Problem - Kevin Tafuro

tory. It is common to see the sticky bit applied to directories such as /tmp so that any
user may create temporary files, but other users may not muck with them.
Historically, application of the sticky bit to executable files also had meaning. Applying
the sticky bit to an executable file would cause the operating system to treat the
executable in a special way by keeping the executable image resident in memory
once it was loaded, even after the image was no longer in use. This optimization is no
longer necessary because of faster hardware and widespread support for and adoption
of shared libraries. As a result, most modern Unix variants no longer honor the
sticky bit for executable files.
The setuid bit
Normally, when an executable file loads and runs, it runs with the effective user, real
user, and saved user IDs of the process that started it running. Under normal circumstances,
all three of these user IDs are the same value, which means that the process
cannot adjust its user IDs unless the process is running as the superuser.
If the setuid bit is set on an executable, this behavior changes significantly. Instead of
inheriting or maintaining the user IDs of the process that started it, the process’s
effective user and saved user IDs will be adjusted to the user ID that owns the executable
file. This works for any user ID, but the most common use of setuid is to use the
superuser ID, which grants the executable superuser privileges regardless of the user
that executes it.
Applying the setuid bit to an executable has serious security considerations and consequences.
If possible, avoid using setuid. Unfortunately, that is not always possible;
Recipes 1.3 and 1.4 discuss the setuid bit and the safe handling of it in more detail.
The setgid bit
Applied to an executable file, the setgid bit behaves similarly to the setuid bit.
Instead of altering the assignment of user IDs, the setgid bit alters the assignment of
group IDs. However, the same semantics apply for group IDs as they do for user IDs
with respect to initialization of a process’s group IDs when a new program starts.
Unlike the setuid bit, the setgid bit also has meaning when applied to a directory.
Ordinarily, the group owner of a newly created file is the same as the effective group
ID of the process that creates the file. However, when the setgid bit is set on the
directory in which a new file is created, the group owner of the newly created file will
instead be the group owner of the directory. In addition, Linux will set the setgid bit
on directories created within a directory having the setgid bit set.
On systems that support mandatory locking, the setgid bit also has special meaning
on nonexecutable files. We discuss its meaning in the context of mandatory locking
in Recipe 2.8.
40 | Chapter 2: Access Control
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