Defusing Fork Bomb

Due to their nature, fork bombs can be difficult to stop once started. Stopping a fork bomb from reproducing further requires the termination of all running copies, which can be difficult to achieve. One problem faced is that a separate program to terminate the fork bomb cannot execute if the process table is fully saturated. The second major problem is that in the time taken between finding the processes to terminate and actually terminating them, more may have been created.
Some fork bombs can be stopped relatively easily. Consider the shell fork bomb:
:(){ :|: & };:
By replacing the function identifier and re-indenting, the code reads:
bomb() { 
  bomb | bomb & 
The fork bomb in this case is a recursive function that runs in the background, thanks to the ampersand operator. This ensures that the child process does not die and keeps forking new copies of the function, consuming system resources.
One important "feature" in this computer code means that a fork bomb process which can no longer fork doesn't stick around, but rather exits. In this situation, if we also try to run a new process often enough, eventually one will successfully start. If the new process does nothing, each new do-nothing process we run reduces the number of rampant "fork bomb" processes by one, until eventually all of them can be eradicated. At this point the do-nothing processes can exit. The following short Z Shell code might get rid of the above fork bomb in about a minute:
while (sleep 100 &) do; done
Alternatively, stopping (“freezing”) the bomb's processes can be used so that a subsequent kill/killall can terminate them without any of the parts re-replicating due to newly available process slots:
killall -STOP processWithBombName
killall -KILL processWithBombName
When a system is low on free PIDs (in Linux the maximum number of pids can be obtained from /proc/sys/kernel/pid_max), defusing a fork bomb becomes more difficult:
$ killall -9 processWithBombName
bash: fork: Cannot allocate memory
In this case, defusing the fork bomb is only possible if at least one shell is open. Processes may not be forked, but one can execve() any program from the current shell. Typically, only one attempt is possible.
killall -9 is not executed directly from the shell because the command is not atomic and doesn't hold locks on the process list, so by the time it finishes the fork bomb will advance some generations ahead. So one must launch a couple of killall processes, for example:
while :; do killall -9 processWithBombName; done
On Linux, because the process table is made accessible through the /proc filesystem, it is possible to defuse the fork bomb using bash builtins which do not require forking new processes. The following example identifies offending processes, and suspends them in order to prevent their continuing to fork while they are killed one at a time. This avoids the race condition of other examples, which can fail if the offending processes can fork faster than they are killed.
cd /proc;
for p in [0-9]*; do read CMDLINE < $p/cmdline; if [[ $CMDLINE == "processWithBombName" ]];
then kill -s SIGSTOP $p; fi; done
for p in [0-9]*; do read CMDLINE < $p/cmdline; if [[ $CMDLINE == "processWithBombName" ]];
then kill -s SIGKILL $p; fi; done
Part 1: Fork Bomb In-Depth
Part 3: Preventing Fork Bomb

Hardeep Singh aka cyb3r.gladiat0r


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