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

I said:
> Alex Pilosov <alex(at)pilosoft(dot)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