/*
 *              INETPEER - A storage for permanent information about peers
 *
 *  This source is covered by the GNU GPL, the same as all kernel sources.
 *
 *  Authors:    Andrey V. Savochkin <saw@msu.ru>
 */

#include <linux/module.h>
#include <linux/types.h>
#include <linux/slab.h>
#include <linux/interrupt.h>
#include <linux/spinlock.h>
#include <linux/random.h>
#include <linux/timer.h>
#include <linux/time.h>
#include <linux/kernel.h>
#include <linux/mm.h>
#include <linux/net.h>
#include <net/ip.h>
#include <net/inetpeer.h>

/*
 *  Theory of operations.
 *  We keep one entry for each peer IP address.  The nodes contains long-living
 *  information about the peer which doesn't depend on routes.
 *  At this moment this information consists only of ID field for the next
 *  outgoing IP packet.  This field is incremented with each packet as encoded
 *  in inet_getid() function (include/net/inetpeer.h).
 *  At the moment of writing this notes identifier of IP packets is generated
 *  to be unpredictable using this code only for packets subjected
 *  (actually or potentially) to defragmentation.  I.e. DF packets less than
 *  PMTU in size uses a constant ID and do not use this code (see
 *  ip_select_ident() in include/net/ip.h).
 *
 *  Route cache entries hold references to our nodes.
 *  New cache entries get references via lookup by destination IP address in
 *  the avl tree.  The reference is grabbed only when it's needed i.e. only
 *  when we try to output IP packet which needs an unpredictable ID (see
 *  __ip_select_ident() in net/ipv4/route.c).
 *  Nodes are removed only when reference counter goes to 0.
 *  When it's happened the node may be removed when a sufficient amount of
 *  time has been passed since its last use.  The less-recently-used entry can
 *  also be removed if the pool is overloaded i.e. if the total amount of
 *  entries is greater-or-equal than the threshold.
 *
 *  Node pool is organised as an AVL tree.
 *  Such an implementation has been chosen not just for fun.  It's a way to
 *  prevent easy and efficient DoS attacks by creating hash collisions.  A huge
 *  amount of long living nodes in a single hash slot would significantly delay
 *  lookups performed with disabled BHs.
 *
 *  Serialisation issues.
 *  1.  Nodes may appear in the tree only with the pool write lock held.
 *  2.  Nodes may disappear from the tree only with the pool write lock held
 *      AND reference count being 0.
 *  3.  Nodes appears and disappears from unused node list only under
 *      "inet_peer_unused_lock".
 *  4.  Global variable peer_total is modified under the pool lock.
 *  5.  struct inet_peer fields modification:
 *              avl_left, avl_right, avl_parent, avl_height: pool lock
 *              unused: unused node list lock
 *              refcnt: atomically against modifications on other CPU;
 *                 usually under some other lock to prevent node disappearing
 *              dtime: unused node list lock
 *              v4daddr: unchangeable
 *              ip_id_count: idlock
 */

static struct kmem_cache *peer_cachep __read_mostly;

#define node_height(x) x->avl_height
static struct inet_peer peer_fake_node = {
        .avl_left       = &peer_fake_node,
        .avl_right      = &peer_fake_node,
        .avl_height     = 0
};
#define peer_avl_empty (&peer_fake_node)
static struct inet_peer *peer_root = peer_avl_empty;
static DEFINE_RWLOCK(peer_pool_lock);
#define PEER_MAXDEPTH 40 /* sufficient for about 2^27 nodes */

static int peer_total;
/* Exported for sysctl_net_ipv4.  */
int inet_peer_threshold __read_mostly = 65536 + 128;    /* start to throw entries more
                                         * aggressively at this stage */
int inet_peer_minttl __read_mostly = 120 * HZ;  /* TTL under high load: 120 sec */
int inet_peer_maxttl __read_mostly = 10 * 60 * HZ;      /* usual time to live: 10 min */
int inet_peer_gc_mintime __read_mostly = 10 * HZ;
int inet_peer_gc_maxtime __read_mostly = 120 * HZ;

static LIST_HEAD(unused_peers);
static DEFINE_SPINLOCK(inet_peer_unused_lock);

static void peer_check_expire(unsigned long dummy);
static DEFINE_TIMER(peer_periodic_timer, peer_check_expire, 0, 0);


/* Called from ip_output.c:ip_init  */
void __init inet_initpeers(void)
{
        struct sysinfo si;

        /* Use the straight interface to information about memory. */
        si_meminfo(&si);
        /* The values below were suggested by Alexey Kuznetsov
         * <kuznet@ms2.inr.ac.ru>.  I don't have any opinion about the values
         * myself.  --SAW
         */
        if (si.totalram <= (32768*1024)/PAGE_SIZE)
                inet_peer_threshold >>= 1; /* max pool size about 1MB on IA32 */
        if (si.totalram <= (16384*1024)/PAGE_SIZE)
                inet_peer_threshold >>= 1; /* about 512KB */
        if (si.totalram <= (8192*1024)/PAGE_SIZE)
                inet_peer_threshold >>= 2; /* about 128KB */

        peer_cachep = kmem_cache_create("inet_peer_cache",
                        sizeof(struct inet_peer),
                        0, SLAB_HWCACHE_ALIGN|SLAB_PANIC,
                        NULL);

        /* All the timers, started at system startup tend
           to synchronize. Perturb it a bit.
         */
        peer_periodic_timer.expires = jiffies
                + net_random() % inet_peer_gc_maxtime
                + inet_peer_gc_maxtime;
        add_timer(&peer_periodic_timer);
}

/* Called with or without local BH being disabled. */
static void unlink_from_unused(struct inet_peer *p)
{
        spin_lock_bh(&inet_peer_unused_lock);
        list_del_init(&p->unused);
        spin_unlock_bh(&inet_peer_unused_lock);
}

/*
 * Called with local BH disabled and the pool lock held.
 * _stack is known to be NULL or not at compile time,
 * so compiler will optimize the if (_stack) tests.
 */
#define lookup(_daddr, _stack)                                  \
({                                                              \
        struct inet_peer *u, **v;                               \
        if (_stack != NULL) {                                   \
                stackptr = _stack;                              \
                *stackptr++ = &peer_root;                       \
        }                                                       \
        for (u = peer_root; u != peer_avl_empty; ) {            \
                if (_daddr == u->v4daddr)                       \
                        break;                                  \
                if ((__force __u32)_daddr < (__force __u32)u->v4daddr)  \
                        v = &u->avl_left;                       \
                else                                            \
                        v = &u->avl_right;                      \
                if (_stack != NULL)                             \
                        *stackptr++ = v;                        \
                u = *v;                                         \
        }                                                       \
        u;                                                      \
})

/* Called with local BH disabled and the pool write lock held. */
#define lookup_rightempty(start)                                \
({                                                              \
        struct inet_peer *u, **v;                               \
        *stackptr++ = &start->avl_left;                         \
        v = &start->avl_left;                                   \
        for (u = *v; u->avl_right != peer_avl_empty; ) {        \
                v = &u->avl_right;                              \
                *stackptr++ = v;                                \
                u = *v;                                         \
        }                                                       \
        u;                                                      \
})

/* Called with local BH disabled and the pool write lock held.
 * Variable names are the proof of operation correctness.
 * Look into mm/map_avl.c for more detail description of the ideas.  */
static void peer_avl_rebalance(struct inet_peer **stack[],
                struct inet_peer ***stackend)
{
        struct inet_peer **nodep, *node, *l, *r;
        int lh, rh;

        while (stackend > stack) {
                nodep = *--stackend;
                node = *nodep;
                l = node->avl_left;
                r = node->avl_right;
                lh = node_height(l);
                rh = node_height(r);
                if (lh > rh + 1) { /* l: RH+2 */
                        struct inet_peer *ll, *lr, *lrl, *lrr;
                        int lrh;
                        ll = l->avl_left;
                        lr = l->avl_right;
                        lrh = node_height(lr);
                        if (lrh <= node_height(ll)) {   /* ll: RH+1 */
                                node->avl_left = lr;    /* lr: RH or RH+1 */
                                node->avl_right = r;    /* r: RH */
                                node->avl_height = lrh + 1; /* RH+1 or RH+2 */
                                l->avl_left = ll;       /* ll: RH+1 */
                                l->avl_right = node;    /* node: RH+1 or RH+2 */
                                l->avl_height = node->avl_height + 1;
                                *nodep = l;
                        } else { /* ll: RH, lr: RH+1 */
                                lrl = lr->avl_left;     /* lrl: RH or RH-1 */
                                lrr = lr->avl_right;    /* lrr: RH or RH-1 */
                                node->avl_left = lrr;   /* lrr: RH or RH-1 */
                                node->avl_right = r;    /* r: RH */
                                node->avl_height = rh + 1; /* node: RH+1 */
                                l->avl_left = ll;       /* ll: RH */
                                l->avl_right = lrl;     /* lrl: RH or RH-1 */
                                l->avl_height = rh + 1; /* l: RH+1 */
                                lr->avl_left = l;       /* l: RH+1 */
                                lr->avl_right = node;   /* node: RH+1 */
                                lr->avl_height = rh + 2;
                                *nodep = lr;
                        }
                } else if (rh > lh + 1) { /* r: LH+2 */
                        struct inet_peer *rr, *rl, *rlr, *rll;
                        int rlh;
                        rr = r->avl_right;
                        rl = r->avl_left;
                        rlh = node_height(rl);
                        if (rlh <= node_height(rr)) {   /* rr: LH+1 */
                                node->avl_right = rl;   /* rl: LH or LH+1 */
                                node->avl_left = l;     /* l: LH */
                                node->avl_height = rlh + 1; /* LH+1 or LH+2 */
                                r->avl_right = rr;      /* rr: LH+1 */
                                r->avl_left = node;     /* node: LH+1 or LH+2 */
                                r->avl_height = node->avl_height + 1;
                                *nodep = r;
                        } else { /* rr: RH, rl: RH+1 */
                                rlr = rl->avl_right;    /* rlr: LH or LH-1 */
                                rll = rl->avl_left;     /* rll: LH or LH-1 */
                                node->avl_right = rll;  /* rll: LH or LH-1 */
                                node->avl_left = l;     /* l: LH */
                                node->avl_height = lh + 1; /* node: LH+1 */
                                r->avl_right = rr;      /* rr: LH */
                                r->avl_left = rlr;      /* rlr: LH or LH-1 */
                                r->avl_height = lh + 1; /* r: LH+1 */
                                rl->avl_right = r;      /* r: LH+1 */
                                rl->avl_left = node;    /* node: LH+1 */
                                rl->avl_height = lh + 2;
                                *nodep = rl;
                        }
                } else {
                        node->avl_height = (lh > rh ? lh : rh) + 1;
                }
        }
}

/* Called with local BH disabled and the pool write lock held. */
#define link_to_pool(n)                                         \
do {                                                            \
        n->avl_height = 1;                                      \
        n->avl_left = peer_avl_empty;                           \
        n->avl_right = peer_avl_empty;                          \
        **--stackptr = n;                                       \
        peer_avl_rebalance(stack, stackptr);                    \
} while(0)

/* May be called with local BH enabled. */
static void unlink_from_pool(struct inet_peer *p)
{
        int do_free;

        do_free = 0;

        write_lock_bh(&peer_pool_lock);
        /* Check the reference counter.  It was artificially incremented by 1
         * in cleanup() function to prevent sudden disappearing.  If the
         * reference count is still 1 then the node is referenced only as `p'
         * here and from the pool.  So under the exclusive pool lock it's safe
         * to remove the node and free it later. */
        if (atomic_read(&p->refcnt) == 1) {
                struct inet_peer **stack[PEER_MAXDEPTH];
                struct inet_peer ***stackptr, ***delp;
                if (lookup(p->v4daddr, stack) != p)
                        BUG();
                delp = stackptr - 1; /* *delp[0] == p */
                if (p->avl_left == peer_avl_empty) {
                        *delp[0] = p->avl_right;
                        --stackptr;
                } else {
                        /* look for a node to insert instead of p */
                        struct inet_peer *t;
                        t = lookup_rightempty(p);
                        BUG_ON(*stackptr[-1] != t);
                        **--stackptr = t->avl_left;
                        /* t is removed, t->v4daddr > x->v4daddr for any
                         * x in p->avl_left subtree.
                         * Put t in the old place of p. */
                        *delp[0] = t;
                        t->avl_left = p->avl_left;
                        t->avl_right = p->avl_right;
                        t->avl_height = p->avl_height;
                        BUG_ON(delp[1] != &p->avl_left);
                        delp[1] = &t->avl_left; /* was &p->avl_left */
                }
                peer_avl_rebalance(stack, stackptr);
                peer_total--;
                do_free = 1;
        }
        write_unlock_bh(&peer_pool_lock);

        if (do_free)
                kmem_cache_free(peer_cachep, p);
        else
                /* The node is used again.  Decrease the reference counter
                 * back.  The loop "cleanup -> unlink_from_unused
                 *   -> unlink_from_pool -> putpeer -> link_to_unused
                 *   -> cleanup (for the same node)"
                 * doesn't really exist because the entry will have a
                 * recent deletion time and will not be cleaned again soon. */
                inet_putpeer(p);
}

/* May be called with local BH enabled. */
static int cleanup_once(unsigned long ttl)
{
        struct inet_peer *p = NULL;

        /* Remove the first entry from the list of unused nodes. */
        spin_lock_bh(&inet_peer_unused_lock);
        if (!list_empty(&unused_peers)) {
                __u32 delta;

                p = list_first_entry(&unused_peers, struct inet_peer, unused);
                delta = (__u32)jiffies - p->dtime;

                if (delta < ttl) {
                        /* Do not prune fresh entries. */
                        spin_unlock_bh(&inet_peer_unused_lock);
                        return -1;
                }

                list_del_init(&p->unused);

                /* Grab an extra reference to prevent node disappearing
                 * before unlink_from_pool() call. */
                atomic_inc(&p->refcnt);
        }
        spin_unlock_bh(&inet_peer_unused_lock);

        if (p == NULL)
                /* It means that the total number of USED entries has
                 * grown over inet_peer_threshold.  It shouldn't really
                 * happen because of entry limits in route cache. */
                return -1;

        unlink_from_pool(p);
        return 0;
}

/* Called with or without local BH being disabled. */
struct inet_peer *inet_getpeer(__be32 daddr, int create)
{
        struct inet_peer *p, *n;
        struct inet_peer **stack[PEER_MAXDEPTH], ***stackptr;

        /* Look up for the address quickly. */
        read_lock_bh(&peer_pool_lock);
        p = lookup(daddr, NULL);
        if (p != peer_avl_empty)
                atomic_inc(&p->refcnt);
        read_unlock_bh(&peer_pool_lock);

        if (p != peer_avl_empty) {
                /* The existing node has been found. */
                /* Remove the entry from unused list if it was there. */
                unlink_from_unused(p);
                return p;
        }

        if (!create)
                return NULL;

        /* Allocate the space outside the locked region. */
        n = kmem_cache_alloc(peer_cachep, GFP_ATOMIC);
        if (n == NULL)
                return NULL;
        n->v4daddr = daddr;
        atomic_set(&n->refcnt, 1);
        atomic_set(&n->rid, 0);
        atomic_set(&n->ip_id_count, secure_ip_id(daddr));
        n->tcp_ts_stamp = 0;

        write_lock_bh(&peer_pool_lock);
        /* Check if an entry has suddenly appeared. */
        p = lookup(daddr, stack);
        if (p != peer_avl_empty)
                goto out_free;

        /* Link the node. */
        link_to_pool(n);
        INIT_LIST_HEAD(&n->unused);
        peer_total++;
        write_unlock_bh(&peer_pool_lock);

        if (peer_total >= inet_peer_threshold)
                /* Remove one less-recently-used entry. */
                cleanup_once(0);

        return n;

out_free:
        /* The appropriate node is already in the pool. */
        atomic_inc(&p->refcnt);
        write_unlock_bh(&peer_pool_lock);
        /* Remove the entry from unused list if it was there. */
        unlink_from_unused(p);
        /* Free preallocated the preallocated node. */
        kmem_cache_free(peer_cachep, n);
        return p;
}

/* Called with local BH disabled. */
static void peer_check_expire(unsigned long dummy)
{
        unsigned long now = jiffies;
        int ttl;

        if (peer_total >= inet_peer_threshold)
                ttl = inet_peer_minttl;
        else
                ttl = inet_peer_maxttl
                                - (inet_peer_maxttl - inet_peer_minttl) / HZ *
                                        peer_total / inet_peer_threshold * HZ;
        while (!cleanup_once(ttl)) {
                if (jiffies != now)
                        break;
        }

        /* Trigger the timer after inet_peer_gc_mintime .. inet_peer_gc_maxtime
         * interval depending on the total number of entries (more entries,
         * less interval). */
        if (peer_total >= inet_peer_threshold)
                peer_periodic_timer.expires = jiffies + inet_peer_gc_mintime;
        else
                peer_periodic_timer.expires = jiffies
                        + inet_peer_gc_maxtime
                        - (inet_peer_gc_maxtime - inet_peer_gc_mintime) / HZ *
                                peer_total / inet_peer_threshold * HZ;
        add_timer(&peer_periodic_timer);
}

void inet_putpeer(struct inet_peer *p)
{
        spin_lock_bh(&inet_peer_unused_lock);
        if (atomic_dec_and_test(&p->refcnt)) {
                list_add_tail(&p->unused, &unused_peers);
                p->dtime = (__u32)jiffies;
        }
        spin_unlock_bh(&inet_peer_unused_lock);
}