/*
 *              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 lock held.
 *  2.  Nodes may disappear from the tree only with the pool 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
 *              daddr: unchangeable
 *              ip_id_count: atomic value (no lock needed)
 */

static struct kmem_cache *peer_cachep __read_mostly;

#define node_height(x) x->avl_height

#define peer_avl_empty ((struct inet_peer *)&peer_fake_node)
#define peer_avl_empty_rcu ((struct inet_peer __rcu __force *)&peer_fake_node)
static const struct inet_peer peer_fake_node = {
        .avl_left       = peer_avl_empty_rcu,
        .avl_right      = peer_avl_empty_rcu,
        .avl_height     = 0
};

struct inet_peer_base {
        struct inet_peer __rcu *root;
        spinlock_t      lock;
        int             total;
};

static struct inet_peer_base v4_peers = {
        .root           = peer_avl_empty_rcu,
        .lock           = __SPIN_LOCK_UNLOCKED(v4_peers.lock),
        .total          = 0,
};

static struct inet_peer_base v6_peers = {
        .root           = peer_avl_empty_rcu,
        .lock           = __SPIN_LOCK_UNLOCKED(v6_peers.lock),
        .total          = 0,
};

#define PEER_MAXDEPTH 40 /* sufficient for about 2^27 nodes */

/* 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 struct {
        struct list_head        list;
        spinlock_t              lock;
} unused_peers = {
        .list                   = LIST_HEAD_INIT(unused_peers.list),
        .lock                   = __SPIN_LOCK_UNLOCKED(unused_peers.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)
{
        if (!list_empty(&p->unused)) {
                spin_lock_bh(&unused_peers.lock);
                list_del_init(&p->unused);
                spin_unlock_bh(&unused_peers.lock);
        }
}

static int addr_compare(const struct inetpeer_addr *a,
                        const struct inetpeer_addr *b)
{
        int i, n = (a->family == AF_INET ? 1 : 4);

        for (i = 0; i < n; i++) {
                if (a->a6[i] == b->a6[i])
                        continue;
                if (a->a6[i] < b->a6[i])
                        return -1;
                return 1;
        }

        return 0;
}

/*
 * Called with local BH disabled and the pool lock held.
 */
#define lookup(_daddr, _stack, _base)                           \
({                                                              \
        struct inet_peer *u;                                    \
        struct inet_peer __rcu **v;                             \
                                                                \
        stackptr = _stack;                                      \
        *stackptr++ = &_base->root;                             \
        for (u = rcu_dereference_protected(_base->root,         \
                        lockdep_is_held(&_base->lock));         \
             u != peer_avl_empty; ) {                           \
                int cmp = addr_compare(_daddr, &u->daddr);      \
                if (cmp == 0)                                   \
                        break;                                  \
                if (cmp == -1)                                  \
                        v = &u->avl_left;                       \
                else                                            \
                        v = &u->avl_right;                      \
                *stackptr++ = v;                                \
                u = rcu_dereference_protected(*v,               \
                        lockdep_is_held(&_base->lock));         \
        }                                                       \
        u;                                                      \
})

/*
 * Called with rcu_read_lock_bh()
 * Because we hold no lock against a writer, its quite possible we fall
 * in an endless loop.
 * But every pointer we follow is guaranteed to be valid thanks to RCU.
 * We exit from this function if number of links exceeds PEER_MAXDEPTH
 */
static struct inet_peer *lookup_rcu_bh(const struct inetpeer_addr *daddr,
                                       struct inet_peer_base *base)
{
        struct inet_peer *u = rcu_dereference_bh(base->root);
        int count = 0;

        while (u != peer_avl_empty) {
                int cmp = addr_compare(daddr, &u->daddr);
                if (cmp == 0) {
                        /* Before taking a reference, check if this entry was
                         * deleted, unlink_from_pool() sets refcnt=-1 to make
                         * distinction between an unused entry (refcnt=0) and
                         * a freed one.
                         */
                        if (unlikely(!atomic_add_unless(&u->refcnt, 1, -1)))
                                u = NULL;
                        return u;
                }
                if (cmp == -1)
                        u = rcu_dereference_bh(u->avl_left);
                else
                        u = rcu_dereference_bh(u->avl_right);
                if (unlikely(++count == PEER_MAXDEPTH))
                        break;
        }
        return NULL;
}

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

/* Called with local BH disabled and the pool 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 __rcu **stack[],
                               struct inet_peer __rcu ***stackend,
                               struct inet_peer_base *base)
{
        struct inet_peer __rcu **nodep;
        struct inet_peer *node, *l, *r;
        int lh, rh;

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

/* Called with local BH disabled and the pool lock held. */
#define link_to_pool(n, base)                                   \
do {                                                            \
        n->avl_height = 1;                                      \
        n->avl_left = peer_avl_empty_rcu;                       \
        n->avl_right = peer_avl_empty_rcu;                      \
        /* lockless readers can catch us now */                 \
        rcu_assign_pointer(**--stackptr, n);                    \
        peer_avl_rebalance(stack, stackptr, base);              \
} while (0)

static void inetpeer_free_rcu(struct rcu_head *head)
{
        kmem_cache_free(peer_cachep, container_of(head, struct inet_peer, rcu));
}

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

        do_free = 0;

        spin_lock_bh(&base->lock);
        /* Check the reference counter.  It was artificially incremented by 1
         * in cleanup() function to prevent sudden disappearing.  If we can
         * atomically (because of lockless readers) take this last reference,
         * it's safe to remove the node and free it later.
         * We use refcnt=-1 to alert lockless readers this entry is deleted.
         */
        if (atomic_cmpxchg(&p->refcnt, 1, -1) == 1) {
                struct inet_peer __rcu **stack[PEER_MAXDEPTH];
                struct inet_peer __rcu ***stackptr, ***delp;
                if (lookup(&p->daddr, stack, base) != p)
                        BUG();
                delp = stackptr - 1; /* *delp[0] == p */
                if (p->avl_left == peer_avl_empty_rcu) {
                        *delp[0] = p->avl_right;
                        --stackptr;
                } else {
                        /* look for a node to insert instead of p */
                        struct inet_peer *t;
                        t = lookup_rightempty(p, base);
                        BUG_ON(rcu_dereference_protected(*stackptr[-1],
                                        lockdep_is_held(&base->lock)) != t);
                        **--stackptr = t->avl_left;
                        /* t is removed, t->daddr > x->daddr for any
                         * x in p->avl_left subtree.
                         * Put t in the old place of p. */
                        RCU_INIT_POINTER(*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, base);
                base->total--;
                do_free = 1;
        }
        spin_unlock_bh(&base->lock);

        if (do_free)
                call_rcu_bh(&p->rcu, inetpeer_free_rcu);
        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);
}

static struct inet_peer_base *family_to_base(int family)
{
        return (family == AF_INET ? &v4_peers : &v6_peers);
}

static struct inet_peer_base *peer_to_base(struct inet_peer *p)
{
        return family_to_base(p->daddr.family);
}

/* 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(&unused_peers.lock);
        if (!list_empty(&unused_peers.list)) {
                __u32 delta;

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

                if (delta < ttl) {
                        /* Do not prune fresh entries. */
                        spin_unlock_bh(&unused_peers.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(&unused_peers.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, peer_to_base(p));
        return 0;
}

/* Called with or without local BH being disabled. */
struct inet_peer *inet_getpeer(struct inetpeer_addr *daddr, int create)
{
        struct inet_peer __rcu **stack[PEER_MAXDEPTH], ***stackptr;
        struct inet_peer_base *base = family_to_base(daddr->family);
        struct inet_peer *p;

        /* Look up for the address quickly, lockless.
         * Because of a concurrent writer, we might not find an existing entry.
         */
        rcu_read_lock_bh();
        p = lookup_rcu_bh(daddr, base);
        rcu_read_unlock_bh();

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

        /* retry an exact lookup, taking the lock before.
         * At least, nodes should be hot in our cache.
         */
        spin_lock_bh(&base->lock);
        p = lookup(daddr, stack, base);
        if (p != peer_avl_empty) {
                atomic_inc(&p->refcnt);
                spin_unlock_bh(&base->lock);
                /* Remove the entry from unused list if it was there. */
                unlink_from_unused(p);
                return p;
        }
        p = create ? kmem_cache_alloc(peer_cachep, GFP_ATOMIC) : NULL;
        if (p) {
                p->daddr = *daddr;
                atomic_set(&p->refcnt, 1);
                atomic_set(&p->rid, 0);
                atomic_set(&p->ip_id_count, secure_ip_id(daddr->a4));
                p->tcp_ts_stamp = 0;
                INIT_LIST_HEAD(&p->unused);


                /* Link the node. */
                link_to_pool(p, base);
                base->total++;
        }
        spin_unlock_bh(&base->lock);

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

        return p;
}

static int compute_total(void)
{
        return v4_peers.total + v6_peers.total;
}
EXPORT_SYMBOL_GPL(inet_getpeer);

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

        total = compute_total();
        if (total >= inet_peer_threshold)
                ttl = inet_peer_minttl;
        else
                ttl = inet_peer_maxttl
                                - (inet_peer_maxttl - inet_peer_minttl) / HZ *
                                        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). */
        total = compute_total();
        if (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 *
                                total / inet_peer_threshold * HZ;
        add_timer(&peer_periodic_timer);
}

void inet_putpeer(struct inet_peer *p)
{
        local_bh_disable();

        if (atomic_dec_and_lock(&p->refcnt, &unused_peers.lock)) {
                list_add_tail(&p->unused, &unused_peers.list);
                p->dtime = (__u32)jiffies;
                spin_unlock(&unused_peers.lock);
        }

        local_bh_enable();
}
EXPORT_SYMBOL_GPL(inet_putpeer);