random() always returns 0 with srandom(0)
My co-worker was troubleshooting why some of our unittests (that work on
multiple operating systems and architectures) failed on OpenBSD and saw
that if you call srandom(0) to initialize the RNG, random() will always
return 0. (I was able to reproduce this.)
If this is expected behaviour, please point to explanation.
He posted his tests and discussed the code related to this at:
https://banu.com/blog/42/openbsd-bug-in-the-random-function/
He suggests two ways to fix this:
1) if (x == 0)
x = 1;
upon entry in srandom(). That's the glibc way and he thinks is the
better solution as it falls back to the default behavior (when srandom
is not called) when the impossible seed=0 is passed.
2) the second way to fix it is the FreeBSD way:
for (i = 1; i < rand_deg; i++) {
+if (state[i-1] == 0)
+ state[i-1] = 123459876;
val = div(state[i-1], 127773);
Re: random() always returns 0 with srandom(0)
On 2012/03/21 12:37, Jeremy C. Reed wrote: > My co-worker was troubleshooting why some of our unittests (that work on > multiple operating systems and architectures) failed on OpenBSD and saw > that if you call srandom(0) to initialize the RNG, random() will always > return 0. (I was able to reproduce this.) Update src/lib/libc/random.c to r1.16.
Re: random() always returns 0 with srandom(0)
> My co-worker was troubleshooting why some of our unittests (that work on
> multiple operating systems and architectures) failed on OpenBSD and saw
> that if you call srandom(0) to initialize the RNG, random() will always
> return 0. (I was able to reproduce this.)
>
> If this is expected behaviour, please point to explanation.
>
> He posted his tests and discussed the code related to this at:
>
> https://banu.com/blog/42/openbsd-bug-in-the-random-function/
>
> He suggests two ways to fix this:
>
> 1) if (x == 0)
>x = 1;
>
> upon entry in srandom(). That's the glibc way and he thinks is the
> better solution as it falls back to the default behavior (when srandom
> is not called) when the impossible seed=0 is passed.
>
> 2) the second way to fix it is the FreeBSD way:
>
>for (i = 1; i < rand_deg; i++) {
> +if (state[i-1] == 0)
> + state[i-1] = 123459876;
>val = div(state[i-1], 127773);
Already fixed.
CVSROOT:/cvs
Module name:src
Changes by: mill...@cvs.openbsd.org 2012/03/21 06:36:49
Modified files:
lib/libc/stdlib: random.c
Log message:
Fix a bug where random() always returns 0 when srandom() is seeded
with 0. Use 1 and not 0 as the first element of the state array,
similar to what glibc does. OK nicm@
Re: random() always returns 0 with srandom(0)
On 2012/03/21 17:41, Stuart Henderson wrote: > On 2012/03/21 12:37, Jeremy C. Reed wrote: > > My co-worker was troubleshooting why some of our unittests (that work on > > multiple operating systems and architectures) failed on OpenBSD and saw > > that if you call srandom(0) to initialize the RNG, random() will always > > return 0. (I was able to reproduce this.) > > Update src/lib/libc/random.c to r1.16. > er sorry, src/lib/libc/stdlib/random.c.
Re: random() always returns 0 with srandom(0)
On 21/03/12 1:37 PM, Jeremy C. Reed wrote:
My co-worker was troubleshooting why some of our unittests (that work on
multiple operating systems and architectures) failed on OpenBSD and saw
that if you call srandom(0) to initialize the RNG, random() will always
return 0. (I was able to reproduce this.)
If this is expected behaviour, please point to explanation.
He posted his tests and discussed the code related to this at:
https://banu.com/blog/42/openbsd-bug-in-the-random-function/
He suggests two ways to fix this:
1) if (x == 0)
x = 1;
upon entry in srandom(). That's the glibc way and he thinks is the
better solution as it falls back to the default behavior (when srandom
is not called) when the impossible seed=0 is passed.
2) the second way to fix it is the FreeBSD way:
for (i = 1; i< rand_deg; i++) {
+if (state[i-1] == 0)
+ state[i-1] = 123459876;
val = div(state[i-1], 127773);
I assume you don't read the source-changes list.
CVSROOT:/cvs
Module name:src
Changes by: mill...@cvs.openbsd.org 2012/03/21 06:36:49
Modified files:
lib/libc/stdlib: random.c
Log message:
Fix a bug where random() always returns 0 when srandom() is seeded
with 0. Use 1 and not 0 as the first element of the state array,
similar to what glibc does. OK nicm@
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Re: random() always returns 0 with srandom(0)
On Wed, Mar 21, 2012 at 12:37:47PM -0500, Jeremy C. Reed wrote: > My co-worker was troubleshooting why some of our unittests (that work on > multiple operating systems and architectures) failed on OpenBSD and saw > that if you call srandom(0) to initialize the RNG, random() will always > return 0. (I was able to reproduce this.) Nice work by your co-working for pinpointing the problem, finding how the issue was dealt with elsewhere, and proposing solutions, and then making sure word got to an appropriate list. Please pass along my thanks as an OpenBSD user. -- http://code.phxbsd.com/
Re: random() always returns 0 with srandom(0)
Hi Jeremy, On Wed, Mar 21, 2012 at 12:37:47PM -0500, Jeremy C. Reed wrote: | My co-worker was troubleshooting why some of our unittests (that work on | multiple operating systems and architectures) failed on OpenBSD and saw | that if you call srandom(0) to initialize the RNG, random() will always | return 0. (I was able to reproduce this.) | | If this is expected behaviour, please point to explanation. | | He posted his tests and discussed the code related to this at: | | https://banu.com/blog/42/openbsd-bug-in-the-random-function/ As already mentioned by others on this list, this was fixed in the tree very recently, thanks to you and your co-worker for pointing it out. I'm curious though - how does a continuous stream of zeroes break your unittests ? I agree that it's pretty debianesque randomness (random() is bad, but not *THAT* bad ;), but keep in mind that any RNG could in theory produce an arbitrarily long sequence of zeroes too. Can you provide some insight ? Cheers, Paul 'WEiRD' de Weerd -- >[<++>-]<+++.>+++[<-->-]<.>+++[<+ +++>-]<.>++[<>-]<+.--.[-] http://www.weirdnet.nl/
