filter_kubernetes: fix uninitialized mem edge case

[runderwo]

In some cases of malformed input, the k8s filter attempts to process an input of length 0. In those cases, the root type would be left uninitialized, but subsequently relied upon anyway. This change treats a length 0 input as an invalid JSON document, and additionally initializes the root type ahead of time as a guard condition.

Example valgrind trace that is fixed:

==1== Thread 2 flb-pipeline:
==1== Conditional jump or move depends on uninitialised value(s)
==1==    at 0x3CA765: UnknownInlinedFun (kubernetes.c:188)
==1==    by 0x3CA765: UnknownInlinedFun (kubernetes.c:324)
==1==    by 0x3CA765: cb_kube_filter.lto_priv.0 (kubernetes.c:765)
==1==    by 0x2323ED: flb_filter_do (flb_filter.c:202)
==1==    by 0x2283CA: input_chunk_append_raw.lto_priv.0 (flb_input_chunk.c:3078)
==1==    by 0x22BFDD: UnknownInlinedFun (flb_input_chunk.c:3441)
==1==    by 0x22BFDD: input_log_append_processed_internal.lto_priv.0 (flb_input_log.c:1512)
==1==    by 0x22E7C5: input_log_append.lto_priv.0 (flb_input_log.c:1594)
==1==    by 0x2E0BA6: UnknownInlinedFun (flb_input_log.c:1616)
==1==    by 0x2E0BA6: UnknownInlinedFun (tail_file.c:570)
==1==    by 0x2E0BA6: process_content (tail_file.c:985)
==1==    by 0x2E106B: flb_tail_file_chunk (tail_file.c:2056)
==1==    by 0x2D5C64: in_tail_collect_static.lto_priv.0 (tail.c:200)
==1==    by 0x224DDA: flb_input_collector_fd (flb_input.c:2522)
==1==    by 0x252902: UnknownInlinedFun (flb_engine.c:742)
==1==    by 0x252902: flb_engine_start (flb_engine.c:1146)
==1==    by 0x216730: flb_lib_worker (flb_lib.c:909)
==1==    by 0x545F2E9: start_thread (in /usr/lib64/libc.so.6)

No new valgrind traces emerged.

While I am happy to do the extensive due diligence below if really necessary, it doesn't occur to me as immediately necessary.

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Testing
Before we can approve your change; please submit the following in a comment:

• [ N/A ] Example configuration file for the change

• [ N/A ] Debug log output from testing the change

• [ N/A ] Attached Valgrind output that shows no leaks or memory corruption was found

• [ N/A ] Run local packaging test showing all targets (including any new ones) build.

• [ N/A ] Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• [ N/A ] Documentation required for this feature

Backporting

• [ N/A ] Backport to latest stable release.

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

Summary by CodeRabbit

• Bug Fixes
  • Improved JSON handling to avoid unpredictable behavior by ensuring internal state is properly initialized before conditional merge logic.
  • Tightened JSON-to-MsgPack input validation: zero-length JSON inputs are now rejected earlier with an error return, rather than continuing processing.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 22ef705a-29cc-479e-9b40-d41f1cd247e2

📥 Commits

Reviewing files that changed from the base of the PR and between 3eadb8f and 3277f76.

📒 Files selected for processing (2)

• plugins/filter_kubernetes/kubernetes.c
• src/flb_pack.c

✅ Files skipped from review due to trivial changes (1)

• plugins/filter_kubernetes/kubernetes.c

🚧 Files skipped from review as they are similar to previous changes (1)

• src/flb_pack.c

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📝 Walkthrough

Walkthrough

This PR initializes root_type in the Kubernetes filter's merge log handler and adds a len == 0 validation check to the yyjson-based JSON-to-MsgPack packer.

Changes

Defensive JSON Parsing Fixes

Layer / File(s)	Summary
Uninitialized variable and input validation fixes plugins/filter_kubernetes/kubernetes.c, src/flb_pack.c	root_type is initialized before use in merge_log_handler(), and pack_json_to_msgpack_yyjson now rejects zero-length input along with existing null checks.

Estimated code review effort: 1 (Trivial) | ~3 minutes

🚥 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%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title matches the main change by describing a filter_kubernetes uninitialized-memory edge case fix.
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.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[runderwo]

The commit subject linter seems to be broken.

[cosmo0920]

Our commit Linter complains as follows:

[2](https://github.com/fluent/fluent-bit/actions/runs/29025702669/job/86145102178?pr=12078#step:6:13)
❌ Commit 3eadb8f00f failed:
Subject prefix 'filter_kubernetes:' does not match files changed.
Expected one of: filter_kubernetes:, pack:


Commit prefix validation failed.
Error: Process completed with exit code 1.

Could you split your commit into 2 commits? This is because your commit includes core component and plugin component changes in the one commit. We didn't want to mix these changes in commits.

[runderwo]

@cosmo0920 I split it. I think the 3 failing tests might be flaky, feel free to re-run them.