[LTP] [PATCH v2] getrandom02: relax check for returned data

[LTP] [PATCH v2] getrandom02: relax check for returned data

[Jan Stancek]

"nb * 0.1" can easily fail for nb < 20, since all we need
are 2 identical bytes. Worst case (nb == 19) is almost
identical to "birthday problem", but with smaller pool.
Chance of hitting 2 identical bytes in pool of 19
is close to 50%.

Adjust formula to allow small repetitions in small pools.
Chance we fail now should be pretty unlikely, ~10^-16.

Signed-off-by: Jan Stancek <jstancek@redhat.com>
---
 testcases/kernel/syscalls/getrandom/getrandom02.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/testcases/kernel/syscalls/getrandom/getrandom02.c b/testcases/kernel/syscalls/getrandom/getrandom02.c
index 0ac8bd28aed0..ec19f0fb32c5 100644
--- a/testcases/kernel/syscalls/getrandom/getrandom02.c
+++ b/testcases/kernel/syscalls/getrandom/getrandom02.c
@@ -83,7 +83,7 @@ static int check_content(int nb)
 
 	memset(table, 0, sizeof(table));
 
-	max = nb * 0.10;
+	max = 6 + nb * 0.2;
 
 	for (i = 0; i < nb; i++) {
 		index = buf[i];
-- 
1.8.3.1

[LTP] [PATCH v2] getrandom02: relax check for returned data

[Cyril Hrubis]

Hi!
> "nb * 0.1" can easily fail for nb < 20, since all we need
> are 2 identical bytes. Worst case (nb == 19) is almost
> identical to "birthday problem", but with smaller pool.
> Chance of hitting 2 identical bytes in pool of 19
> is close to 50%.
> 
> Adjust formula to allow small repetitions in small pools.
> Chance we fail now should be pretty unlikely, ~10^-16.
> 
> Signed-off-by: Jan Stancek <jstancek@redhat.com>

Acked.

-- 
Cyril Hrubis
chrubis@suse.cz

[LTP] [PATCH v2] getrandom02: relax check for returned data

[Jan Stancek]

----- Original Message -----
> From: "Cyril Hrubis" <chrubis@suse.cz>
> To: "Jan Stancek" <jstancek@redhat.com>
> Cc: ltp@lists.linux.it
> Sent: Wednesday, 8 February, 2017 3:12:51 PM
> Subject: Re: [PATCH v2] getrandom02: relax check for returned data
> 
> Hi!
> > "nb * 0.1" can easily fail for nb < 20, since all we need
> > are 2 identical bytes. Worst case (nb == 19) is almost
> > identical to "birthday problem", but with smaller pool.
> > Chance of hitting 2 identical bytes in pool of 19
> > is close to 50%.
> > 
> > Adjust formula to allow small repetitions in small pools.
> > Chance we fail now should be pretty unlikely, ~10^-16.
> > 
> > Signed-off-by: Jan Stancek <jstancek@redhat.com>
> 
> Acked.

Pushed.

Regards,
Jan

> 
> --
> Cyril Hrubis
> chrubis@suse.cz
>