Fork Bomb and why it doesn't happen on BSDs

[Aleksandar Milivojevic]

William Hooper wrote:
> Ow Mun Heng said:
> [snip]
> 
>>The author then compares the results with that of BSD eg: OpenBSD and
>>FreeBSD IIRC, and notes that on a system with Pentium 90 w/ 32MB Ram, on
>>using the forkbomb, doesn't bring the system down to it's knees.
> 
> 
> Following the discussion on the -devel list, the BSDs aren't immune.
> 
> https://www.redhat.com/archives/fedora-devel-list/2005-March/msg01201.html

I gave a try to "while(1) fork();" on OpenBSD (SS5, 64MB RAM).  You 
can't go much simpler than that.  Results:

Running as normal user, max number of processes is limited to 64 by 
default, load average went to 50-something.  Perfectly able to kill 
offending process(es).

Running as root, max number of processes is limited to 340, load average 
went to almost 300.  Not able to do much, since root couldn't create any 
new processes (in order to kill existing), but top worked, and already 
running processes worked.  Anyhow, if somebody does fork bomb as root, 
he could just as well type "halt" or wipe out the disk or do some other 
irreversable damage.

The maximum number of processes per user is hard coded in OpenBSD 
kernel.  It is 340 for root, and 64/128 for normal users in my generic 
kernel (soft limit is 64, hard limit is 128).  If you want more either 
for root or normal users, you need to recompile the kernel.  In *BSD 
world, recompiling the kernel is something you do twice a day anyhow, so 
this isn't much of a problem ;-)

For those interested, these are ulimits on stock OpenBSD (user and root, 
soft and hard).

$ ulimit -Sa
core file size        (blocks, -c) unlimited
data seg size         (kbytes, -d) 76800
file size             (blocks, -f) unlimited
max locked memory     (kbytes, -l) 28430
max memory size       (kbytes, -m) 84964
open files                    (-n) 64
pipe size          (512 bytes, -p) 1
stack size            (kbytes, -s) 4096
cpu time             (seconds, -t) unlimited
max user processes            (-u) 64
virtual memory        (kbytes, -v) 80896

$ ulimit -Ha
core file size        (blocks, -c) unlimited
data seg size         (kbytes, -d) 131072
file size             (blocks, -f) unlimited
max locked memory     (kbytes, -l) 85292
max memory size       (kbytes, -m) 85292
open files                    (-n) 1024
pipe size          (512 bytes, -p) 1
stack size            (kbytes, -s) 131072
cpu time             (seconds, -t) unlimited
max user processes            (-u) 128
virtual memory        (kbytes, -v) 262144

# ulimit -Sa
core file size        (blocks, -c) unlimited
data seg size         (kbytes, -d) 131072
file size             (blocks, -f) unlimited
max locked memory     (kbytes, -l) 28430
max memory size       (kbytes, -m) 84964
open files                    (-n) 128
pipe size          (512 bytes, -p) 1
stack size            (kbytes, -s) 8192
cpu time             (seconds, -t) unlimited
max user processes            (-u) 340
virtual memory        (kbytes, -v) 139264

# ulimit -Ha
core file size        (blocks, -c) unlimited
data seg size         (kbytes, -d) 131072
file size             (blocks, -f) unlimited
max locked memory     (kbytes, -l) 85292
max memory size       (kbytes, -m) 85292
open files                    (-n) 1024
pipe size          (512 bytes, -p) 1
stack size            (kbytes, -s) 131072
cpu time             (seconds, -t) unlimited
max user processes            (-u) 340
virtual memory        (kbytes, -v) 262144

-- 
Aleksandar Milivojevic <amilivojevic at pbl.ca>    Pollard Banknote Limited
Systems Administrator                           1499 Buffalo Place
Tel: (204) 474-2323 ext 276                     Winnipeg, MB  R3T 1L7