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SubjectRe: net: uninitialized loopback addr leaks to userspace
On Sat, 2009-05-30 at 22:23 +0200, Vegard Nossum wrote:
> 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

#define REPEAT_COUNT 10000

int childTask() {
struct ifreq ifreq;
int fd;
unsigned char allBits;

fd = socket(AF_INET, SOCK_DGRAM, 0);
if (fd < 0){
printf("Error %s from socket()\n", strerror(errno));

strncpy(ifreq.ifr_name, "lo", sizeof("lo"));
if (ioctl (fd, SIOCGIFHWADDR, &ifreq) < 0){
printf("Error %s from ioctl(SIOCGIFHWADDR) for %s.\n", strerror(errno), ifreq.ifr_name);

allBits = ifreq.ifr_hwaddr.sa_data[0] |
ifreq.ifr_hwaddr.sa_data[1] |
ifreq.ifr_hwaddr.sa_data[2] |
ifreq.ifr_hwaddr.sa_data[3] |
ifreq.ifr_hwaddr.sa_data[4] |

if (allBits != 0)
printf("Device %s -> Ethernet %02x:%02x:%02x:%02x:%02x:%02x\n", ifreq.ifr_name,
(int) ((unsigned char *) &ifreq.ifr_hwaddr.sa_data)[0],
(int) ((unsigned char *) &ifreq.ifr_hwaddr.sa_data)[1],
(int) ((unsigned char *) &ifreq.ifr_hwaddr.sa_data)[2],
(int) ((unsigned char *) &ifreq.ifr_hwaddr.sa_data)[3],
(int) ((unsigned char *) &ifreq.ifr_hwaddr.sa_data)[4],
(int) ((unsigned char *) &ifreq.ifr_hwaddr.sa_data)[5]);

int main(int argc, char *argv[]) {
void **child_stack;
int pid, i, status;

child_stack = (void **) malloc(16384);

for (i = 0; i < REPEAT_COUNT; i++){

pid = clone(childTask, child_stack, CLONE_NEWNET, NULL);
if (pid < 0){
printf("Error %s from clone()\n", strerror(errno));

pid = waitpid(pid, &status, __WCLONE);
if (pid < 0){
printf("Error %s from waitpid()\n", strerror(errno));

return 0;

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?

-- John

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