LEARN. PRACTICE. GROW.Permissions and path traversal
An operation runs under a specific identity and groups. A readable file is insufficient if a directory on its path lacks traversal permission. For directories, execute permits traversal, read permits listing names, and write participates in modifying entries alongside other conditions. Examine owner, group, mode, and ACLs across the complete path, not only the final file.
Additional access-control layers
SELinux applies additional policy beyond traditional user/group controls. A denial may involve context, policy, or booleans even when file modes appear permissive. Inspect context and audit events and compare them with expected state. Distributions not using SELinux may use different mechanisms; do not assume every server has the same policy.
Fix the cause with the smallest scope
Do not make chmod 777 or disabling SELinux a standard response. Identify the required access and correct ownership, group, ACL, or context configuration under policy. A temporary label change may be lost during relabeling. On SELinux systems, a persistent fcontext rule and application of the expected context address a different issue from simply broadening file mode. Every change requires path analysis and effect review.
Workplace application
When migrating an application to a dedicated account, inventory required configuration, log, and data paths. Validate reading, writing, and traversal with the service identity and context, including groups and ACLs. An administrative session may mask missing authorization. On SELinux systems, investigate denials before creating exceptions and confirm that expected labels survive relabeling.
Mask and effective rights in an ACL
A named-user entry with rwx does not guarantee those rights if the ACL mask restricts them. For a nonowner account, user:batch:rwx and mask::r-x yield effective r-x for that entry. Examine identity, mask, and permissions along the whole path before changing the ACL. The mask constrains named users and the group class; it must not be read as another user. Keep the correction limited to access actually needed.
id appuser
namei -l /srv/funds/config/app.conf
getfacl /srv/funds/config/app.conf
ls -lZ /srv/funds/config/app.conf
getenforceAn administrator sees a readable file mode, but appuser cannot traverse /srv/funds/config. Investigation must use the correct identity and examine directories. Making the file world-writable does not fix missing traversal and creates unnecessary exposure.
Common pitfalls
Validating only as root, overlooking intermediate directories, or fixing SELinux by broadening permissions.
Related topics: Distinguish space, inodes, and mounts · Interpret CPU, memory, and I/O waiting
Analyze identity, path, and policy; broader permissions do not explain a failure.
Reference: path_resolution(7): path traversal · DR Linux 2026.4; networking and Bash manuals reviewed 2026-10-01; cgroup v2 and upstream systemd manuals reviewed 2026-10-01; RHEL 10 examples; Linux man-pages 6.19; OpenSSL 3.5