Re: SHM ids (was running pgsql 7 under Jail'ed virtual machine on FreeBSD 4.2)

I said:
> Alex Pilosov <alex@pilosoft.com> writes:
>> I'd suggest you do this: add a global backend_shmid_offset in ipc.c,
>> initialized to current default, and an option to postgres to put a value
>> there.

> Don't waste your time.  This issue is gone in current sources.  See
> IpcMemoryCreate and IpcSemaphoreCreate in src/backend/storage/ipc/ipc.c.

Actually, now that I think about it, you might still have a problem if
a "jail" is what I think it is.  The current code will step past an
existing shmem segment if it appears to be a non-Postgres memory segment
or if it appears to belong to another running postmaster.  However, the
test to see if the segment owner appears to be alive is

        /*
         * If the creator PID is my own PID or does not belong to any
         * extant process, it's safe to zap it.
         */
        if (hdr->creatorPID != getpid())
        {
            if (kill(hdr->creatorPID, 0) == 0 ||
                errno != ESRCH)
            {
        // segment belongs to a live postmaster, so continue
        }
    }

So the question for you is, what will happen if kill() is given a PID
belonging to a process that's in a different "jail"?  Will it return
some kind of permission failure, or will it claim that there is no
such PID (ie, return ESRCH)?  If the latter, we still have a problem ...

            regards, tom lane