Linux permissions, identity, and access decisions

Mechanism

Linux access depends on credentials, permission bits, ACLs, capabilities, mount options, and the operation being attempted. File write permission differs from directory write permission, which can allow renaming or replacing directory entries. Effective identity may differ from the login identity, especially in set-user-ID programs and privilege transitions.

Practical method

Inspect real and effective identity, groups, the target path, and every parent directory. Check ACLs and mount options before interpreting a permission failure. Reproduce the exact operation with a harmless fixture rather than assuming read access implies write or execution access.

id
namei -l /path/to/fixture
stat /path/to/fixture

Interpreting evidence

Record which credential and path component determines access. A failed open can reflect directory traversal permission rather than the file mode itself.

Pitfalls and limits

Root is still constrained by some namespaces and security policies. ACL masks affect effective entries. Network filesystems may apply server-side identity mapping and different privilege rules.