dev: Fix network device ifindex allocation

There are some problems with network device ifindex handling in the core and veth driver.

* Network device index (ifindex) is signed int, but only values > 0 consodered to be valid. However it is possible for userspace to supply negative ifindex value using following command:

    # ip link add dev dummy2 index -100 up type dummy
    # ip link sh dev dummy2
    Device "dummy2" does not exist.
    # ip li sh |tail -n2
    -100: dummy2: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue \
    state UNKNOWN mode DEFAULT group default
        link/ether 7e:77:1b:49:50:de brd ff:ff:ff:ff:ff:ff

* It is possible to hit infinite loop when number of ifindex in some network namespace get exchausted.

	# There should be plenty of RAM to create INT_MAX netdev.
	# Hitting OOM is much likely with such huge number, but anyway.
	for ((i = 1; i <= 0x7fffffff; i++)); do
		ifname="dummy$i";
		[ ! -d "/sys/class/net/$ifname" ] || continue
		ip link add dev "$ifname" index "$i" type dummy
	done

* (minor) check for dev->ifindex being greater than zero.

Patches in this series tries to address these problems.

Serhey Popovych (3):
dev: Prevent creating network devices with negative ifindex
dev: Avoid infinite loop on network device index exhaustion
veth: Set ifindex only if given and not negative

 drivers/net/veth.c |  2 +-
 net/core/dev.c     | 30 ++++++++++++++++++++++--------
 2 files changed, 23 insertions(+), 9 deletions(-)

1.8.3.1