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

From: François Michel
Date: Mon Aug 14 2023 - 16:16:10 EST


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@xxxxxxxxxxxx> 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.”