurity:v3/masterfrom
fzipi:fix-lmdb-writemap-freelist-crash

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