ackvec.c

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/*
 *  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 <[email protected]>
 *
 *      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);
    }
}

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);
}

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);
}

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(lost_packets, (u32)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\n");
        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);
}

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);
        }
    }
}

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)
{
    if (dccp_ackvec_slab != NULL) {
        kmem_cache_destroy(dccp_ackvec_slab);
        dccp_ackvec_slab = NULL;
    }
    if (dccp_ackvec_record_slab != NULL) {
        kmem_cache_destroy(dccp_ackvec_record_slab);
        dccp_ackvec_record_slab = NULL;
    }
}