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SubjectRe: Bug in random32.c: all-zero outputs with probability 1/2^32, other seeding bugs
[who maintains random32.c ?]

On Thu, Jun 19, 2008 at 5:30 PM, Jack Lloyd <lloyd@randombit.net> wrote:
> Hi,
>
> There appears to be an error in how random seeding is done in the
> random32.c RNG. I am looking at 2.6.25.7.
>
[snip]
>
> An easy and straightforward fix for this that doesn't require changing
> any interfaces is to add
> s &= 0xFFFFFFFF;
> before the check in __set_random32, which ensures this condition will
> be caught by the check. Alternately, you could replace the check for
> s == 0 with some logic like:
> if((s & 0xFFFFFFFF) == 0)
> s += 1;
> since just chopping the seed to 32 bits does throw away some of your
> seed input (with sizeof(long) == 8, at least; doesn't make any
> difference for sizeof(long) == 4)
>

I think it is cleaner to change the interface to account for long != u32

The rest of your patch (ensuring values are big enough) looks valid to me.

Signed-off-by: Benoit Boissinot <benoit.boissinot@ens-lyon.org>

diff -r ced66ca0044f lib/random32.c
--- a/lib/random32.c Mon Jun 30 08:58:09 2008 -0700
+++ b/lib/random32.c Wed Jul 02 01:13:12 2008 +0200
@@ -56,7 +56,7 @@
return (state->s1 ^ state->s2 ^ state->s3);
}

-static void __set_random32(struct rnd_state *state, unsigned long s)
+static void __set_random32(struct rnd_state *state, u32 s)
{
if (s == 0)
s = 1; /* default seed is 1 */
@@ -84,7 +84,7 @@
*/
u32 random32(void)
{
- unsigned long r;
+ u32 r;
struct rnd_state *state = &get_cpu_var(net_rand_state);
r = __random32(state);
put_cpu_var(state);
@@ -122,7 +122,7 @@

for_each_possible_cpu(i) {
struct rnd_state *state = &per_cpu(net_rand_state,i);
- __set_random32(state, i + jiffies);
+ __set_random32(state, (u32) i + jiffies);
}
return 0;
}
@@ -135,7 +135,7 @@
static int __init random32_reseed(void)
{
int i;
- unsigned long seed;
+ u32 seed;

for_each_possible_cpu(i) {
struct rnd_state *state = &per_cpu(net_rand_state,i);

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