/*
 *  net/dccp/ackvec.c
 *
 *  An implementation of Ack Vectors for the DCCP protocol
 *  Copyright (c) 2007 University of Aberdeen, Scotland, UK
 *  Copyright (c) 2005 Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
 *
 *      This program is free software; you can redistribute it and/or modify it
 *      under the terms of the GNU General Public License as published by the
 *      Free Software Foundation; version 2 of the License;
 */
#include "dccp.h"
#include <linux/kernel.h>
#include <linux/slab.h>
#include <linux/export.h>

static struct kmem_cache *dccp_ackvec_slab;
static struct kmem_cache *dccp_ackvec_record_slab;

struct dccp_ackvec *dccp_ackvec_alloc(const gfp_t priority)
{
        struct dccp_ackvec *av = kmem_cache_zalloc(dccp_ackvec_slab, priority);

        if (av != NULL) {
                av->av_buf_head = av->av_buf_tail = DCCPAV_MAX_ACKVEC_LEN - 1;
                INIT_LIST_HEAD(&av->av_records);
        }
        return av;
}

static void dccp_ackvec_purge_records(struct dccp_ackvec *av)
{
        struct dccp_ackvec_record *cur, *next;

        list_for_each_entry_safe(cur, next, &av->av_records, avr_node)
                kmem_cache_free(dccp_ackvec_record_slab, cur);
        INIT_LIST_HEAD(&av->av_records);
}

void dccp_ackvec_free(struct dccp_ackvec *av)
{
        if (likely(av != NULL)) {
                dccp_ackvec_purge_records(av);
                kmem_cache_free(dccp_ackvec_slab, av);
        }
}

/**
 * dccp_ackvec_update_records  -  Record information about sent Ack Vectors
 * @av:         Ack Vector records to update
 * @seqno:      Sequence number of the packet carrying the Ack Vector just sent
 * @nonce_sum:  The sum of all buffer nonces contained in the Ack Vector
 */
int dccp_ackvec_update_records(struct dccp_ackvec *av, u64 seqno, u8 nonce_sum)
{
        struct dccp_ackvec_record *avr;

        avr = kmem_cache_alloc(dccp_ackvec_record_slab, GFP_ATOMIC);
        if (avr == NULL)
                return -ENOBUFS;

        avr->avr_ack_seqno  = seqno;
        avr->avr_ack_ptr    = av->av_buf_head;
        avr->avr_ack_ackno  = av->av_buf_ackno;
        avr->avr_ack_nonce  = nonce_sum;
        avr->avr_ack_runlen = dccp_ackvec_runlen(av->av_buf + av->av_buf_head);
        /*
         * When the buffer overflows, we keep no more than one record. This is
         * the simplest way of disambiguating sender-Acks dating from before the
         * overflow from sender-Acks which refer to after the overflow; a simple
         * solution is preferable here since we are handling an exception.
         */
        if (av->av_overflow)
                dccp_ackvec_purge_records(av);
        /*
         * Since GSS is incremented for each packet, the list is automatically
         * arranged in descending order of @ack_seqno.
         */
        list_add(&avr->avr_node, &av->av_records);

        dccp_pr_debug("Added Vector, ack_seqno=%llu, ack_ackno=%llu (rl=%u)\n",
                      (unsigned long long)avr->avr_ack_seqno,
                      (unsigned long long)avr->avr_ack_ackno,
                      avr->avr_ack_runlen);
        return 0;
}

static struct dccp_ackvec_record *dccp_ackvec_lookup(struct list_head *av_list,
                                                     const u64 ackno)
{
        struct dccp_ackvec_record *avr;
        /*
         * Exploit that records are inserted in descending order of sequence
         * number, start with the oldest record first. If @ackno is `before'
         * the earliest ack_ackno, the packet is too old to be considered.
         */
        list_for_each_entry_reverse(avr, av_list, avr_node) {
                if (avr->avr_ack_seqno == ackno)
                        return avr;
                if (before48(ackno, avr->avr_ack_seqno))
                        break;
        }
        return NULL;
}

/*
 * Buffer index and length computation using modulo-buffersize arithmetic.
 * Note that, as pointers move from right to left, head is `before' tail.
 */
static inline u16 __ackvec_idx_add(const u16 a, const u16 b)
{
        return (a + b) % DCCPAV_MAX_ACKVEC_LEN;
}

static inline u16 __ackvec_idx_sub(const u16 a, const u16 b)
{
        return __ackvec_idx_add(a, DCCPAV_MAX_ACKVEC_LEN - b);
}

u16 dccp_ackvec_buflen(const struct dccp_ackvec *av)
{
        if (unlikely(av->av_overflow))
                return DCCPAV_MAX_ACKVEC_LEN;
        return __ackvec_idx_sub(av->av_buf_tail, av->av_buf_head);
}

/**
 * dccp_ackvec_update_old  -  Update previous state as per RFC 4340, 11.4.1
 * @av:         non-empty buffer to update
 * @distance:   negative or zero distance of @seqno from buf_ackno downward
 * @seqno:      the (old) sequence number whose record is to be updated
 * @state:      state in which packet carrying @seqno was received
 */
static void dccp_ackvec_update_old(struct dccp_ackvec *av, s64 distance,
                                   u64 seqno, enum dccp_ackvec_states state)
{
        u16 ptr = av->av_buf_head;

        BUG_ON(distance > 0);
        if (unlikely(dccp_ackvec_is_empty(av)))
                return;

        do {
                u8 runlen = dccp_ackvec_runlen(av->av_buf + ptr);

                if (distance + runlen >= 0) {
                        /*
                         * Only update the state if packet has not been received
                         * yet. This is OK as per the second table in RFC 4340,
                         * 11.4.1; i.e. here we are using the following table:
                         *                     RECEIVED
                         *                      0   1   3
                         *              S     +---+---+---+
                         *              T   0 | 0 | 0 | 0 |
                         *              O     +---+---+---+
                         *              R   1 | 1 | 1 | 1 |
                         *              E     +---+---+---+
                         *              D   3 | 0 | 1 | 3 |
                         *                    +---+---+---+
                         * The "Not Received" state was set by reserve_seats().
                         */
                        if (av->av_buf[ptr] == DCCPAV_NOT_RECEIVED)
                                av->av_buf[ptr] = state;
                        else
                                dccp_pr_debug("Not changing %llu state to %u\n",
                                              (unsigned long long)seqno, state);
                        break;
                }

                distance += runlen + 1;
                ptr       = __ackvec_idx_add(ptr, 1);

        } while (ptr != av->av_buf_tail);
}

/* Mark @num entries after buf_head as "Not yet received". */
static void dccp_ackvec_reserve_seats(struct dccp_ackvec *av, u16 num)
{
        u16 start = __ackvec_idx_add(av->av_buf_head, 1),
            len   = DCCPAV_MAX_ACKVEC_LEN - start;

        /* check for buffer wrap-around */
        if (num > len) {
                memset(av->av_buf + start, DCCPAV_NOT_RECEIVED, len);
                start = 0;
                num  -= len;
        }
        if (num)
                memset(av->av_buf + start, DCCPAV_NOT_RECEIVED, num);
}

/**
 * dccp_ackvec_add_new  -  Record one or more new entries in Ack Vector buffer
 * @av:          container of buffer to update (can be empty or non-empty)
 * @num_packets: number of packets to register (must be >= 1)
 * @seqno:       sequence number of the first packet in @num_packets
 * @state:       state in which packet carrying @seqno was received
 */
static void dccp_ackvec_add_new(struct dccp_ackvec *av, u32 num_packets,
                                u64 seqno, enum dccp_ackvec_states state)
{
        u32 num_cells = num_packets;

        if (num_packets > DCCPAV_BURST_THRESH) {
                u32 lost_packets = num_packets - 1;

                DCCP_WARN("Warning: large burst loss (%u)\n", lost_packets);
                /*
                 * We received 1 packet and have a loss of size "num_packets-1"
                 * which we squeeze into num_cells-1 rather than reserving an
                 * entire byte for each lost packet.
                 * The reason is that the vector grows in O(burst_length); when
                 * it grows too large there will no room left for the payload.
                 * This is a trade-off: if a few packets out of the burst show
                 * up later, their state will not be changed; it is simply too
                 * costly to reshuffle/reallocate/copy the buffer each time.
                 * Should such problems persist, we will need to switch to a
                 * different underlying data structure.
                 */
                for (num_packets = num_cells = 1; lost_packets; ++num_cells) {
                        u8 len = min_t(u32, lost_packets, DCCPAV_MAX_RUNLEN);

                        av->av_buf_head = __ackvec_idx_sub(av->av_buf_head, 1);
                        av->av_buf[av->av_buf_head] = DCCPAV_NOT_RECEIVED | len;

                        lost_packets -= len;
                }
        }

        if (num_cells + dccp_ackvec_buflen(av) >= DCCPAV_MAX_ACKVEC_LEN) {
                DCCP_CRIT("Ack Vector buffer overflow: dropping old entries");
                av->av_overflow = true;
        }

        av->av_buf_head = __ackvec_idx_sub(av->av_buf_head, num_packets);
        if (av->av_overflow)
                av->av_buf_tail = av->av_buf_head;

        av->av_buf[av->av_buf_head] = state;
        av->av_buf_ackno            = seqno;

        if (num_packets > 1)
                dccp_ackvec_reserve_seats(av, num_packets - 1);
}

/**
 * dccp_ackvec_input  -  Register incoming packet in the buffer
 */
void dccp_ackvec_input(struct dccp_ackvec *av, struct sk_buff *skb)
{
        u64 seqno = DCCP_SKB_CB(skb)->dccpd_seq;
        enum dccp_ackvec_states state = DCCPAV_RECEIVED;

        if (dccp_ackvec_is_empty(av)) {
                dccp_ackvec_add_new(av, 1, seqno, state);
                av->av_tail_ackno = seqno;

        } else {
                s64 num_packets = dccp_delta_seqno(av->av_buf_ackno, seqno);
                u8 *current_head = av->av_buf + av->av_buf_head;

                if (num_packets == 1 &&
                    dccp_ackvec_state(current_head) == state &&
                    dccp_ackvec_runlen(current_head) < DCCPAV_MAX_RUNLEN) {

                        *current_head   += 1;
                        av->av_buf_ackno = seqno;

                } else if (num_packets > 0) {
                        dccp_ackvec_add_new(av, num_packets, seqno, state);
                } else {
                        dccp_ackvec_update_old(av, num_packets, seqno, state);
                }
        }
}

/**
 * dccp_ackvec_clear_state  -  Perform house-keeping / garbage-collection
 * This routine is called when the peer acknowledges the receipt of Ack Vectors
 * up to and including @ackno. While based on on section A.3 of RFC 4340, here
 * are additional precautions to prevent corrupted buffer state. In particular,
 * we use tail_ackno to identify outdated records; it always marks the earliest
 * packet of group (2) in 11.4.2.
 */
void dccp_ackvec_clear_state(struct dccp_ackvec *av, const u64 ackno)
{
        struct dccp_ackvec_record *avr, *next;
        u8 runlen_now, eff_runlen;
        s64 delta;

        avr = dccp_ackvec_lookup(&av->av_records, ackno);
        if (avr == NULL)
                return;
        /*
         * Deal with outdated acknowledgments: this arises when e.g. there are
         * several old records and the acks from the peer come in slowly. In
         * that case we may still have records that pre-date tail_ackno.
         */
        delta = dccp_delta_seqno(av->av_tail_ackno, avr->avr_ack_ackno);
        if (delta < 0)
                goto free_records;
        /*
         * Deal with overlapping Ack Vectors: don't subtract more than the
         * number of packets between tail_ackno and ack_ackno.
         */
        eff_runlen = delta < avr->avr_ack_runlen ? delta : avr->avr_ack_runlen;

        runlen_now = dccp_ackvec_runlen(av->av_buf + avr->avr_ack_ptr);
        /*
         * The run length of Ack Vector cells does not decrease over time. If
         * the run length is the same as at the time the Ack Vector was sent, we
         * free the ack_ptr cell. That cell can however not be freed if the run
         * length has increased: in this case we need to move the tail pointer
         * backwards (towards higher indices), to its next-oldest neighbour.
         */
        if (runlen_now > eff_runlen) {

                av->av_buf[avr->avr_ack_ptr] -= eff_runlen + 1;
                av->av_buf_tail = __ackvec_idx_add(avr->avr_ack_ptr, 1);

                /* This move may not have cleared the overflow flag. */
                if (av->av_overflow)
                        av->av_overflow = (av->av_buf_head == av->av_buf_tail);
        } else {
                av->av_buf_tail = avr->avr_ack_ptr;
                /*
                 * We have made sure that avr points to a valid cell within the
                 * buffer. This cell is either older than head, or equals head
                 * (empty buffer): in both cases we no longer have any overflow.
                 */
                av->av_overflow = 0;
        }

        /*
         * The peer has acknowledged up to and including ack_ackno. Hence the
         * first packet in group (2) of 11.4.2 is the successor of ack_ackno.
         */
        av->av_tail_ackno = ADD48(avr->avr_ack_ackno, 1);

free_records:
        list_for_each_entry_safe_from(avr, next, &av->av_records, avr_node) {
                list_del(&avr->avr_node);
                kmem_cache_free(dccp_ackvec_record_slab, avr);
        }
}

/*
 *      Routines to keep track of Ack Vectors received in an skb
 */
int dccp_ackvec_parsed_add(struct list_head *head, u8 *vec, u8 len, u8 nonce)
{
        struct dccp_ackvec_parsed *new = kmalloc(sizeof(*new), GFP_ATOMIC);

        if (new == NULL)
                return -ENOBUFS;
        new->vec   = vec;
        new->len   = len;
        new->nonce = nonce;

        list_add_tail(&new->node, head);
        return 0;
}
EXPORT_SYMBOL_GPL(dccp_ackvec_parsed_add);

void dccp_ackvec_parsed_cleanup(struct list_head *parsed_chunks)
{
        struct dccp_ackvec_parsed *cur, *next;

        list_for_each_entry_safe(cur, next, parsed_chunks, node)
                kfree(cur);
        INIT_LIST_HEAD(parsed_chunks);
}
EXPORT_SYMBOL_GPL(dccp_ackvec_parsed_cleanup);

int __init dccp_ackvec_init(void)
{
        dccp_ackvec_slab = kmem_cache_create("dccp_ackvec",
                                             sizeof(struct dccp_ackvec), 0,
                                             SLAB_HWCACHE_ALIGN, NULL);
        if (dccp_ackvec_slab == NULL)
                goto out_err;

        dccp_ackvec_record_slab = kmem_cache_create("dccp_ackvec_record",
                                             sizeof(struct dccp_ackvec_record),
                                             0, SLAB_HWCACHE_ALIGN, NULL);
        if (dccp_ackvec_record_slab == NULL)
                goto out_destroy_slab;

        return 0;

out_destroy_slab:
        kmem_cache_destroy(dccp_ackvec_slab);
        dccp_ackvec_slab = NULL;
out_err:
        DCCP_CRIT("Unable to create Ack Vector slab cache");
        return -ENOBUFS;
}

void dccp_ackvec_exit(void)
{
        kmem_cache_destroy(dccp_ackvec_slab);
        dccp_ackvec_slab = NULL;
        kmem_cache_destroy(dccp_ackvec_record_slab);
        dccp_ackvec_record_slab = NULL;
}