evading ulimits

[John Richard Moser <nigelenki@comcast.net>]

I've set up some stuff on my box where /etc/security/limits.conf
contains the following:

@users          soft    nproc           3072
@users          hard    nproc           4096

I'm in group users, and a simple fork bomb is easily quashed by this:

bluefox@icebox:~$ :(){ :|:; };:
bash: fork: Resource temporarily unavailable
Terminated

Oddly enough, trying this again and again yields the same results; but,
I can kill the box (eventually; about 1 minute in I managed to `/exec
killall -9 bash` from x-chat, since I couldn't get a new shell open)
with the below:

bluefox@icebox:~$ :(){ :|:|:; };:

How exactly does the ulimit work?  Why do I seem to be able to evade
limits on maximum number of processes by doing a bigger fork bomb?  I
would have thought that the above would have terminated much sooner,
since it was spawning x^3 processes instead of x^2 for iteration x and
should have hit 4096 a lot sooner.


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