Re: random PID generation

["H. Peter Anvin" <hpa@zytor.com>]

Followup to:  <20010222232423.A18448@home.ds9a.nl>
By author:    bert hubert <ahu@ds9a.nl>
In newsgroup: linux.dev.kernel
> 
> Well - I'm not sure that this is a good idea. When PIDs increase
> monotonically, chances are very small that the race condition implicit in
> sending any signal to a process results in killing the wrong process (ie, a
> new process, but with the same PID) - you'd need to zoom through 32000 PIDs
> in a very short time to make this happen.
> 
> With truly random PIDs, there is a much larger chance of a new process
> sitting on a recently used PID.
> 
> What would work is to have cryptographically randomly generated PIDs which
> would then guarantee not to return a previously returned number within 32000
> tries, and also not be predictable - there must be algoritms out there which
> do this.
> 

It depends on the size of your number space.  If you have a 31-bit
pid_t (since it apparently must be sign-safe) then you can take random
16-bit numbers from the /dev/urandom code and add to the last used
value instead of simple increment.

	-hpa

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