Unprivileged user can't set sticky bit on a file; why?

[David Malone]

On Thu, Nov 13, 2008 at 11:05:21PM -0800, Nate Eldredge wrote:
> since root is able to do it.  src/sys/ufs/ufs/ufs_vnops.c has the 
> following comment:
> 
>         /*
>          * Privileged processes may set the sticky bit on non-directories,
>          * as well as set the setgid bit on a file with a group that the
>          * process is not a member of.  Both of these are allowed in
>          * jail(8).
>          */
> 
> but does not explain why unprivileged process should be forbidden to set 
> the sticky bit.

I'm guessing a little, but this check has been there since the first
revision of ufs_vnops.c in FreeBSD, see:

	http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/ufs/ufs/ufs_vnops.c?annotate=1.1

line 424. When this was imported, there was still a call to
vnode_pager_uncache based on the sticky bit a few lines down. This
may explain why the check was there at the time - it was to stop
ordinary users giving hints to the pager system.

	David.