[PATCH] uml: assign random MACs to interfaces if necessary

Assign a random MAC to an ethernet interface if one was not provided on the
command line.  This became pressing when distros started bringing interfaces
up before assigning IPs to them.  The previous pattern of assigning an IP then
bringing it up allowed the MAC to be generated from the first IP assigned.
However, once the thing is up, it's probably a bad idea to change the MAC, so
the MAC stayed initialized to fe:fd:0:0:0:0.

Now, if there is no MAC from the command line, one is generated.  We use the
microseconds from gettimeofday (20 bits), plus the low 12 bits of the pid to
seed the random number generator.  random() is called twice, with 16 bits of
each result used.  I didn't want to have to try to fill in 32 bits optimally
given an arbitrary RAND_MAX, so I just assume that it is greater than 65536
and use 16 bits of each random() return.

There is also a bit of reformatting and whitespace cleanup here.

Signed-off-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 30 insertions, 0 deletions

diff --git a/arch/um/drivers/net_user.c b/arch/um/drivers/net_user.c
index 107c5e43fa00fe..142bcb2c7c6ab4 100644
--- a/arch/um/drivers/net_user.c
+++ b/arch/um/drivers/net_user.c
@@ -12,6 +12,7 @@
 #include <string.h>
 #include <sys/socket.h>
 #include <sys/wait.h>
+#include <sys/time.h>
 #include "user.h"
 #include "user_util.h"
 #include "kern_util.h"
@@ -258,3 +259,32 @@ char *split_if_spec(char *str, ...)
 	va_end(ap);
 	return str;
 }
+
+void random_mac(unsigned char *addr)
+{
+	struct timeval tv;
+	long n;
+	unsigned int seed;
+
+	gettimeofday(&tv, NULL);
+
+	/* Assume that 20 bits of microseconds and 12 bits of the pid are
+	 * reasonably unpredictable.
+	 */
+	seed = tv.tv_usec | (os_getpid() << 20);
+	srandom(seed);
+
+	/* Don't care about endianness here - switching endianness
+	 * just rearranges what are hopefully random numbers.
+	 *
+	 * Assume that RAND_MAX > 65536, so random is called twice and
+	 * we use 16 bits of the result.
+	 */
+	n = random();
+	addr[2] = (n >> 8) & 255;
+	addr[3] = n % 255;
+
+	n = random();
+	addr[4] = (n >> 8) & 255;
+	addr[5] = n % 255;
+}