Prevent path traversal through a watched-root handle
A path is validated under the inbox, then a directory is replaced with a symlink before open, escaping the allowed root.
- Focused work estimate
- 6h + prerequisites
- Priority in the scenario
- High
- Engineering practice
- Filesystem security · TOCTOU · Path handling
Estimated field mix
- Systems programming65%
- Security35%
Field percentages are editorial estimates of the ticket's engineering focus. They total 100%; they are not measured time, proficiency scores, or ownership evidence.
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Project context
A fictional local agent watches generated inbox files and schedules bounded processing. Direct system calls are scattered across the code, mishandle interrupted operations, and make tests platform-dependent. Build a Rust OS adapter with temporary fixture directories; no kernel module, elevated privilege, or production host modification is required.
Setup prerequisites
- System calls
- File descriptors
- Monotonic clocks
Preceding work
Complete these dependencies, or supply their agreed outputs before taking this ticket.
- SKERNEL-102 · Write a durable replacement without exposing a half file
- SKERNEL-101 · Read a file completely across short system calls
- SKERNEL-104 · Keep wall-clock changes out of elapsed-time deadlines
- SKERNEL-105 · Normalize watcher bursts into a bounded rescan request
- SKERNEL-106 · Bound open descriptors while scanning a deep inbox
Acceptance criteria
- Operations resolve relative to an already opened trusted root
- Traversal rejects symlinks or verifies each component under declared policy
- Validation and use cannot be separated by an attacker-controlled rename
Implementation constraints
- Use temporary synthetic directories and no elevated privileges.
Verification to include
- Open and replace normal nested fixture files through the root handle.
- Race a directory-to-symlink swap and confirm no outside file is read or changed.
Deliverables
- Root-relative filesystem adapter and race regression
Rollout and recovery
Fail closed on platforms where the required safe primitive is unavailable.
Value of the work
For the engineer: Practice OS error semantics, partial I/O, portability and deterministic abstraction boundaries.
For the team: Review host-facing code whose retries, resource limits, and platform differences are explicit before packaging it as a local agent.
Evidence boundaries
Outcome Evidence: Tests, patches, and runbooks are requested deliverables. They become Outcome Evidence only through a qualified Mission and immutable Evidence IDs.
Ownership Evidence: Independent adaptation must be observed under a declared verification policy and cite immutable Evidence IDs. Completing a planning ticket establishes no Ownership Evidence.