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

[Andreas Bießmann]

Hi Josh,

On 09/16/2015 05:18 AM, Josh Wu wrote:
> As 'time(0) | getpid()' sometimes get same value. That depends on the
> value of getpid().
> So that is not a expected behavior. We expect different value for the
> seed when when run it in many times.

I don't think your change made it better. Here is a snippet from a run
of time(NULL) and getpid():

---8<---
time: 1442389450; pid: 11632; time | pid: 1442397690;
time: 1442389450; pid: 11633; time | pid: 1442397691;
time: 1442389450; pid: 11634; time | pid: 1442397690;
time: 1442389450; pid: 11635; time | pid: 1442397691;
time: 1442389450; pid: 11636; time | pid: 1442397694;
time: 1442389450; pid: 11637; time | pid: 1442397695;
time: 1442389450; pid: 11638; time | pid: 1442397694;
time: 1442389450; pid: 11639; time | pid: 1442397695;
time: 1442389450; pid: 11640; time | pid: 1442397690;
time: 1442389450; pid: 11641; time | pid: 1442397691;
time: 1442389450; pid: 11642; time | pid: 1442397690;
time: 1442389450; pid: 11643; time | pid: 1442397691;
--->8---

While time(NULL) is stable, getpid() is incrementing by one. As you may
expect the OR'ed value is oscillating and the values almost the same. So
calling gen_eth_addr three times within the same second you will get two
time the same MAC.

> So this patch remove the getpid(), just use the time(0) as the seed.

So let's see the effect of your change ...

The output of gen_eth_addr at the current ToT:

% RUN=0; while [ $RUN -lt 10000 ]; do
/tmp/build-uboot-test/tools/gen_eth_addr; RUN=$(($RUN+1)); done | sort |
uniq | wc -l
254

With your change applied:

% RUN=0; while [ $RUN -lt 10000 ]; do
/tmp/build-uboot-test/tools/gen_eth_addr; RUN=$(($RUN+1)); done | sort |
uniq | wc -l
10

Another approach would be to change the algorithm (OR the values) here.
A short test showed that using XOR could be a solution:

---8<---
time: 1442389450; pid: 11632; time ^ pid: 1442394298;
time: 1442389450; pid: 11633; time ^ pid: 1442394299;
time: 1442389450; pid: 11634; time ^ pid: 1442394296;
time: 1442389450; pid: 11635; time ^ pid: 1442394297;
time: 1442389450; pid: 11636; time ^ pid: 1442394302;
time: 1442389450; pid: 11637; time ^ pid: 1442394303;
time: 1442389450; pid: 11638; time ^ pid: 1442394300;
time: 1442389450; pid: 11639; time ^ pid: 1442394301;
time: 1442389450; pid: 11640; time ^ pid: 1442394290;
time: 1442389450; pid: 11641; time ^ pid: 1442394291;
time: 1442389450; pid: 11642; time ^ pid: 1442394288;
time: 1442389450; pid: 11643; time ^ pid: 1442394289;
--->8---

It is the same input but none of the outputs is the same value.

The XOR approach applied to gen_eth_addr:

% RUN=0; while [ $RUN -lt 10000 ]; do
/tmp/build-uboot-test/tools/gen_eth_addr; RUN=$(($RUN+1)); done | sort |
uniq | wc -l
9988

Andreas

> Signed-off-by: Josh Wu <josh.wu at atmel.com>
> ---
> 
>  tools/gen_eth_addr.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/gen_eth_addr.c b/tools/gen_eth_addr.c
> index bf9d935..53b023a 100644
> --- a/tools/gen_eth_addr.c
> +++ b/tools/gen_eth_addr.c
> @@ -15,7 +15,7 @@ main(int argc, char *argv[])
>  {
>      unsigned long ethaddr_low, ethaddr_high;
>  
> -    srand(time(0) | getpid());
> +    srand(time(0));
>  
>      /*
>       * setting the 2nd LSB in the most significant byte of
>