// SPDX-License-Identifier: GPL-2.0
  #include <linux/init.h>
  #include <linux/mm.h>
  #include <linux/security.h>
  #include <linux/sysctl.h>
  
  /* amount of vm to protect from userspace access by both DAC and the LSM*/
  unsigned long mmap_min_addr;
  /* amount of vm to protect from userspace using CAP_SYS_RAWIO (DAC) */
  unsigned long dac_mmap_min_addr = CONFIG_DEFAULT_MMAP_MIN_ADDR;
  /* amount of vm to protect from userspace using the LSM = CONFIG_LSM_MMAP_MIN_ADDR */
  
  /*
   * Update mmap_min_addr = max(dac_mmap_min_addr, CONFIG_LSM_MMAP_MIN_ADDR)
   */
  static void update_mmap_min_addr(void)
  {
  #ifdef CONFIG_LSM_MMAP_MIN_ADDR
  	if (dac_mmap_min_addr > CONFIG_LSM_MMAP_MIN_ADDR)
  		mmap_min_addr = dac_mmap_min_addr;
  	else
  		mmap_min_addr = CONFIG_LSM_MMAP_MIN_ADDR;
  #else
  	mmap_min_addr = dac_mmap_min_addr;
  #endif
  }
  
  /*
   * sysctl handler which just sets dac_mmap_min_addr = the new value and then
   * calls update_mmap_min_addr() so non MAP_FIXED hints get rounded properly
   */
  int mmap_min_addr_handler(struct ctl_table *table, int write,
  			  void __user *buffer, size_t *lenp, loff_t *ppos)
  {
  	int ret;
  
  	if (write && !capable(CAP_SYS_RAWIO))
  		return -EPERM;
  
  	ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
  
  	update_mmap_min_addr();
  
  	return ret;
  }
  
  static int __init init_mmap_min_addr(void)
  {
  	update_mmap_min_addr();
  
  	return 0;
  }
  pure_initcall(init_mmap_min_addr);