LFCS Question #666: Real Exam Question with Answer & Explanation

Question

What do the permissions -rwSr-xr-x mean for a binary file when it is executed as a command?

Options

• AThe command is SetUID and it will be executed with the effective rights of the owner.
• BThe command will be executed with the effective rights of the group instead of the owner.
• CThe execute flag is not set for the owner. Therefore the SetUID flag is ignored.
• DThe command will be executed with the effective rights of the owner and group.

Explanation

The permissions -rwSr-xr-x on a binary file indicate that the SetUID bit is present but ineffective because the owner's execute permission is missing.

Common mistakes.

• A. While the SetUID bit is technically present, the uppercase 'S' indicates the owner's execute permission is missing, which means the command will not be executed with the effective rights of the owner.
• B. The 'S' is in the owner's permission field, not the group's, meaning it pertains to SetUID, not SetGID; thus, it does not relate to executing with the effective rights of the group.
• D. This permission set does not enable execution with the effective rights of both owner and group simultaneously, and specifically, the SetUID bit is ineffective due to the missing owner execute permission.

Concept tested. Linux SetUID permissions and their effectiveness

Reference. https://linux.die.net/man/1/chmod

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