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On 08/16/2013 04:23 PM, Solomon Peachy wrote:
> On Fri, Aug 16, 2013 at 03:16:11PM +0200, Hauke Mehrtens wrote:
>> I tested this on an amd64 system, it could be that it works on
>> some other archs.
>
> Yeah, I was building this on arm.
>
>> Yes you should put it into backport/compat/backport-3.10.c. I am
>> still wondering if get_random_int() is needed for such an
>> algorithm at all.
>
> I'm not sure what you mean by this. This is get_random_int() in
> current kernels: (copied from 3.11-rc)
>
> /* * Get a random word for internal kernel use only. Similar to
> urandom but * with the goal of minimal entropy pool depletion. As a
> result, the random * value is not cryptographically secure but for
> several uses the cost of * depleting entropy is too high */ static
> DEFINE_PER_CPU(__u32 [MD5_DIGEST_WORDS], get_random_int_hash);
> unsigned int get_random_int(void) { __u32 *hash; unsigned int ret;
>
> if (arch_get_random_int(&ret)) return ret;
>
> hash = get_cpu_var(get_random_int_hash);
>
> hash[0] += current->pid + jiffies + get_cycles();
> md5_transform(hash, random_int_secret); ret = hash[0];
> put_cpu_var(get_random_int_hash);
>
> return ret; }
>
> The cw1200 driver calls get_random_int() on every tx paket when
> there is QoS/queue contention to determine random backoff
> intervals. The numbers don't need to be cryptographically secure,
> just "different" -- and there's really no point in sucking the
> kernel entropy pool dry to do this when it could happen on nearly
> every frame we transmit. IOW, this is the perfect situation for
> get_random_int() vs get_random_bytes().
>
> I wonder if it's worth backporting this whole thing (possibly minus
> arch_get_random_int()) or just re-implementing it using
> get_random_bytes() instead:
>
> unsigned int get_random_int(void) { int r; get_random_bytes(&r,
> sizeof(r));
>
> return r; }
>
> After all, this logic is what any modules would have had would have
> had to use prior to 3.10.
>
> Any thoughts on this?
Yes using get_random_bytes() sound good to me, so you would add this
functions somewhere to backports.
I was thinking about if using get_random_int() in cw1200 already has a
too big overhead, but I do not know how long it takes to generate this
random data, but this is unrelated to backports.
>> I uploaded a logfile with some error messages:
>> http://hauke-m.de/files/compat-wireless/ckmake.log
>
> Thanks, I will look into this tonight.
>
> - Solomon
>
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