FreeBSD/Linux Kernel Cross Reference
sys/netinet/ip_id.c

     /*
      * Copyright (c) 2007 The DragonFly Project.  All rights reserved.
      * 
      * This code is derived from software contributed to The DragonFly Project
      * by Matthew Dillon <dillon@backplane.com>
      * 
      * Redistribution and use in source and binary forms, with or without
      * modification, are permitted provided that the following conditions
      * are met:
     * 
     * 1. Redistributions of source code must retain the above copyright
     *    notice, this list of conditions and the following disclaimer.
     * 2. Redistributions in binary form must reproduce the above copyright
     *    notice, this list of conditions and the following disclaimer in
     *    the documentation and/or other materials provided with the
     *    distribution.
     * 3. Neither the name of The DragonFly Project nor the names of its
     *    contributors may be used to endorse or promote products derived
     *    from this software without specific, prior written permission.
     * 
     * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
     * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
     * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
     * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
     * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
     * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
     * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
     * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
     * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
     * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
     * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     * SUCH DAMAGE.
     * 
     * $DragonFly: src/sys/netinet/ip_id.c,v 1.8 2007/11/23 22:29:51 dillon Exp $
     */
    
    /*
     * Random ip sequence number generator.  Use the system PRNG to shuffle the
     * 65536 entry ID space.  We reshuffle the front-side of the array as we
     * index through it, guarenteeing an id will not be reused for at least
     * 32768 calls.
     */
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/kernel.h>
    #include <sys/random.h>
    #include <sys/spinlock.h>
    #include <sys/globaldata.h>
    #include <netinet/ip_var.h>
    
    #include <sys/spinlock2.h>
    
    #define IPRANDCOUNT     32
    
    typedef struct iprandinfo {
            short   randdata[IPRANDCOUNT];
            int     randidx;
            int     isidx;
    } *iprandinfo_t;
    
    struct iprandinfo iprandcpu[MAXCPU];
    
    static u_int16_t ip_shuffle[65536];
    
    /*
     * Initialize the shuffle.  We assume that the system PRNG won't be all that
     * good this early in the boot sequence but use it anyway.  The ids will be
     * reshuffled as they are popped and the PRNG should be better then.
     */
    static void
    ip_initshuffle(void *dummy __unused)
    {
            int i;
    
            for (i = 0; i < 65536; ++i)
                    ip_shuffle[i] = i;
            for (i = 0; i < 65536; ++i)
                    ip_randomid();
            for (i = 0; i < ncpus; ++i) {
                    iprandcpu[i].isidx = i * (65536 / ncpus_fit);
            }
    }
    
    SYSINIT(ipshuffle, SI_SUB_PSEUDO, SI_ORDER_ANY, ip_initshuffle, NULL);
    
    /*
     * Return a random IP id.  Use a forward shuffle over half the index
     * space to avoid duplicates occuring too quickly.  Since the initial
     * shuffle may not have had a good random basis we returned the element
     * at the shuffle target instead of the current element.
     *
     * XXX make per-cpu so the spinlock can be removed?
     */
    u_int16_t
    ip_randomid(void)
    {
            globaldata_t gd = mycpu;
            iprandinfo_t info;
            u_int16_t si, r;
           int i1, i2;
   
           info = &iprandcpu[gd->gd_cpuid];
           crit_enter();
   
           /*
            * Reload random array efficiently
            */
           if (info->randidx == 0) {
                   info->randidx = IPRANDCOUNT;
                   read_random_unlimited(info->randdata, sizeof(info->randdata));
           }
   
           /*
            * Get random number (si) and calculate shuffle.  The shuffle
            * is calculated such that the cpus do not interfere with each
            * other.
            */
           si = info->randdata[--info->randidx];
           i1 = (info->isidx & 0xFFFF & ~ncpus_fit_mask) | gd->gd_cpuid;
           i2 = ((i1 + (si & 0x7FFF)) & 0xFFFF & ~ncpus_fit_mask) | gd->gd_cpuid;
   
           /*
            * Do the shuffle, bump isidx taking into account the cpu
            * partitioning.
            */
           r = ip_shuffle[i2];
           ip_shuffle[i2] = ip_shuffle[i1];
           ip_shuffle[i1] = r;
           info->isidx += ncpus_fit;
   
           crit_exit();
           return(r);
   }
   

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