[PATCH v2] eal: allow setting random number generator seed

[Konstantin Ananyev]

> Den 2026-09-12 kl. 18:16, skrev Stephen Hemminger:
> > Many performance tests use rte_rand() and the random number
> > can perturb the results. Add an ability to override the automatic
> > random seed on DPDK startup.
> 
> There is a way to seed the PRNG already, rte_srand(). Why the tests
> can't use this?
> 
> Provided the test results depend on something that use the PRNG before
> the test driver has had the opportunity to call rte_srand(), controlling
> the *initial* seed may be required.
> 
> I don't think this feature should be controlled by an environment
> variable. There is no precedent for that. If this functionality is
> deemed useful, it should be an EAL command line option, it seems to me.

+1

> 
> >
> > This is not a security problem since rte_rand() is documented
> > as not being cryptographically secure.
> >
> > Signed-off-by: Stephen Hemminger <stephen at networkplumber.org>
> > Reviewed-by: Morten Brørup <mb at smartsharesystems.com>
> > ---
> > v2 - fix header inclusion
> >     - add docbook comment
> >
> >   lib/eal/common/rte_random.c  | 14 +++++++++++++-
> >   lib/eal/include/rte_random.h |  8 +++++---
> >   2 files changed, 18 insertions(+), 4 deletions(-)
> >
> > diff --git a/lib/eal/common/rte_random.c b/lib/eal/common/rte_random.c
> > index 576a32a46c..3cc0e3ec5f 100644
> > --- a/lib/eal/common/rte_random.c
> > +++ b/lib/eal/common/rte_random.c
> > @@ -8,6 +8,7 @@
> >   #endif
> >   #endif
> >   #include <unistd.h>
> > +#include <stdlib.h>
> >
> >   #include <rte_bitops.h>
> >   #include <rte_branch_prediction.h>
> > @@ -247,7 +248,18 @@ eal_rand_init(void)
> >
> >   	RTE_LCORE_VAR_ALLOC(rand_state);
> >
> > -	seed = __rte_random_initial_seed();
> > +	const char *env = getenv("DPDK_RANDOM_SEED");
> > +	if (env != NULL && *env != '\0') {
> > +		char *end;
> > +
> > +		errno = 0;
> > +		seed = strtoull(env, &end, 0);
> > +		if (errno != 0 || *end != '\0')
> > +			rte_exit(EXIT_FAILURE,
> > +				 "invalid DPDK_RANDOM_SEED: %s\n", env);
> 
> Is rte_exit() the way to deal with errors here? Not to be used in DPDK
> libraries, if I recall correctly.

Again, +1
Whole patch looks to me like a strange hack that completely ignores DPDK coding practices. 
My vote is NACK.

> 
> I would think logging an error would suffice. Or rte_eal_init_alert().
> 
> > +	} else {
> > +		seed = __rte_random_initial_seed();
> > +	}
> >
> >   	rte_srand(seed);
> >   }
> > diff --git a/lib/eal/include/rte_random.h b/lib/eal/include/rte_random.h
> > index 15cbe6215a..bdd4001e78 100644
> > --- a/lib/eal/include/rte_random.h
> > +++ b/lib/eal/include/rte_random.h
> > @@ -20,9 +20,11 @@ extern "C" {
> >   /**
> >    * Seed the pseudo-random generator.
> >    *
> > - * The generator is automatically seeded by the EAL init with a timer
> > - * value. It may need to be re-seeded by the user with a real random
> > - * value.
> > + * The generator is automatically seeded by the EAL init with
> > + * a system provided random entropy source. But for testing
> > + * it can be useful to force a repeatable starting point by
> > + * setting the initial seed. This can be done by setting
> > + * the `DPDK_RANDOM_SEED` environment variable.
> >    *
> >    * This function is not multi-thread safe in regards to other
> >    * rte_srand() calls, nor is it in relation to concurrent rte_rand(),