Summary: What SUID, SGID and Sticky bits do on inappropriate files

[Jamie Mason]

	A while back, I posted a message to this newsgropup asking what
effect the Set-User-ID, Set-Group-ID and "Sticky" Bits have on
"Inappropriate" files.

	I asked what effect the SUID and SGID bits had on files which
could not be rightly called 'executable' -- special files such as Block
Devices, Character Devices, Symbolic Links, Named Pipes, Sockets and
Directories.

	I asked what effect the sticky bit had on files other than
directories.  (I knew that on directories, the sticky bit indicates that
files in the directory cannot be deleted by unprivliged users...  The
rules of which are a bit long.  But /tmp is a good example.)  I asked
what the sticky bit does on text files, executable files, and the special
files mentionned above.

	I asked for reply by Mail and promised a summary.  So far, I have
only received two replies, but it has been a long time, so I will
summarize.

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From:	gordon at sneaky.lonestar.org (Gordon Burditt)
Date:	Tue, 25 Dec 1990 17:39:00 -0500

The ORIGINAL use for the "sticky" bit was on executable files.  It
indicated that the text segment of the file should be kept loaded (in
memory or swap space) in anticipation of the file being used again.  
This was a speedup hack, and the sticky bit was often applied to things 
like the shell and 'ls'.  This may be somewhat obsoleted by systems
with virtual memory, or maybe there's still a saving from not having
to set up the file in memory again.

On some systems, the sticky bit on NON-executable files means that
file locking should be mandatory instead of advisory.  That is, a
read() or write() may block if another process has the file locked.


[ About the Sticky Bit on special files ]
No use that I know of.


The set-uid or set-gid bit on directories can often be used as a
"someone's been modifying my directory" flag, since on some systems it's
reset when the directory is modified.  It also annoys superusers who
carefully keep track of unauthorized setuid programs with not-too-smart
find commands.

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From:	p554mve at c1a.mpifr-bonn.mpg.de (Michael van Elst)
Date:	Wed, 2 Jan 1991 13:35:29 -0500
Organization: Max-Planck-Institut fuer Radioastronomie, Bonn

Hello, this won't be a complete answer but....

[ About Sticky Bit... ]

Nothing for regular files. The NFS server usually doesn't force inodes
to be written to disk for these files which is used for NFS swapping.

[ ... on executables ]
The program (text segment) is forced to be left in virtual memory
(swap space) after the process running the program has finished.
Starting the program again will be faster.


Set-Group-ID is used in BSD4.3 (and maybe SVR4) to indicate that newly
created files in this directory should inherit the group id of this
directory instead of the group ID of the process. Some systems allow
the reversal of this definition.

          --------------------------------------------------

	That's the summary.  I thank the two people who contributed.  It
would seem then that these bits have no meaning on certian types of
files...

	Just as none of the bits of the mode have any real meaning on
symbolic links.

	And why would one possibly make a file of mode 266, or anything
other mode where a permission is granted to GROUP or OTHER which is not
granted to USER, or where a permission is granted to OTHER which is not
granted to GROUP.  And what is mode 000 for??

	It seems that file modes are not logical, afterall. :-)

Jamie  ...  "Who was that Masked Interrupt?"
Written On  Monday, February 4, 1991  at  07:37:16am EST