[U-Boot] [PATCH] tools: gen_eth_addr: remove getpid() operation for the random seed

[Josh Wu]

Hi, Wolfgang

Thanks for the reply.

On 9/16/2015 2:37 PM, Wolfgang Denk wrote:
> Dear Josh Wu,
>
> In message <1442373526-842-1-git-send-email-josh.wu at atmel.com> you wrote:
>> As 'time(0) | getpid()' sometimes get same value. That depends on the
>> value of getpid().
> I think removing some "random input" from the way how we compute the
> seed is a bad idea.
>
>> So that is not a expected behavior. We expect different value for the
>> seed when when run it in many times.
> What is your execution environment? In any sane OS it is higly
> unlikely that you will see the same or even similar PIDs for
> successive runs of the program - each run will start a new process,
> which will get a new PID.
my system is Ubuntu 14.04

#uname -a
Linux melon 3.13.0-45-generic #74-Ubuntu SMP Tue Jan 13 19:36:28 UTC 
2015 x86_64 x86_64 x86_64 GNU/Linux

Following is my test history:

➜  tools  date && ./gen_eth_addr
Wed Sep 16 14:48:53 CST 2015
4a:c3:21:45:17:b2
➜  tools  date && ./gen_eth_addr
Wed Sep 16 14:48:56 CST 2015
a6:29:4b:0b:e6:d0
➜  tools  date && ./gen_eth_addr
Wed Sep 16 14:49:02 CST 2015
d2:41:66:54:64:aa
➜  tools  date && ./gen_eth_addr
Wed Sep 16 14:49:06 CST 2015
2a:58:1d:b0:f0:c5
➜  tools  date && ./gen_eth_addr
Wed Sep 16 14:49:13 CST 2015
1e:8e:6f:0e:16:b8
➜  tools  date && ./gen_eth_addr
Wed Sep 16 14:49:19 CST 2015
56:4f:58:67:ad:30
➜  tools  date && ./gen_eth_addr
Wed Sep 16 14:49:21 CST 2015
2e:53:29:97:6a:8a
➜  tools  date && ./gen_eth_addr
Wed Sep 16 14:49:26 CST 2015
d2:41:66:54:64:aa
➜  tools  date && ./gen_eth_addr
Wed Sep 16 14:49:35 CST 2015
d2:41:66:54:64:aa

➜  tools  date && ./gen_eth_addr
Wed Sep 16 14:49:50 CST 2015
92:33:16:3f:0a:56
➜  tools  date && ./gen_eth_addr
Wed Sep 16 14:49:58 CST 2015
92:33:16:3f:0a:56

In above commands, I have two duplicated eth addr:
92:33:16:3f:0a:56
d2:41:66:54:64:aa

>
> [On a mostly idle Linux system (4.1.6 kernel) I see zero dupes in a
> set of 30,000 invocations of getpid().]
>
> One can argue if ORing the values is the most clever idea, or if for
> example ADDing them would result in more "randomness".
Sure. The ORing seems has big chance to get same result in my machine.


>   But completely
> removing the pid() is bad - any parallel runs of the program on any
> machines with synchronized times would predictably result in the same
> seeds which is definitely worse behaviour than what we have now.
I understand your concern. My intention is make it harder to generate 
the duplicated result.

Maybe we can ORing the MSB of time(0)?
I'll investigate it little more.

>
>> So this patch remove the getpid(), just use the time(0) as the seed.
> NAK.  This is a bad idea.
>
> Best regards,
>
> Wolfgang Denk
>
Best Regards,
Josh Wu