On Mon, Apr 16, 2001 at 10:05:57AM -0700, Linus Torvalds wrote:
>
>
> On Mon, 16 Apr 2001 yodaiken@fsmlabs.com wrote:
> >
> > I'm trying to imagine a case where 32,000 sharing a semaphore was anything but a
> > major failure and I can't. To me: the result of an attempt by the 32,768th locker
> > should be a kernel panic. Is there a reasonable scenario where this is wrong?
>
> Hint: "I'm trying to imagine a case when writing all zeroes to /etc/passwd
> is anything but a major failure, but I can't. So why don't we make
> /etc/passwd world-writable?"
>
> Right. Security.
The analogy is too subtle for me,
but my question was not whether the correct error
response should be to panic, but whether there was a good reason for allowing
such a huge number of users of a lock.
> There is _never_ any excuse for panic'ing because of some inherent
> limitation of the data structures. You can return -ENOMEM, -EAGAIN or
> somehting like that, but you must _not_ allow a panic (or a roll-over,
> which would just result in corrupted kernel data structures).
There's a difference between a completely reasonable situation in which
all of some resource has been committed
and a situation which in itself indicates some sort of fundamental error.
If 32K+ users of a lock is an errror, then returning -ENOMEM may be
inadequate.
>
> Note that the limit is probably really easy to work around even without
> extending the number of bits: a sleeper that notices that the count is
> even _halfway_ to rolling around could easily do something like:
>
> - undo "this process" action
> - sleep for 1 second
> - try again from the beginning.
>
> I certainly agree that no _reasonable_ pattern can cause the failure, but
> we need to worry about people who are malicious. The above trivial
> approach would take care of that, while not penalizing any non-malicious
> users.
Ok. I'm too nice a guy to think about malicious users so I simply considered
the kernel error case.
You probably want a diagnostic so people who get mysterious slowdowns can
report:
/var/log/messages included the message "Too many users on lock 0x..."
>
> So I'm not worried about this at all. I just want people _always_ to think
> about "how could I mis-use this if I was _truly_ evil", and making sure it
> doesn't cause problems for others on the system.
>
> (NOTE: This does not mean that the kernel has to do anything _reasonable_
> under all circumstances. There are cases where Linux has decided that
> "this is not something a reasonable program can do, and if you try to do
> it, we'll give you random results back - but they will not be _security_
> holes". We don't need to be _nice_ to unreasonable requests. We just must
> never panic, otherwise crash or allow unreasonable requests to mess up
> _other_ people)
>
> Linus
-- --------------------------------------------------------- Victor Yodaiken Finite State Machine Labs: The RTLinux Company. www.fsmlabs.com www.rtlinux.com- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
This archive was generated by hypermail 2b29 : Mon Apr 23 2001 - 21:00:19 EST