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@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