ptg10805159108 Files and Directories Chapter 4
On Solaris, thelscommand displays anlinstead of anSto indicate that mandatory file and
recordlocking has been enabled for this file. This behavior applies only to regular files, but
we’ll discuss this more in Section 14.3.
Finally,note that the time and date listed by thelscommand did not change after
we ran the program in Figure4.12. We’ll see in Section 4.19 that thechmodfunction
updates only the time that the i-node was last changed. By default, thels -llists the
time when the contents of the file werelast modified.Thechmod functions automatically clear two of the permission bits under the
following conditions:
•Onsystems, such as Solaris, that place special meaning on the sticky bit when
used with regular files, if we try to set the sticky bit (S_ISVTX) on a regular file
and do not have superuser privileges, the sticky bit in themodeis automatically
turned off. (Wedescribe the sticky bit in the next section.)To prevent malicious
users from setting the sticky bit and adversely affecting system performance,
only the superuser can set the sticky bit of a regular file.In FreeBSD 8.0 and Solaris 10, only the superuser can set the sticky bit on a regular file.
Linux 3.2.0 and Mac OS X 10.6.8 place no such restriction on the setting of the sticky bit,
because the bit has no meaning when applied to regular files on these systems. Although the
bit also has no meaning when applied to regular files on FreeBSD, everyone except the
superuser is prevented from setting it on a regular file.•The group ID of a newly created file might potentially be a group that the calling
process does not belong to. Recall from Section 4.6 that it’s possible for the
group ID of the new file to be the group ID of the parent directory.Specifically,
if the group ID of the new file does not equal either the effective group ID of the
process or one of the process’s supplementary group IDs and if the process does
not have superuser privileges, then the set-group-ID bit is automatically turned
off. This prevents a user from creating a set-group-ID file owned by a group that
the user doesn’t belong to.FreeBSD 8.0 fails an attempt to set the set-group-ID in this case. The other systems silently
turn the bit off, but don’t fail the attempt to change the file access permissions.
FreeBSD 8.0, Linux 3.2.0, Mac OS X 10.6.8, and Solaris 10 add another security feature to try to
prevent misuse of some of the protection bits. If a process that does not have superuser
privileges writes to a file, the set-user-ID and set-group-ID bits areautomatically turned off. If
malicious users find a set-group-ID or a set-user-ID file they can write to, even though they can
modify the file, they lose the special privileges of the file.4.10 Sticky Bit
TheS_ISVTXbit has an interesting history.Onversions of the UNIX System that
predated demand paging, this bit was known as thesticky bit.If it was set for an
executable program file, then the first time the program was executed, a copy of the
program’s text was saved in the swap area when the process terminated. (The text