Re: top delay value

Dr. Markus Waldeck wrote:
Oliver Fromme wrote:
Well, an unprivileged user can achieve the same effect by
typing "while :; do :; done". There are a thousand ways
to waste CPU time, and there is no way to prevent a user
from doing it.

It is not the same effect.

You describe fork bombing.

No. What I write above is not a "fork bomb", it's a single
process which is wasting CPU in a busy loop. It's exactly
equivalent to top(1) with zero delay, except that top
produces some output, while a busy loop does nothing useful
at all.

I could limit the number of process via
:maxproc=100: \
in /etc/login.conf

Which doesn't help against a busy loop.

If you want to make top more secure, type "chmod 700 /usr/bin/top".

:-)

Actually I was serious. Normal users don't really need to
run top (which is only contributed third-party software
anyway). It doesn't provide any information that you
can't get with other regular tools, such as ps(1) which
is a native FreeBSD tools.

By the way, you can "emulate" top(1) with run ps(1) in a
shell loop like this (sh/zsh/ksh/bash syntax):

while :; do clear; ps -a; sleep 1; done

Do get zero delay, simply remove the sleep command from the
loop ... That's actually _worse_ than top(1) with zero
delay, because kernel cycles are wasted for the fork() and
exec() calls, not to mention I/O and other syscalls. An
empty shell loop ("while :; do :; done") doesn't perform
any syscalls into the kernel.

Bottom line: Disabling zero-delay in top doesn't buy you
anything at all. In fact, it might cause your users to
invent work-arounds (for example shell loops) that waste
even more resources.

Best regards
Oliver

--
Oliver Fromme, secnetix GmbH & Co. KG, Marktplatz 29, 85567 Grafing b. M.
Handelsregister: Registergericht Muenchen, HRA 74606, USt-Id: DE204219783
Any opinions expressed in this message are personal to the author and may
not necessarily reflect the opinions of secnetix GmbH & Co KG in any way.
FreeBSD-Dienstleistungen, -Produkte und mehr: http://www.secnetix.de/bsd

"... there are two ways of constructing a software design: One way
is to make it so simple that there are _obviously_ no deficiencies and
the other way is to make it so complicated that there are no _obvious_
deficiencies." -- C.A.R. Hoare, ACM Turing Award Lecture, 1980
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