net: ethernet: xscale: Check for PTP support properly

In the Linux kernel, the following vulnerability has been resolved:

net: ethernet: xscale: Check for PTP support properly

In ixp4xx_get_ts_info() ixp46x_ptp_find() is called

unconditionally despite this feature only existing on

ixp46x, leading to the following splat from tcpdump:

root@OpenWrt:~# tcpdump -vv -X -i eth0

(...)

Unable to handle kernel NULL pointer dereference at virtual address

00000238 when read

(...)

Call trace:

ptp_clock_index from ixp46x_ptp_find+0x1c/0x38

ixp46x_ptp_find from ixp4xx_get_ts_info+0x4c/0x64

ixp4xx_get_ts_info from __ethtool_get_ts_info+0x90/0x108

__ethtool_get_ts_info from __dev_ethtool+0xa00/0x2648

__dev_ethtool from dev_ethtool+0x160/0x234

dev_ethtool from dev_ioctl+0x2cc/0x460

dev_ioctl from sock_ioctl+0x1ec/0x524

sock_ioctl from sys_ioctl+0x51c/0xa94

sys_ioctl from ret_fast_syscall+0x0/0x44

(...)

Segmentation fault

Check for ixp46x in ixp46x_ptp_find() before trying to set up

PTP to avoid this.

To avoid altering the returned error code from ixp4xx_hwtstamp_set()

which before this patch was -EOPNOTSUPP, we return -EOPNOTSUPP

from ixp4xx_hwtstamp_set() if ixp46x_ptp_find() fails no matter

the error code. The helper function ixp46x_ptp_find() helper

returns -ENODEV.