Re: [PATCH 4/4] ndp: Use high quality entropy in NDP timer even if not needed

On Thu, 16 Jul 2026 15:45:30 +1000 David Gibson wrote:

On Thu, Jul 16, 2026 at 01:25:23AM +0200, Stefano Brivio wrote:

...

...instead of calling random(), to make static checkers happy.

I don't think that an attacker could actually gain anything by making router advertisement intervals predictable, but a doubt remains, and this is cheap enough that we might just want to do this to get rid of the noise from static checkers informing us that random() shouldn't be used.

Signed-off-by: Stefano Brivio --- ndp.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/ndp.c b/ndp.c index 1f2bcb0..43457b3 100644 --- a/ndp.c +++ b/ndp.c @@ -413,6 +413,7 @@ void ndp_timer(const struct ctx *c, const struct timespec *now) { time_t max_rtr_adv_interval = DEFAULT_MAX_RTR_ADV_INTERVAL; time_t min_rtr_adv_interval, interval; + long random_part;

if (!tap_is_ready(c) || c->no_ra || now->tv_sec < next_ra) return; @@ -433,15 +434,17 @@ void ndp_timer(const struct ctx *c, const struct timespec *now) * and causing flurries of RAs at the same time. * * This random doesn't need to be cryptographically strong, so random(3) - * is fine. Other routers on the link also want to avoid - * synchronisation, and anything malicious has much easier ways to cause - * trouble. + * would be fine. Other routers on the link also want to avoid + * synchronisation, and anything malicious would have much easier ways + * to cause trouble. However, for the sake of static checkers, use high + * quality entropy as provided by raw_random(). * * The modulus also makes this not strictly a uniform distribution, but, * again, it's close enough for our purposes. */ + raw_random(&random_part, sizeof(random_part)); interval = min_rtr_adv_interval + - random() % (max_rtr_adv_interval - min_rtr_adv_interval); + random_part % (max_rtr_adv_interval - min_rtr_adv_interval);

Although it returns a signed long, random() is explicitly defined to only return values between 0 and 2^31-1.

Oops, I missed that part. I would have naturally used uint32_t here but then I looked (too quickly) at the prototype of random() and concluded it would be better to make it equivalent... except it's not.

Using raw_random() means we can get anything in the full range of a 'long', including negative numbers. Is that going to mess up our calculations here?

I don't think in any catastrophic way, but it might, yes.

Might be safer to make random_part a uint32_t, then cast it to a time_t for the arithmetic.

Right, v2 does that. -- Stefano