Problem - Kevin Tafuro

and a saved user ID. * The effective user ID is the user ID used for most permission
checks. The real user and saved user IDs are used primarily for determining whether
a process can legally change its effective user ID (see Recipe 1.3).
In addition to user IDs, each process also has a group ID. As with user IDs, there are
actually three group IDs: an effective group ID, a real group ID, and a saved group
ID. Processes may belong to more than a single group. The operating system maintains
a list of groups to which a process belongs for each process. Group-based permission
checks check the effective group ID as well as the process’s group list.
The operating system performs a series of tests to determine whether a process has
permission to access a particular file on the filesystem or some other resource (such
as a semaphore or shared memory segment). By far, the most common permission
check performed is for file access.
When a process creates a file or some other resource, the operating system assigns a
user ID and a group ID as the owner of the file or resource. The user ID is assigned
the process’s effective user ID, and the group ID is assigned the process’s effective
group ID.
To define the accessibility of a file or resource, each file or resource has three sets of
three permission bits assigned to it. For the owning user, the owning group, and
everyone else (often referred to as “world” or “other”), read, write, and execute permissions
are stored.
If the process attempting to access a file or resource shares its effective user ID with
the owning user ID of the file or resource, the first set of permission bits is used. If
the process shares its effective group ID with the owning group ID of the file or
resource, the second set of permission bits is used. In addition, if the file or
resource’s group owner is in the process’s group membership list, the second set of
permission bits is used. If neither the user ID nor the group ID match, the third set of
bits is used. User ownership always trumps group ownership.
Files also have an additional set of bits: the sticky bit, the setuid bit, and the setgid
bit. The sticky and setgid bits are defined for directories; the setuid and setgid bits
are defined for executable files; and all three bits are ignored for any other type of
file. In no case are all three bits defined to have meaning for a single type of file.
The sticky bit
Under normal circumstances, a user may delete or rename any file in a directory that
the user owns, regardless of whether the user owns the file. Applying the sticky bit to
a directory alters this behavior such that a user may only delete or rename files in the
directory if the user owns the file and additionally has write permission in the direc-
* Saved user IDs may not be available on some very old Unix platforms, but are available on all modern
Unixes.
Understanding the Unix Access Control Model | 39
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