Re: Concurrent access to /dev/urandom

["Bernard Normier" <bernard@zeroc.com>]

In which version of 2.6 did this bug get fixed? I am seeing these duplicates 
with 2.6.9-1.667smp (FC3)?
I am just trying to generate UUIDs (without duplicates, obviously).

Thanks,
Bernard

> On Tue, Dec 07, 2004 at 06:41:12PM -0500, Bernard Normier wrote:
>> Reading concurrently /proc/sys/kernel/random/uuid also returns duplicates
>> quite quickly ... which definitely looks like a bug. I included a small
>> python test-case below.
>> Can anybody suggest a work-around, for example a simple way to serialize
>> access to /dev/urandom from multiple threads/processes on the same box?
>
> This has been fixed in 2.6, but not yet in 2.4.  Really, this should
> be fixed in the kernel, but if you need to worry about this from the
> perspective of user-level programs that might be running on unfixed
> distribution kernels, the best way to deal with the problem is to use
> /dev/urandom to seed a cryptographic random number generator, and then
> mix in your pid and time/date into the CRNG.
>
> For example (in Pseudo code):
>
> char key[16];
> int  counter;
>
> seed_random_number_generator()
> {
> key <- SHA(20 bytes from /dev/random || pid || time(0));
> counter = 0;
> }
>
> get_random_bytes()
> {
> return SHA(counter++ || key);
> }
>
> This by the way is a generally a good thing to do in all cases;
> /dev/urandom is designed to be used to seed a cryptographic random
> generator.  If you need a large number of cryptographic random
> numbers, it's much faster to do it in user space than to try to do it
> in the kernel.
>
> - Ted
> -
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