/*
 *              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/cache.h>
#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 <linux/workqueue.h>
#include <net/ip.h>
#include <net/inetpeer.h>
#include <net/secure_seq.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.
 *
 *  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 RB 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.  Global variable peer_total is modified under the pool lock.
 *  4.  struct inet_peer fields modification:
 *              rb_node: pool lock
 *              refcnt: atomically against modifications on other CPU;
 *                 usually under some other lock to prevent node disappearing
 *              daddr: unchangeable
 */

static struct kmem_cache *peer_cachep __ro_after_init;

void inet_peer_base_init(struct inet_peer_base *bp)
{
        bp->rb_root = RB_ROOT;
        seqlock_init(&bp->lock);
        bp->total = 0;
}
EXPORT_SYMBOL_GPL(inet_peer_base_init);

#define PEER_MAX_GC 32

/* Exported for sysctl_net_ipv4.  */
int inet_peer_threshold __read_mostly;  /* 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 */

/* Called from ip_output.c:ip_init  */
void __init inet_initpeers(void)
{
        u64 nr_entries;

         /* 1% of physical memory */
        nr_entries = div64_ul((u64)totalram_pages() << PAGE_SHIFT,
                              100 * L1_CACHE_ALIGN(sizeof(struct inet_peer)));

        inet_peer_threshold = clamp_val(nr_entries, 4096, 65536 + 128);

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

/* Called with rcu_read_lock() or base->lock held */
static struct inet_peer *lookup(const struct inetpeer_addr *daddr,
                                struct inet_peer_base *base,
                                unsigned int seq,
                                struct inet_peer *gc_stack[],
                                unsigned int *gc_cnt,
                                struct rb_node **parent_p,
                                struct rb_node ***pp_p)
{
        struct rb_node **pp, *parent, *next;
        struct inet_peer *p;

        pp = &base->rb_root.rb_node;
        parent = NULL;
        while (1) {
                int cmp;

                next = rcu_dereference_raw(*pp);
                if (!next)
                        break;
                parent = next;
                p = rb_entry(parent, struct inet_peer, rb_node);
                cmp = inetpeer_addr_cmp(daddr, &p->daddr);
                if (cmp == 0) {
                        if (!refcount_inc_not_zero(&p->refcnt))
                                break;
                        return p;
                }
                if (gc_stack) {
                        if (*gc_cnt < PEER_MAX_GC)
                                gc_stack[(*gc_cnt)++] = p;
                } else if (unlikely(read_seqretry(&base->lock, seq))) {
                        break;
                }
                if (cmp == -1)
                        pp = &next->rb_left;
                else
                        pp = &next->rb_right;
        }
        *parent_p = parent;
        *pp_p = pp;
        return NULL;
}

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

/* perform garbage collect on all items stacked during a lookup */
static void inet_peer_gc(struct inet_peer_base *base,
                         struct inet_peer *gc_stack[],
                         unsigned int gc_cnt)
{
        struct inet_peer *p;
        __u32 delta, ttl;
        int i;

        if (base->total >= inet_peer_threshold)
                ttl = 0; /* be aggressive */
        else
                ttl = inet_peer_maxttl
                                - (inet_peer_maxttl - inet_peer_minttl) / HZ *
                                        base->total / inet_peer_threshold * HZ;
        for (i = 0; i < gc_cnt; i++) {
                p = gc_stack[i];

                /* The READ_ONCE() pairs with the WRITE_ONCE()
                 * in inet_putpeer()
                 */
                delta = (__u32)jiffies - READ_ONCE(p->dtime);

                if (delta < ttl || !refcount_dec_if_one(&p->refcnt))
                        gc_stack[i] = NULL;
        }
        for (i = 0; i < gc_cnt; i++) {
                p = gc_stack[i];
                if (p) {
                        rb_erase(&p->rb_node, &base->rb_root);
                        base->total--;
                        call_rcu(&p->rcu, inetpeer_free_rcu);
                }
        }
}

struct inet_peer *inet_getpeer(struct inet_peer_base *base,
                               const struct inetpeer_addr *daddr,
                               int create)
{
        struct inet_peer *p, *gc_stack[PEER_MAX_GC];
        struct rb_node **pp, *parent;
        unsigned int gc_cnt, seq;
        int invalidated;

        /* Attempt a lockless lookup first.
         * Because of a concurrent writer, we might not find an existing entry.
         */
        rcu_read_lock();
        seq = read_seqbegin(&base->lock);
        p = lookup(daddr, base, seq, NULL, &gc_cnt, &parent, &pp);
        invalidated = read_seqretry(&base->lock, seq);
        rcu_read_unlock();

        if (p)
                return p;

        /* If no writer did a change during our lookup, we can return early. */
        if (!create && !invalidated)
                return NULL;

        /* retry an exact lookup, taking the lock before.
         * At least, nodes should be hot in our cache.
         */
        parent = NULL;
        write_seqlock_bh(&base->lock);

        gc_cnt = 0;
        p = lookup(daddr, base, seq, gc_stack, &gc_cnt, &parent, &pp);
        if (!p && create) {
                p = kmem_cache_alloc(peer_cachep, GFP_ATOMIC);
                if (p) {
                        p->daddr = *daddr;
                        p->dtime = (__u32)jiffies;
                        refcount_set(&p->refcnt, 2);
                        atomic_set(&p->rid, 0);
                        p->metrics[RTAX_LOCK-1] = INETPEER_METRICS_NEW;
                        p->rate_tokens = 0;
                        p->n_redirects = 0;
                        /* 60*HZ is arbitrary, but chosen enough high so that the first
                         * calculation of tokens is at its maximum.
                         */
                        p->rate_last = jiffies - 60*HZ;

                        rb_link_node(&p->rb_node, parent, pp);
                        rb_insert_color(&p->rb_node, &base->rb_root);
                        base->total++;
                }
        }
        if (gc_cnt)
                inet_peer_gc(base, gc_stack, gc_cnt);
        write_sequnlock_bh(&base->lock);

        return p;
}
EXPORT_SYMBOL_GPL(inet_getpeer);

void inet_putpeer(struct inet_peer *p)
{
        /* The WRITE_ONCE() pairs with itself (we run lockless)
         * and the READ_ONCE() in inet_peer_gc()
         */
        WRITE_ONCE(p->dtime, (__u32)jiffies);

        if (refcount_dec_and_test(&p->refcnt))
                call_rcu(&p->rcu, inetpeer_free_rcu);
}
EXPORT_SYMBOL_GPL(inet_putpeer);

/*
 *      Check transmit rate limitation for given message.
 *      The rate information is held in the inet_peer entries now.
 *      This function is generic and could be used for other purposes
 *      too. It uses a Token bucket filter as suggested by Alexey Kuznetsov.
 *
 *      Note that the same inet_peer fields are modified by functions in
 *      route.c too, but these work for packet destinations while xrlim_allow
 *      works for icmp destinations. This means the rate limiting information
 *      for one "ip object" is shared - and these ICMPs are twice limited:
 *      by source and by destination.
 *
 *      RFC 1812: 4.3.2.8 SHOULD be able to limit error message rate
 *                        SHOULD allow setting of rate limits
 *
 *      Shared between ICMPv4 and ICMPv6.
 */
#define XRLIM_BURST_FACTOR 6
bool inet_peer_xrlim_allow(struct inet_peer *peer, int timeout)
{
        unsigned long now, token;
        bool rc = false;

        if (!peer)
                return true;

        token = peer->rate_tokens;
        now = jiffies;
        token += now - peer->rate_last;
        peer->rate_last = now;
        if (token > XRLIM_BURST_FACTOR * timeout)
                token = XRLIM_BURST_FACTOR * timeout;
        if (token >= timeout) {
                token -= timeout;
                rc = true;
        }
        peer->rate_tokens = token;
        return rc;
}
EXPORT_SYMBOL(inet_peer_xrlim_allow);

void inetpeer_invalidate_tree(struct inet_peer_base *base)
{
        struct rb_node *p = rb_first(&base->rb_root);

        while (p) {
                struct inet_peer *peer = rb_entry(p, struct inet_peer, rb_node);

                p = rb_next(p);
                rb_erase(&peer->rb_node, &base->rb_root);
                inet_putpeer(peer);
                cond_resched();
        }

        base->total = 0;
}
EXPORT_SYMBOL(inetpeer_invalidate_tree);