Vegard Nossum a écrit :

2009/6/7 John Dykstra [off-list ref]:

quoted

On Sat, 2009-05-30 at 22:23 +0200, Vegard Nossum wrote:

quoted

It seems that loopback's hardware address is never initialized by the
kernel. So if userspace attempts to read this address before it has
been set, the kernel will return some uninitialized data (only 6
bytes, though).

Thank you for the report, Vegard.

I've been unable to reproduce the problem you describe, using
2.6-30-rc8, this test program and a couple of kernel builds for system
load:

[...]

quoted

------------------------------------------------------------------

Looking at the kernel code, it appears that all bytes of struct
net_device, including the L2 address, are initialized to zeros at
interface creation time.

Can you spot a difference between your test procedures and mine that
would enable me to reproduce the problem?

Hi,

I just tried your test program on a linux-next kernel, it works beautifully :-)

(I made one change: The stack grows downwards on x86, so I think you
should put child_stack + 16386 as the stack to clone()?)

As I wrote in reply to Stephen Hemminger, this problem seems to be
caused by a particular patch in linux-next:

commit f001fde5eadd915f4858d22ed70d7040f48767cf
Author: Jiri Pirko [off-list ref]
Date:   Tue May 5 02:48:28 2009 +0000

   net: introduce a list of device addresses dev_addr_list (v6)

I believe following patch should fix this problem.

Thank you

[PATCH net-next-2.6] net: loopback device dev->addr_len fix

commit f001fde5eadd915f4858d22ed70d7040f48767cf 
(net: introduce a list of device addresses dev_addr_list (v6))
added one regression Vegard Nossum found in its testings.

loopback device doesnt have a hw address, we should set its
dev->addr_len to 0, not ETH_ALEN.

Reported-by: Vegard Nossum <redacted>
Signed-off-by: Eric Dumazet <redacted>
---

diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
index da472c6..40ded4e 100644
--- a/drivers/net/loopback.c
+++ b/drivers/net/loopback.c

@@ -166,7 +166,7 @@ static void loopback_setup(struct net_device *dev)
 {
 	dev->mtu		= (16 * 1024) + 20 + 20 + 12;
 	dev->hard_header_len	= ETH_HLEN;	/* 14	*/
-	dev->addr_len		= ETH_ALEN;	/* 6	*/
+	dev->addr_len		= 0;
 	dev->tx_queue_len	= 0;
 	dev->type		= ARPHRD_LOOPBACK;	/* 0x0001*/
 	dev->flags		= IFF_LOOPBACK;