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Thursday, August 29, 2013

finding the netdev_priv struct

In addition to the net_device struct within the kernel, network drivers also have their own private or device-specific struct which stores stats unique to the individual hardware.

The name of the struct varies for each device type, however the location remains the same, right after net_device:

 *      netdev_priv - access network device private data
 *      @dev: network device
 * Get network device private data
static inline void *netdev_priv(const struct net_device *dev)
        return (char *)dev + ALIGN(sizeof(struct net_device), NETDEV_ALIGN);
A search in cscope for netdev_priv will show you many functions within drivers which update the priv structure via pointers, for example this one in e1000_main.c:
struct e1000_adapter *adapter = netdev_priv(netdev);
So, given that we know what the device-specific struct is called, and where the device-specific private struct is, how do you find it and read it?

We'll need the ability to read kernel memory using crash, either run on a live system, or in a vmcore captured with kdump:
crash /usr/lib/debug/lib/modules/2.6.32-358.14.1.el6.x86_64/vmlinux /var/crash/2013-08-26/vmcore
We'll also need the debugging symbols for the driver in question:
crash> mod -s e1000 /usr/lib/debug/lib/modules/2.6.32-358.14.1.el6.x86_64/kernel/drivers/net/e1000/e1000.ko.debug
     MODULE       NAME                 SIZE  OBJECT FILE
ffffffffa01550e0  e1000              170678  /usr/lib/debug/lib/modules/2.6.32-358.14.1.el6.x86_64/kernel/drivers/net/e1000/e1000.ko.debug 
The net command will show you the network devices in the system:
crash> net
ffff88007e76b820  lo
ffff8800372c0020  eth0
We can then cast net_device against this to see the in-kernel device struct:
crash> net_device 0xffff8800372c0020
struct net_device {
  name = "eth0\000\000\060:03.0\000\000\000",
But we want to get after this struct and cast it against the struct in the driver.

We need to know how big net_device is:
crash> struct -o net_device
struct net_device {
SIZE: 1728
We then find the net address plus the size of net_device:
crash> px 0xffff8800372c0020+1728
$1 = 0xffff8800372c06e0
We can now cast the device-specific private struct against this new address:
crash> e1000_adapter 0xffff8800372c06e0
struct e1000_adapter {
  vlgrp = 0x0,
  mng_vlan_id = 65535,
  bd_number = 0,
  rx_buffer_len = 1522,
and we're done!