[HACKERS] semaphore usage "port based"?

[Andrew Thompson]

On Mon, Apr 03, 2006 at 01:57:17AM -0300, Marc G. Fournier wrote:
> On Mon, 3 Apr 2006, Andrew Thompson wrote:
> 
> >On Mon, Apr 03, 2006 at 01:23:59AM -0300, Marc G. Fournier wrote:
> >>
> >>in kern/kern_jail.c, I can see the prison_check() call ... wouldn't one
> >>want to make the change a bit further up?  say in kern_prot.c?  wouldn't
> >>you want to change just cr_cansignal() to allow *just* for 'case 0', when
> >>someone is just checking to see if a process is already running?  I
> >>wouldn't want to be able to SIGKILL the process from a different jail,
> >>mind you ... maybe move the check for SIG0 to just before the
> >>prison_check, since, unless I'm missing something, other then determining
> >>that a process is, in fact, running, SIG0 is a benign signal?
> >>
> >
> >I think the suggestion was to make this EPERM rather than ESRCH to make
> >postgres a bit happier, not remove the check entirely. Im not familiar
> >with that part of the kernel at all, so I cant say what the consequences
> >will be apart from the obvious information leak.
> 
> 'k, first question is 'what information leak' are we trying to protect 
> from?  to 'make postgres a bit happier', all that needs to be fixed, from 
> what I can tell, is that cr_cansignal() needs to work for signal 0, but no 
> other signals ... what risk of information leak does that create?

By returning EPERM instead then the kernel is acknowledging that the pid
exists. In theory jails shouldnt be able to find out info about other
jails or the base system, postgres just relies on this.

cr_cansignal() doesnt need to work, it just needs to return an error
that isnt ESRCH. Postgres is just trying to find out of the pid is valid
and EPERM is just as useful as if the call actually succeeded.

backend/utils/init/miscinit.c:
 /*
  * Check to see if the other process still exists
  *
  * Normally kill() will fail with ESRCH if the given PID doesn't
  * exist.  BeOS returns EINVAL for some silly reason, however.
  */
 if (other_pid != my_pid)
 {
   if (kill(other_pid, 0) == 0 || (errno != ESRCH)) 
   ...


Andrew