netdev - Re: [PATCH net-next 2/3] [PATCH 2/3] netem: allow using a seeded PRNG for generating random losses

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Date: Mon, 14 Aug 2023 22:14:53 +0200
From: François Michel <francois.michel@...ouvain.be>
To: Stephen Hemminger <stephen@...workplumber.org>
Cc: Jamal Hadi Salim <jhs@...atatu.com>, Cong Wang
 <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 2/3] [PATCH 2/3] netem: allow using a seeded PRNG
 for generating random losses

Hi,

Le 14/08/23 à 17:49, Stephen Hemminger a écrit :
> On Mon, 14 Aug 2023 04:31:39 +0200
> Francois Michel <francois.michel@...ouvain.be> wrote:
> 
>> +/* netem_get_random_u32 - polls a new random 32-bits integer from
>> + * the prng.
>> + * Uses a deterministic seeded prng if p->deterministic_rng is true.
>> + * Uses get_random_u32() underneath if p is NULL or if p->deterministic_rng
>> + * is false.
>> + */
>> +static u32 netem_get_random_u32(struct prng *p)
> 
> Overall I am fine with this patch, but the function name is getting excessively
> long. It is a local function, so no need for netem_ prefix.
> 
> Checking for p == NULL is redundant, all callers are passing a valid pointer.
> 
> For logical consistency, put the new wrapper before init_crandom() and after netem_skb_cb().
> 
> Since this is not security related, the change could also be simplified to just
> always prandom_u32_state() and initialize the state on first use with either
> get_random or provided seed.  This would also simplify the code around storing
> original seed and boolean.

Thank you very much for your comment.

I do not use prandom_u32_state() directly in order to ensure
that the original netem behaviour is preserved when no seed is specified.

But I agree that it would be cleaner to directly use prandom_u32_state() 
instead of get_random_u32(), if we are sure that we won't have problems 
(e.g. short prng cycles) with the randomly generated seeds when no seed 
is explicitly provided. If it is okay, then
I don't see a reason to not use prandom_u32_state() directly.

I'll make an update of the patch taking these comments into account and 
simplifying the patch.

Thank you !

François

> 
> Reminds me of the quote attributed to Mark Twain:
> “I apologize for such a long letter - I didn't have time to write a short one.”