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

[Stephen Hemminger <stephen@networkplumber.org>]

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.

This is not a security problem since rte_rand() is documented
as not being cryptographically secure.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Reviewed-by: Morten Brørup <mb@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);
+	} 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(),
-- 
2.53.0