svn commit: r334543 - head/usr.bin/top

[Warner Losh]

On Mon, Jun 4, 2018 at 9:13 AM, Rodney W. Grimes <
freebsd at pdx.rh.cn85.dnsmgr.net> wrote:

> > On  2 Jun, Rodney W. Grimes wrote:
> > >> Author: eadler
> > >> Date: Sat Jun  2 22:06:27 2018
> > >> New Revision: 334543
> > >> URL: https://svnweb.freebsd.org/changeset/base/334543
> > >>
> > >> Log:
> > >>   top(1): chdir to / as init; remove unneeded comment
> > >>
> > >>   - chdir to / to allow unmounting of wd
> > >>   - remove warning about running top(1) as setuid. If this is a
> concern we
> > >>   should just drop privs instead.
> > >>
> > >> Modified:
> > >>   head/usr.bin/top/machine.c
> > >>   head/usr.bin/top/top.c
> > >>
> > >> Modified: head/usr.bin/top/machine.c
> > >> ============================================================
> ==================
> > >> --- head/usr.bin/top/machine.c     Sat Jun  2 21:50:00 2018
> (r334542)
> > >> +++ head/usr.bin/top/machine.c     Sat Jun  2 22:06:27 2018
> (r334543)
> > >> @@ -1613,11 +1613,6 @@ compare_ivcsw(const void *arg1, const void
> *arg2)
> > >>  /*
> > >>   * proc_owner(pid) - returns the uid that owns process "pid", or -1
> if
> > >>   *                the process does not exist.
> > >> - *                It is EXTREMELY IMPORTANT that this function work
> correctly.
> > >> - *                If top runs setuid root (as in SVR4), then this
> function
> > >> - *                is the only thing that stands in the way of a
> serious
> > >> - *                security problem.  It validates requests for the
> "kill"
> > >> - *                and "renice" commands.
> > >>   */
> > >>
> > >>  int
> > >>
> > >> Modified: head/usr.bin/top/top.c
> > >> ============================================================
> ==================
> > >> --- head/usr.bin/top/top.c Sat Jun  2 21:50:00 2018        (r334542)
> > >> +++ head/usr.bin/top/top.c Sat Jun  2 22:06:27 2018        (r334543)
> > >> @@ -260,6 +260,15 @@ main(int argc, char *argv[])
> > >>  #define CMD_order 26
> > >>  #define CMD_pid           27
> > >>
> > >> +    /*
> > >> +     * Since top(1) is often long running and
> > >> +     * doesn't typically care about where its running from
> > >> +     * chdir to the root to allow unmounting of its
> > >> +     * originall wd. Failure is alright as this is
> > >> +     * just a courtesy for users.
> > >> +     */
> > >> +    chdir("/");
> > >> +
> > >
> > > Bad side effect of doing that is it is not hard to get a "core"
> > > from top when run as a user, as it is going to try to write
> > > to /, and it probably does not have permission for that.
> > >
> > > Better might be a cd to /tmp, or /var/tmp, which are usually
> > > hard to unmount for these reasons anyway.
> >
> > Unless you start top using the exec shell builtin, the shell that you
> > use to launch top will also be long running and will also prevent its
> > $cwd from being unmounted.
>
> Thats a good point, so that makes the chdir worthless.


Turns out it wasn't completely useless, but the usefulness ended before
FreeBSD 1.0 was released.


> > If you do use exec, then you will get logged out when you kill top ...
>
> :-(.
>
> The long standing (30 years) solution is to use lsof and find
> the processes that have cwd's in what ever it is you want to
> unmount.
>

30 years is a bit too long. lsof didn't exist until 1991. :) The issues
that prompted top to cd to / didn't get fixed until SysVr4 / early BSD
kernels in the early 90s, and didn't make it into some vendor code until
the mid 90's.


> Special casing top(1) is just a none solution to the
> can not unmount foo problem.
>

True. It used to be critically important to do.   Now, it's irrelevant. I
posted a longer version why after doing some research. Basically, through
the early System V releases, rebooting was weird and long running processes
had to take actions to ensure they didn't accidentally hold references to
non / filesystems. That did get fixed by the late 80's / early 90's, so
it's pointless these days. I'd misremembered the details over the weekend,
so forget I said it was a good change :)

Warner