[ipxe-devel] [PATCH] [device] Improve the seed of the pseudo-random number generator

[Bernhard Kohl]

When (re)starting a couple of virtual machines (about 10) on the same host
it frequently happens that some of them use the same DHCP transaction ID.

This problem can be solved by combining the current timer ticks and the
least significant bits of the hardware address(es) to get a better
randomness for the seed.

Signed-off-by: Bernhard Kohl <bernhard.kohl at nsn.com>
---
 src/core/device.c |   21 +++++++++++++++++++++
 1 files changed, 21 insertions(+), 0 deletions(-)

diff --git a/src/core/device.c b/src/core/device.c
index 330f95c..ba7d26c 100644
--- a/src/core/device.c
+++ b/src/core/device.c
@@ -20,11 +20,15 @@
 FILE_LICENCE ( GPL2_OR_LATER );
 
 #include <string.h>
+#include <stdlib.h>
+#include <byteswap.h>
 #include <ipxe/list.h>
 #include <ipxe/tables.h>
 #include <ipxe/init.h>
 #include <ipxe/interface.h>
 #include <ipxe/device.h>
+#include <ipxe/netdevice.h>
+#include <ipxe/timer.h>
 
 /**
  * @file
@@ -78,6 +82,9 @@ static void rootdev_remove ( struct root_device *rootdev ) {
 static void probe_devices ( void ) {
 	struct root_device *rootdev;
 	int rc;
+	struct net_device *netdev;
+	unsigned int seed;
+	unsigned int ll_addr;
 
 	for_each_table_entry ( rootdev, ROOT_DEVICES ) {
 		list_add ( &rootdev->dev.siblings, &devices );
@@ -85,6 +92,20 @@ static void probe_devices ( void ) {
 		if ( ( rc = rootdev_probe ( rootdev ) ) != 0 )
 			list_del ( &rootdev->dev.siblings );
 	}
+
+	/* Seed the pseudo-random number generator. Combine the current
+	 * timer ticks and the least significant bits of the hardware
+	 * address(es) to get a high degree of randomness for the seed.
+	 */
+	seed = (unsigned int)currticks();
+	for_each_netdev ( netdev ) {
+		memcpy ( &ll_addr, ( netdev->ll_addr + netdev->ll_protocol->ll_addr_len
+				 - sizeof ( ll_addr ) ), sizeof ( ll_addr ) );
+		ll_addr = ntohl ( ll_addr );
+		seed <<= 1;
+		seed ^= ll_addr;
+	}
+	srand ( seed );
 }
 
 /**
-- 
1.7.7.6