A new backup utility has been installed as /usr/local/bin/backup and is owned by root:operators. Its current permissions are 0750 (rwx r-x ---). Members of the operators group must be able to run the program with root privileges, but it must remain non-executable for all other users and no additional special permission bits should be added. Which octal mode should the administrator set (for example with chmod <mode> /usr/local/bin/backup) to meet these requirements?
The requirement calls for adding only the set-user-ID bit so that the executable runs with the file owner's (root's) privileges, while keeping the existing owner permissions (rwx), group permissions (r-x), and no access for others. The setuid flag is represented by the value 4 in the high-order (special) octal digit. Adding this 4 to the original 0750 produces 4750.
4750 therefore yields permissions of rwsr-x---:
r, w, x for the owner (root)
r, x for the group (operators)
no rights for others
setuid bit set (s in the owner-execute position).
2750 sets only the setgid bit, not setuid. 4755 would make the binary executable by everyone, violating the requirement to keep others from executing it. 6750 adds both setuid and setgid, introducing an extra special bit that was explicitly not desired.
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