/*
   * data_breakpoint.c - Sample HW Breakpoint file to watch kernel data address
   *
   * This program is free software; you can redistribute it and/or modify
   * it under the terms of the GNU General Public License as published by
   * the Free Software Foundation; either version 2 of the License, or
   * (at your option) any later version.
   *
   * This program is distributed in the hope that it will be useful,
   * but WITHOUT ANY WARRANTY; without even the implied warranty of
   * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   * GNU General Public License for more details.
   *
   * You should have received a copy of the GNU General Public License
   * along with this program; if not, write to the Free Software
   * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
   *
   * usage: insmod data_breakpoint.ko ksym=<ksym_name>
   *
   * This file is a kernel module that places a breakpoint over ksym_name kernel
   * variable using Hardware Breakpoint register. The corresponding handler which

   * prints a backtrace is invoked every time a write operation is performed on

   * that variable.
   *
   * Copyright (C) IBM Corporation, 2009

   *
   * Author: K.Prasad <prasad@linux.vnet.ibm.com>

   */
  #include <linux/module.h>	/* Needed by all modules */
  #include <linux/kernel.h>	/* Needed for KERN_INFO */
  #include <linux/init.h>		/* Needed for the macros */

  #include <linux/kallsyms.h>

  #include <linux/perf_event.h>
  #include <linux/hw_breakpoint.h>

  struct perf_event * __percpu *sample_hbp;

  
  static char ksym_name[KSYM_NAME_LEN] = "pid_max";
  module_param_string(ksym, ksym_name, KSYM_NAME_LEN, S_IRUGO);
  MODULE_PARM_DESC(ksym, "Kernel symbol to monitor; this module will report any"
  			" write operations on the kernel symbol");

  static void sample_hbp_handler(struct perf_event *bp,

  			       struct perf_sample_data *data,
  			       struct pt_regs *regs)

  {
  	printk(KERN_INFO "%s value is changed
  ", ksym_name);
  	dump_stack();
  	printk(KERN_INFO "Dump stack from sample_hbp_handler
  ");
  }
  
  static int __init hw_break_module_init(void)
  {
  	int ret;

  	struct perf_event_attr attr;

  	hw_breakpoint_init(&attr);

  	attr.bp_addr = kallsyms_lookup_name(ksym_name);
  	attr.bp_len = HW_BREAKPOINT_LEN_4;
  	attr.bp_type = HW_BREAKPOINT_W | HW_BREAKPOINT_R;

  	sample_hbp = register_wide_hw_breakpoint(&attr, sample_hbp_handler, NULL);

  	if (IS_ERR((void __force *)sample_hbp)) {
  		ret = PTR_ERR((void __force *)sample_hbp);

  		goto fail;

  	}

  	printk(KERN_INFO "HW Breakpoint for %s write installed
  ", ksym_name);

  
  	return 0;

  
  fail:
  	printk(KERN_INFO "Breakpoint registration failed
  ");
  
  	return ret;

  }
  
  static void __exit hw_break_module_exit(void)
  {

  	unregister_wide_hw_breakpoint(sample_hbp);

  	printk(KERN_INFO "HW Breakpoint for %s write uninstalled
  ", ksym_name);
  }
  
  module_init(hw_break_module_init);
  module_exit(hw_break_module_exit);
  
  MODULE_LICENSE("GPL");
  MODULE_AUTHOR("K.Prasad");
  MODULE_DESCRIPTION("ksym breakpoint");