The directory /srv/incoming is drwxr-xr-x and owned by user bob. Inside it sits report.log, owned by root with mode 644. Bob runs `rm /srv/incoming/report.log` and it succeeds. Why?
LPIC-1 Exam 101-500, objective 104. Devices, Linux filesystems, filesystem hierarchy standard medium
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The options
Correct Removing a name requires write and execute permission on the directory that holds it; the file's own mode and owner do not decide the matter.
Correct. Deletion edits the directory, not the file, and bob has write permission on the directory because he owns it and it is mode 755.
Not correct Mode 644 grants write permission to the owner, and rm consults that bit, so any process running as a user with write access to the file may unlink it.
Wrong on both counts. The file's write bit governs modifying its contents, not removing its name, and 644 grants write to root only in any case.
Not correct The directory has no sticky bit, so any user on the system may delete files in it.
Wrong. Without the sticky bit the ordinary rule applies, and that rule still demands write permission on the directory — which a user other than bob would not have here.
Not correct rm only decrements the inode's link count, an operation that ownership never restricts.
Wrong. Unlinking does decrement the link count, but the permission check happens on the directory entry being removed, and it is a real check.
Why
A directory is a list of names mapped to inodes, so creating, renaming and deleting a file are all writes to the directory and are checked against the directory's mode: write plus execute. This is why a read-only file can be deleted by someone with no rights over it at all, and why /tmp needs the sticky bit — with it set, only the file's owner, the directory's owner or root may remove an entry, despite the directory being world-writable.
Where this comes from
- Cited
- manual page unlink(2)
Practise this
Reading one question is not practice. The trainer will draw a short set from objective 104 and space the ones you get wrong.
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