feat.c

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/*
 *  net/dccp/feat.c
 *
 *  Feature negotiation for the DCCP protocol (RFC 4340, section 6)
 *
 *  Copyright (c) 2008 Gerrit Renker <[email protected]>
 *  Rewrote from scratch, some bits from earlier code by
 *  Copyright (c) 2005 Andrea Bittau <[email protected]>
 *
 *
 *  ASSUMPTIONS
 *  -----------
 *  o Feature negotiation is coordinated with connection setup (as in TCP), wild
 *    changes of parameters of an established connection are not supported.
 *  o Changing non-negotiable (NN) values is supported in state OPEN/PARTOPEN.
 *  o All currently known SP features have 1-byte quantities. If in the future
 *    extensions of RFCs 4340..42 define features with item lengths larger than
 *    one byte, a feature-specific extension of the code will be required.
 *
 *  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; either version
 *  2 of the License, or (at your option) any later version.
 */
#include <linux/module.h>
#include <linux/slab.h>
#include "ccid.h"
#include "feat.h"

/* feature-specific sysctls - initialised to the defaults from RFC 4340, 6.4 */
unsigned long   sysctl_dccp_sequence_window __read_mostly = 100;
int     sysctl_dccp_rx_ccid     __read_mostly = 2,
        sysctl_dccp_tx_ccid     __read_mostly = 2;

/*
 * Feature activation handlers.
 *
 * These all use an u64 argument, to provide enough room for NN/SP features. At
 * this stage the negotiated values have been checked to be within their range.
 */
static int dccp_hdlr_ccid(struct sock *sk, u64 ccid, bool rx)
{
    struct dccp_sock *dp = dccp_sk(sk);
    struct ccid *new_ccid = ccid_new(ccid, sk, rx);

    if (new_ccid == NULL)
        return -ENOMEM;

    if (rx) {
        ccid_hc_rx_delete(dp->dccps_hc_rx_ccid, sk);
        dp->dccps_hc_rx_ccid = new_ccid;
    } else {
        ccid_hc_tx_delete(dp->dccps_hc_tx_ccid, sk);
        dp->dccps_hc_tx_ccid = new_ccid;
    }
    return 0;
}

static int dccp_hdlr_seq_win(struct sock *sk, u64 seq_win, bool rx)
{
    struct dccp_sock *dp = dccp_sk(sk);

    if (rx) {
        dp->dccps_r_seq_win = seq_win;
        /* propagate changes to update SWL/SWH */
        dccp_update_gsr(sk, dp->dccps_gsr);
    } else {
        dp->dccps_l_seq_win = seq_win;
        /* propagate changes to update AWL */
        dccp_update_gss(sk, dp->dccps_gss);
    }
    return 0;
}

static int dccp_hdlr_ack_ratio(struct sock *sk, u64 ratio, bool rx)
{
    if (rx)
        dccp_sk(sk)->dccps_r_ack_ratio = ratio;
    else
        dccp_sk(sk)->dccps_l_ack_ratio = ratio;
    return 0;
}

static int dccp_hdlr_ackvec(struct sock *sk, u64 enable, bool rx)
{
    struct dccp_sock *dp = dccp_sk(sk);

    if (rx) {
        if (enable && dp->dccps_hc_rx_ackvec == NULL) {
            dp->dccps_hc_rx_ackvec = dccp_ackvec_alloc(gfp_any());
            if (dp->dccps_hc_rx_ackvec == NULL)
                return -ENOMEM;
        } else if (!enable) {
            dccp_ackvec_free(dp->dccps_hc_rx_ackvec);
            dp->dccps_hc_rx_ackvec = NULL;
        }
    }
    return 0;
}

static int dccp_hdlr_ndp(struct sock *sk, u64 enable, bool rx)
{
    if (!rx)
        dccp_sk(sk)->dccps_send_ndp_count = (enable > 0);
    return 0;
}

/*
 * Minimum Checksum Coverage is located at the RX side (9.2.1). This means that
 * `rx' holds when the sending peer informs about his partial coverage via a
 * ChangeR() option. In the other case, we are the sender and the receiver
 * announces its coverage via ChangeL() options. The policy here is to honour
 * such communication by enabling the corresponding partial coverage - but only
 * if it has not been set manually before; the warning here means that all
 * packets will be dropped.
 */
static int dccp_hdlr_min_cscov(struct sock *sk, u64 cscov, bool rx)
{
    struct dccp_sock *dp = dccp_sk(sk);

    if (rx)
        dp->dccps_pcrlen = cscov;
    else {
        if (dp->dccps_pcslen == 0)
            dp->dccps_pcslen = cscov;
        else if (cscov > dp->dccps_pcslen)
            DCCP_WARN("CsCov %u too small, peer requires >= %u\n",
                  dp->dccps_pcslen, (u8)cscov);
    }
    return 0;
}

static const struct {
    u8          feat_num;       /* DCCPF_xxx */
    enum dccp_feat_type rxtx;           /* RX or TX  */
    enum dccp_feat_type reconciliation;     /* SP or NN  */
    u8          default_value;      /* as in 6.4 */
    int (*activation_hdlr)(struct sock *sk, u64 val, bool rx);
/*
 *    Lookup table for location and type of features (from RFC 4340/4342)
 *  +--------------------------+----+-----+----+----+---------+-----------+
 *  | Feature                  | Location | Reconc. | Initial |  Section  |
 *  |                          | RX | TX  | SP | NN |  Value  | Reference |
 *  +--------------------------+----+-----+----+----+---------+-----------+
 *  | DCCPF_CCID               |    |  X  | X  |    |   2     | 10        |
 *  | DCCPF_SHORT_SEQNOS       |    |  X  | X  |    |   0     |  7.6.1    |
 *  | DCCPF_SEQUENCE_WINDOW    |    |  X  |    | X  | 100     |  7.5.2    |
 *  | DCCPF_ECN_INCAPABLE      | X  |     | X  |    |   0     | 12.1      |
 *  | DCCPF_ACK_RATIO          |    |  X  |    | X  |   2     | 11.3      |
 *  | DCCPF_SEND_ACK_VECTOR    | X  |     | X  |    |   0     | 11.5      |
 *  | DCCPF_SEND_NDP_COUNT     |    |  X  | X  |    |   0     |  7.7.2    |
 *  | DCCPF_MIN_CSUM_COVER     | X  |     | X  |    |   0     |  9.2.1    |
 *  | DCCPF_DATA_CHECKSUM      | X  |     | X  |    |   0     |  9.3.1    |
 *  | DCCPF_SEND_LEV_RATE      | X  |     | X  |    |   0     | 4342/8.4  |
 *  +--------------------------+----+-----+----+----+---------+-----------+
 */
} dccp_feat_table[] = {
    { DCCPF_CCID,        FEAT_AT_TX, FEAT_SP, 2,   dccp_hdlr_ccid     },
    { DCCPF_SHORT_SEQNOS,    FEAT_AT_TX, FEAT_SP, 0,   NULL },
    { DCCPF_SEQUENCE_WINDOW, FEAT_AT_TX, FEAT_NN, 100, dccp_hdlr_seq_win  },
    { DCCPF_ECN_INCAPABLE,   FEAT_AT_RX, FEAT_SP, 0,   NULL },
    { DCCPF_ACK_RATIO,   FEAT_AT_TX, FEAT_NN, 2,   dccp_hdlr_ack_ratio},
    { DCCPF_SEND_ACK_VECTOR, FEAT_AT_RX, FEAT_SP, 0,   dccp_hdlr_ackvec   },
    { DCCPF_SEND_NDP_COUNT,  FEAT_AT_TX, FEAT_SP, 0,   dccp_hdlr_ndp      },
    { DCCPF_MIN_CSUM_COVER,  FEAT_AT_RX, FEAT_SP, 0,   dccp_hdlr_min_cscov},
    { DCCPF_DATA_CHECKSUM,   FEAT_AT_RX, FEAT_SP, 0,   NULL },
    { DCCPF_SEND_LEV_RATE,   FEAT_AT_RX, FEAT_SP, 0,   NULL },
};
#define DCCP_FEAT_SUPPORTED_MAX     ARRAY_SIZE(dccp_feat_table)

static int dccp_feat_index(u8 feat_num)
{
    /* The first 9 entries are occupied by the types from RFC 4340, 6.4 */
    if (feat_num > DCCPF_RESERVED && feat_num <= DCCPF_DATA_CHECKSUM)
        return feat_num - 1;

    /*
     * Other features: add cases for new feature types here after adding
     * them to the above table.
     */
    switch (feat_num) {
    case DCCPF_SEND_LEV_RATE:
            return DCCP_FEAT_SUPPORTED_MAX - 1;
    }
    return -1;
}

static u8 dccp_feat_type(u8 feat_num)
{
    int idx = dccp_feat_index(feat_num);

    if (idx < 0)
        return FEAT_UNKNOWN;
    return dccp_feat_table[idx].reconciliation;
}

static int dccp_feat_default_value(u8 feat_num)
{
    int idx = dccp_feat_index(feat_num);
    /*
     * There are no default values for unknown features, so encountering a
     * negative index here indicates a serious problem somewhere else.
     */
    DCCP_BUG_ON(idx < 0);

    return idx < 0 ? 0 : dccp_feat_table[idx].default_value;
}

/*
 *  Debugging and verbose-printing section
 */
static const char *dccp_feat_fname(const u8 feat)
{
    static const char *const feature_names[] = {
        [DCCPF_RESERVED]    = "Reserved",
        [DCCPF_CCID]        = "CCID",
        [DCCPF_SHORT_SEQNOS]    = "Allow Short Seqnos",
        [DCCPF_SEQUENCE_WINDOW] = "Sequence Window",
        [DCCPF_ECN_INCAPABLE]   = "ECN Incapable",
        [DCCPF_ACK_RATIO]   = "Ack Ratio",
        [DCCPF_SEND_ACK_VECTOR] = "Send ACK Vector",
        [DCCPF_SEND_NDP_COUNT]  = "Send NDP Count",
        [DCCPF_MIN_CSUM_COVER]  = "Min. Csum Coverage",
        [DCCPF_DATA_CHECKSUM]   = "Send Data Checksum",
    };
    if (feat > DCCPF_DATA_CHECKSUM && feat < DCCPF_MIN_CCID_SPECIFIC)
        return feature_names[DCCPF_RESERVED];

    if (feat ==  DCCPF_SEND_LEV_RATE)
        return "Send Loss Event Rate";
    if (feat >= DCCPF_MIN_CCID_SPECIFIC)
        return "CCID-specific";

    return feature_names[feat];
}

static const char *const dccp_feat_sname[] = {
    "DEFAULT", "INITIALISING", "CHANGING", "UNSTABLE", "STABLE",
};

#ifdef CONFIG_IP_DCCP_DEBUG
static const char *dccp_feat_oname(const u8 opt)
{
    switch (opt) {
    case DCCPO_CHANGE_L:  return "Change_L";
    case DCCPO_CONFIRM_L: return "Confirm_L";
    case DCCPO_CHANGE_R:  return "Change_R";
    case DCCPO_CONFIRM_R: return "Confirm_R";
    }
    return NULL;
}

static void dccp_feat_printval(u8 feat_num, dccp_feat_val const *val)
{
    u8 i, type = dccp_feat_type(feat_num);

    if (val == NULL || (type == FEAT_SP && val->sp.vec == NULL))
        dccp_pr_debug_cat("(NULL)");
    else if (type == FEAT_SP)
        for (i = 0; i < val->sp.len; i++)
            dccp_pr_debug_cat("%s%u", i ? " " : "", val->sp.vec[i]);
    else if (type == FEAT_NN)
        dccp_pr_debug_cat("%llu", (unsigned long long)val->nn);
    else
        dccp_pr_debug_cat("unknown type %u", type);
}

static void dccp_feat_printvals(u8 feat_num, u8 *list, u8 len)
{
    u8 type = dccp_feat_type(feat_num);
    dccp_feat_val fval = { .sp.vec = list, .sp.len = len };

    if (type == FEAT_NN)
        fval.nn = dccp_decode_value_var(list, len);
    dccp_feat_printval(feat_num, &fval);
}

static void dccp_feat_print_entry(struct dccp_feat_entry const *entry)
{
    dccp_debug("   * %s %s = ", entry->is_local ? "local" : "remote",
                    dccp_feat_fname(entry->feat_num));
    dccp_feat_printval(entry->feat_num, &entry->val);
    dccp_pr_debug_cat(", state=%s %s\n", dccp_feat_sname[entry->state],
              entry->needs_confirm ? "(Confirm pending)" : "");
}

#define dccp_feat_print_opt(opt, feat, val, len, mandatory) do {          \
    dccp_pr_debug("%s(%s, ", dccp_feat_oname(opt), dccp_feat_fname(feat));\
    dccp_feat_printvals(feat, val, len);                      \
    dccp_pr_debug_cat(") %s\n", mandatory ? "!" : "");  } while (0)

#define dccp_feat_print_fnlist(fn_list)  {      \
    const struct dccp_feat_entry *___entry;     \
                            \
    dccp_pr_debug("List Dump:\n");          \
    list_for_each_entry(___entry, fn_list, node)    \
        dccp_feat_print_entry(___entry);    \
}
#else   /* ! CONFIG_IP_DCCP_DEBUG */
#define dccp_feat_print_opt(opt, feat, val, len, mandatory)
#define dccp_feat_print_fnlist(fn_list)
#endif

static int __dccp_feat_activate(struct sock *sk, const int idx,
                const bool is_local, dccp_feat_val const *fval)
{
    bool rx;
    u64 val;

    if (idx < 0 || idx >= DCCP_FEAT_SUPPORTED_MAX)
        return -1;
    if (dccp_feat_table[idx].activation_hdlr == NULL)
        return 0;

    if (fval == NULL) {
        val = dccp_feat_table[idx].default_value;
    } else if (dccp_feat_table[idx].reconciliation == FEAT_SP) {
        if (fval->sp.vec == NULL) {
            /*
             * This can happen when an empty Confirm is sent
             * for an SP (i.e. known) feature. In this case
             * we would be using the default anyway.
             */
            DCCP_CRIT("Feature #%d undefined: using default", idx);
            val = dccp_feat_table[idx].default_value;
        } else {
            val = fval->sp.vec[0];
        }
    } else {
        val = fval->nn;
    }

    /* Location is RX if this is a local-RX or remote-TX feature */
    rx = (is_local == (dccp_feat_table[idx].rxtx == FEAT_AT_RX));

    dccp_debug("   -> activating %s %s, %sval=%llu\n", rx ? "RX" : "TX",
           dccp_feat_fname(dccp_feat_table[idx].feat_num),
           fval ? "" : "default ",  (unsigned long long)val);

    return dccp_feat_table[idx].activation_hdlr(sk, val, rx);
}

static int dccp_feat_activate(struct sock *sk, u8 feat_num, bool local,
                  dccp_feat_val const *fval)
{
    return __dccp_feat_activate(sk, dccp_feat_index(feat_num), local, fval);
}

/* Test for "Req'd" feature (RFC 4340, 6.4) */
static inline int dccp_feat_must_be_understood(u8 feat_num)
{
    return  feat_num == DCCPF_CCID || feat_num == DCCPF_SHORT_SEQNOS ||
        feat_num == DCCPF_SEQUENCE_WINDOW;
}

/* copy constructor, fval must not already contain allocated memory */
static int dccp_feat_clone_sp_val(dccp_feat_val *fval, u8 const *val, u8 len)
{
    fval->sp.len = len;
    if (fval->sp.len > 0) {
        fval->sp.vec = kmemdup(val, len, gfp_any());
        if (fval->sp.vec == NULL) {
            fval->sp.len = 0;
            return -ENOBUFS;
        }
    }
    return 0;
}

static void dccp_feat_val_destructor(u8 feat_num, dccp_feat_val *val)
{
    if (unlikely(val == NULL))
        return;
    if (dccp_feat_type(feat_num) == FEAT_SP)
        kfree(val->sp.vec);
    memset(val, 0, sizeof(*val));
}

static struct dccp_feat_entry *
          dccp_feat_clone_entry(struct dccp_feat_entry const *original)
{
    struct dccp_feat_entry *new;
    u8 type = dccp_feat_type(original->feat_num);

    if (type == FEAT_UNKNOWN)
        return NULL;

    new = kmemdup(original, sizeof(struct dccp_feat_entry), gfp_any());
    if (new == NULL)
        return NULL;

    if (type == FEAT_SP && dccp_feat_clone_sp_val(&new->val,
                              original->val.sp.vec,
                              original->val.sp.len)) {
        kfree(new);
        return NULL;
    }
    return new;
}

static void dccp_feat_entry_destructor(struct dccp_feat_entry *entry)
{
    if (entry != NULL) {
        dccp_feat_val_destructor(entry->feat_num, &entry->val);
        kfree(entry);
    }
}

/*
 * List management functions
 *
 * Feature negotiation lists rely on and maintain the following invariants:
 * - each feat_num in the list is known, i.e. we know its type and default value
 * - each feat_num/is_local combination is unique (old entries are overwritten)
 * - SP values are always freshly allocated
 * - list is sorted in increasing order of feature number (faster lookup)
 */
static struct dccp_feat_entry *dccp_feat_list_lookup(struct list_head *fn_list,
                             u8 feat_num, bool is_local)
{
    struct dccp_feat_entry *entry;

    list_for_each_entry(entry, fn_list, node) {
        if (entry->feat_num == feat_num && entry->is_local == is_local)
            return entry;
        else if (entry->feat_num > feat_num)
            break;
    }
    return NULL;
}

static struct dccp_feat_entry *
          dccp_feat_entry_new(struct list_head *head, u8 feat, bool local)
{
    struct dccp_feat_entry *entry;

    list_for_each_entry(entry, head, node)
        if (entry->feat_num == feat && entry->is_local == local) {
            dccp_feat_val_destructor(entry->feat_num, &entry->val);
            return entry;
        } else if (entry->feat_num > feat) {
            head = &entry->node;
            break;
        }

    entry = kmalloc(sizeof(*entry), gfp_any());
    if (entry != NULL) {
        entry->feat_num = feat;
        entry->is_local = local;
        list_add_tail(&entry->node, head);
    }
    return entry;
}

static int dccp_feat_push_change(struct list_head *fn_list, u8 feat, u8 local,
                 u8 mandatory, dccp_feat_val *fval)
{
    struct dccp_feat_entry *new = dccp_feat_entry_new(fn_list, feat, local);

    if (new == NULL)
        return -ENOMEM;

    new->feat_num        = feat;
    new->is_local        = local;
    new->state       = FEAT_INITIALISING;
    new->needs_confirm   = false;
    new->empty_confirm   = false;
    new->val         = *fval;
    new->needs_mandatory = mandatory;

    return 0;
}

static int dccp_feat_push_confirm(struct list_head *fn_list, u8 feat, u8 local,
                  dccp_feat_val *fval)
{
    struct dccp_feat_entry *new = dccp_feat_entry_new(fn_list, feat, local);

    if (new == NULL)
        return DCCP_RESET_CODE_TOO_BUSY;

    new->feat_num        = feat;
    new->is_local        = local;
    new->state       = FEAT_STABLE; /* transition in 6.6.2 */
    new->needs_confirm   = true;
    new->empty_confirm   = (fval == NULL);
    new->val.nn      = 0;       /* zeroes the whole structure */
    if (!new->empty_confirm)
        new->val     = *fval;
    new->needs_mandatory = false;

    return 0;
}

static int dccp_push_empty_confirm(struct list_head *fn_list, u8 feat, u8 local)
{
    return dccp_feat_push_confirm(fn_list, feat, local, NULL);
}

static inline void dccp_feat_list_pop(struct dccp_feat_entry *entry)
{
    list_del(&entry->node);
    dccp_feat_entry_destructor(entry);
}

void dccp_feat_list_purge(struct list_head *fn_list)
{
    struct dccp_feat_entry *entry, *next;

    list_for_each_entry_safe(entry, next, fn_list, node)
        dccp_feat_entry_destructor(entry);
    INIT_LIST_HEAD(fn_list);
}
EXPORT_SYMBOL_GPL(dccp_feat_list_purge);

/* generate @to as full clone of @from - @to must not contain any nodes */
int dccp_feat_clone_list(struct list_head const *from, struct list_head *to)
{
    struct dccp_feat_entry *entry, *new;

    INIT_LIST_HEAD(to);
    list_for_each_entry(entry, from, node) {
        new = dccp_feat_clone_entry(entry);
        if (new == NULL)
            goto cloning_failed;
        list_add_tail(&new->node, to);
    }
    return 0;

cloning_failed:
    dccp_feat_list_purge(to);
    return -ENOMEM;
}

static u8 dccp_feat_valid_nn_length(u8 feat_num)
{
    if (feat_num == DCCPF_ACK_RATIO)    /* RFC 4340, 11.3 and 6.6.8 */
        return 2;
    if (feat_num == DCCPF_SEQUENCE_WINDOW)  /* RFC 4340, 7.5.2 and 6.5  */
        return 6;
    return 0;
}

static u8 dccp_feat_is_valid_nn_val(u8 feat_num, u64 val)
{
    switch (feat_num) {
    case DCCPF_ACK_RATIO:
        return val <= DCCPF_ACK_RATIO_MAX;
    case DCCPF_SEQUENCE_WINDOW:
        return val >= DCCPF_SEQ_WMIN && val <= DCCPF_SEQ_WMAX;
    }
    return 0;   /* feature unknown - so we can't tell */
}

/* check that SP values are within the ranges defined in RFC 4340 */
static u8 dccp_feat_is_valid_sp_val(u8 feat_num, u8 val)
{
    switch (feat_num) {
    case DCCPF_CCID:
        return val == DCCPC_CCID2 || val == DCCPC_CCID3;
    /* Type-check Boolean feature values: */
    case DCCPF_SHORT_SEQNOS:
    case DCCPF_ECN_INCAPABLE:
    case DCCPF_SEND_ACK_VECTOR:
    case DCCPF_SEND_NDP_COUNT:
    case DCCPF_DATA_CHECKSUM:
    case DCCPF_SEND_LEV_RATE:
        return val < 2;
    case DCCPF_MIN_CSUM_COVER:
        return val < 16;
    }
    return 0;           /* feature unknown */
}

static u8 dccp_feat_sp_list_ok(u8 feat_num, u8 const *sp_list, u8 sp_len)
{
    if (sp_list == NULL || sp_len < 1)
        return 0;
    while (sp_len--)
        if (!dccp_feat_is_valid_sp_val(feat_num, *sp_list++))
            return 0;
    return 1;
}

int dccp_feat_insert_opts(struct dccp_sock *dp, struct dccp_request_sock *dreq,
              struct sk_buff *skb)
{
    struct list_head *fn = dreq ? &dreq->dreq_featneg : &dp->dccps_featneg;
    struct dccp_feat_entry *pos, *next;
    u8 opt, type, len, *ptr, nn_in_nbo[DCCP_OPTVAL_MAXLEN];
    bool rpt;

    /* put entries into @skb in the order they appear in the list */
    list_for_each_entry_safe_reverse(pos, next, fn, node) {
        opt  = dccp_feat_genopt(pos);
        type = dccp_feat_type(pos->feat_num);
        rpt  = false;

        if (pos->empty_confirm) {
            len = 0;
            ptr = NULL;
        } else {
            if (type == FEAT_SP) {
                len = pos->val.sp.len;
                ptr = pos->val.sp.vec;
                rpt = pos->needs_confirm;
            } else if (type == FEAT_NN) {
                len = dccp_feat_valid_nn_length(pos->feat_num);
                ptr = nn_in_nbo;
                dccp_encode_value_var(pos->val.nn, ptr, len);
            } else {
                DCCP_BUG("unknown feature %u", pos->feat_num);
                return -1;
            }
        }
        dccp_feat_print_opt(opt, pos->feat_num, ptr, len, 0);

        if (dccp_insert_fn_opt(skb, opt, pos->feat_num, ptr, len, rpt))
            return -1;
        if (pos->needs_mandatory && dccp_insert_option_mandatory(skb))
            return -1;

        if (skb->sk->sk_state == DCCP_OPEN &&
            (opt == DCCPO_CONFIRM_R || opt == DCCPO_CONFIRM_L)) {
            /*
             * Confirms don't get retransmitted (6.6.3) once the
             * connection is in state OPEN
             */
            dccp_feat_list_pop(pos);
        } else {
            /*
             * Enter CHANGING after transmitting the Change
             * option (6.6.2).
             */
            if (pos->state == FEAT_INITIALISING)
                pos->state = FEAT_CHANGING;
        }
    }
    return 0;
}

static int __feat_register_nn(struct list_head *fn, u8 feat,
                  u8 mandatory, u64 nn_val)
{
    dccp_feat_val fval = { .nn = nn_val };

    if (dccp_feat_type(feat) != FEAT_NN ||
        !dccp_feat_is_valid_nn_val(feat, nn_val))
        return -EINVAL;

    /* Don't bother with default values, they will be activated anyway. */
    if (nn_val - (u64)dccp_feat_default_value(feat) == 0)
        return 0;

    return dccp_feat_push_change(fn, feat, 1, mandatory, &fval);
}

static int __feat_register_sp(struct list_head *fn, u8 feat, u8 is_local,
                  u8 mandatory, u8 const *sp_val, u8 sp_len)
{
    dccp_feat_val fval;

    if (dccp_feat_type(feat) != FEAT_SP ||
        !dccp_feat_sp_list_ok(feat, sp_val, sp_len))
        return -EINVAL;

    /* Avoid negotiating alien CCIDs by only advertising supported ones */
    if (feat == DCCPF_CCID && !ccid_support_check(sp_val, sp_len))
        return -EOPNOTSUPP;

    if (dccp_feat_clone_sp_val(&fval, sp_val, sp_len))
        return -ENOMEM;

    return dccp_feat_push_change(fn, feat, is_local, mandatory, &fval);
}

int dccp_feat_register_sp(struct sock *sk, u8 feat, u8 is_local,
              u8 const *list, u8 len)
{    /* any changes must be registered before establishing the connection */
    if (sk->sk_state != DCCP_CLOSED)
        return -EISCONN;
    if (dccp_feat_type(feat) != FEAT_SP)
        return -EINVAL;
    return __feat_register_sp(&dccp_sk(sk)->dccps_featneg, feat, is_local,
                  0, list, len);
}

u64 dccp_feat_nn_get(struct sock *sk, u8 feat)
{
    if (dccp_feat_type(feat) == FEAT_NN) {
        struct dccp_sock *dp = dccp_sk(sk);
        struct dccp_feat_entry *entry;

        entry = dccp_feat_list_lookup(&dp->dccps_featneg, feat, 1);
        if (entry != NULL)
            return entry->val.nn;

        switch (feat) {
        case DCCPF_ACK_RATIO:
            return dp->dccps_l_ack_ratio;
        case DCCPF_SEQUENCE_WINDOW:
            return dp->dccps_l_seq_win;
        }
    }
    DCCP_BUG("attempt to look up unsupported feature %u", feat);
    return 0;
}
EXPORT_SYMBOL_GPL(dccp_feat_nn_get);

int dccp_feat_signal_nn_change(struct sock *sk, u8 feat, u64 nn_val)
{
    struct list_head *fn = &dccp_sk(sk)->dccps_featneg;
    dccp_feat_val fval = { .nn = nn_val };
    struct dccp_feat_entry *entry;

    if (sk->sk_state != DCCP_OPEN && sk->sk_state != DCCP_PARTOPEN)
        return 0;

    if (dccp_feat_type(feat) != FEAT_NN ||
        !dccp_feat_is_valid_nn_val(feat, nn_val))
        return -EINVAL;

    if (nn_val == dccp_feat_nn_get(sk, feat))
        return 0;   /* already set or negotiation under way */

    entry = dccp_feat_list_lookup(fn, feat, 1);
    if (entry != NULL) {
        dccp_pr_debug("Clobbering existing NN entry %llu -> %llu\n",
                  (unsigned long long)entry->val.nn,
                  (unsigned long long)nn_val);
        dccp_feat_list_pop(entry);
    }

    inet_csk_schedule_ack(sk);
    return dccp_feat_push_change(fn, feat, 1, 0, &fval);
}
EXPORT_SYMBOL_GPL(dccp_feat_signal_nn_change);

/*
 *  Tracking features whose value depend on the choice of CCID
 *
 * This is designed with an extension in mind so that a list walk could be done
 * before activating any features. However, the existing framework was found to
 * work satisfactorily up until now, the automatic verification is left open.
 * When adding new CCIDs, add a corresponding dependency table here.
 */
static const struct ccid_dependency *dccp_feat_ccid_deps(u8 ccid, bool is_local)
{
    static const struct ccid_dependency ccid2_dependencies[2][2] = {
        /*
         * CCID2 mandates Ack Vectors (RFC 4341, 4.): as CCID is a TX
         * feature and Send Ack Vector is an RX feature, `is_local'
         * needs to be reversed.
         */
        {   /* Dependencies of the receiver-side (remote) CCID2 */
            {
                .dependent_feat = DCCPF_SEND_ACK_VECTOR,
                .is_local   = true,
                .is_mandatory   = true,
                .val        = 1
            },
            { 0, 0, 0, 0 }
        },
        {   /* Dependencies of the sender-side (local) CCID2 */
            {
                .dependent_feat = DCCPF_SEND_ACK_VECTOR,
                .is_local   = false,
                .is_mandatory   = true,
                .val        = 1
            },
            { 0, 0, 0, 0 }
        }
    };
    static const struct ccid_dependency ccid3_dependencies[2][5] = {
        {   /*
             * Dependencies of the receiver-side CCID3
             */
            {   /* locally disable Ack Vectors */
                .dependent_feat = DCCPF_SEND_ACK_VECTOR,
                .is_local   = true,
                .is_mandatory   = false,
                .val        = 0
            },
            {   /* see below why Send Loss Event Rate is on */
                .dependent_feat = DCCPF_SEND_LEV_RATE,
                .is_local   = true,
                .is_mandatory   = true,
                .val        = 1
            },
            {   /* NDP Count is needed as per RFC 4342, 6.1.1 */
                .dependent_feat = DCCPF_SEND_NDP_COUNT,
                .is_local   = false,
                .is_mandatory   = true,
                .val        = 1
            },
            { 0, 0, 0, 0 },
        },
        {   /*
             * CCID3 at the TX side: we request that the HC-receiver
             * will not send Ack Vectors (they will be ignored, so
             * Mandatory is not set); we enable Send Loss Event Rate
             * (Mandatory since the implementation does not support
             * the Loss Intervals option of RFC 4342, 8.6).
             * The last two options are for peer's information only.
            */
            {
                .dependent_feat = DCCPF_SEND_ACK_VECTOR,
                .is_local   = false,
                .is_mandatory   = false,
                .val        = 0
            },
            {
                .dependent_feat = DCCPF_SEND_LEV_RATE,
                .is_local   = false,
                .is_mandatory   = true,
                .val        = 1
            },
            {   /* this CCID does not support Ack Ratio */
                .dependent_feat = DCCPF_ACK_RATIO,
                .is_local   = true,
                .is_mandatory   = false,
                .val        = 0
            },
            {   /* tell receiver we are sending NDP counts */
                .dependent_feat = DCCPF_SEND_NDP_COUNT,
                .is_local   = true,
                .is_mandatory   = false,
                .val        = 1
            },
            { 0, 0, 0, 0 }
        }
    };
    switch (ccid) {
    case DCCPC_CCID2:
        return ccid2_dependencies[is_local];
    case DCCPC_CCID3:
        return ccid3_dependencies[is_local];
    default:
        return NULL;
    }
}

static int dccp_feat_propagate_ccid(struct list_head *fn, u8 id, bool is_local)
{
    const struct ccid_dependency *table = dccp_feat_ccid_deps(id, is_local);
    int i, rc = (table == NULL);

    for (i = 0; rc == 0 && table[i].dependent_feat != DCCPF_RESERVED; i++)
        if (dccp_feat_type(table[i].dependent_feat) == FEAT_SP)
            rc = __feat_register_sp(fn, table[i].dependent_feat,
                            table[i].is_local,
                            table[i].is_mandatory,
                            &table[i].val, 1);
        else
            rc = __feat_register_nn(fn, table[i].dependent_feat,
                            table[i].is_mandatory,
                            table[i].val);
    return rc;
}

int dccp_feat_finalise_settings(struct dccp_sock *dp)
{
    struct list_head *fn = &dp->dccps_featneg;
    struct dccp_feat_entry *entry;
    int i = 2, ccids[2] = { -1, -1 };

    /*
     * Propagating CCIDs:
     * 1) not useful to propagate CCID settings if this host advertises more
     *    than one CCID: the choice of CCID  may still change - if this is
     *    the client, or if this is the server and the client sends
     *    singleton CCID values.
     * 2) since is that propagate_ccid changes the list, we defer changing
     *    the sorted list until after the traversal.
     */
    list_for_each_entry(entry, fn, node)
        if (entry->feat_num == DCCPF_CCID && entry->val.sp.len == 1)
            ccids[entry->is_local] = entry->val.sp.vec[0];
    while (i--)
        if (ccids[i] > 0 && dccp_feat_propagate_ccid(fn, ccids[i], i))
            return -1;
    dccp_feat_print_fnlist(fn);
    return 0;
}

int dccp_feat_server_ccid_dependencies(struct dccp_request_sock *dreq)
{
    struct list_head *fn = &dreq->dreq_featneg;
    struct dccp_feat_entry *entry;
    u8 is_local, ccid;

    for (is_local = 0; is_local <= 1; is_local++) {
        entry = dccp_feat_list_lookup(fn, DCCPF_CCID, is_local);

        if (entry != NULL && !entry->empty_confirm)
            ccid = entry->val.sp.vec[0];
        else
            ccid = dccp_feat_default_value(DCCPF_CCID);

        if (dccp_feat_propagate_ccid(fn, ccid, is_local))
            return -1;
    }
    return 0;
}

/* Select the first entry in @servlist that also occurs in @clilist (6.3.1) */
static int dccp_feat_preflist_match(u8 *servlist, u8 slen, u8 *clilist, u8 clen)
{
    u8 c, s;

    for (s = 0; s < slen; s++)
        for (c = 0; c < clen; c++)
            if (servlist[s] == clilist[c])
                return servlist[s];
    return -1;
}

static u8 dccp_feat_prefer(u8 preferred_value, u8 *array, u8 array_len)
{
    u8 i, does_occur = 0;

    if (array != NULL) {
        for (i = 0; i < array_len; i++)
            if (array[i] == preferred_value) {
                array[i] = array[0];
                does_occur++;
            }
        if (does_occur)
            array[0] = preferred_value;
    }
    return does_occur;
}

static int dccp_feat_reconcile(dccp_feat_val *fv, u8 *arr, u8 len,
                   bool is_server, bool reorder)
{
    int rc;

    if (!fv->sp.vec || !arr) {
        DCCP_CRIT("NULL feature value or array");
        return 0;
    }

    if (is_server)
        rc = dccp_feat_preflist_match(fv->sp.vec, fv->sp.len, arr, len);
    else
        rc = dccp_feat_preflist_match(arr, len, fv->sp.vec, fv->sp.len);

    if (!reorder)
        return rc;
    if (rc < 0)
        return 0;

    /*
     * Reorder list: used for activating features and in dccp_insert_fn_opt.
     */
    return dccp_feat_prefer(rc, fv->sp.vec, fv->sp.len);
}

static u8 dccp_feat_change_recv(struct list_head *fn, u8 is_mandatory, u8 opt,
                u8 feat, u8 *val, u8 len, const bool server)
{
    u8 defval, type = dccp_feat_type(feat);
    const bool local = (opt == DCCPO_CHANGE_R);
    struct dccp_feat_entry *entry;
    dccp_feat_val fval;

    if (len == 0 || type == FEAT_UNKNOWN)       /* 6.1 and 6.6.8 */
        goto unknown_feature_or_value;

    dccp_feat_print_opt(opt, feat, val, len, is_mandatory);

    /*
     *  Negotiation of NN features: Change R is invalid, so there is no
     *  simultaneous negotiation; hence we do not look up in the list.
     */
    if (type == FEAT_NN) {
        if (local || len > sizeof(fval.nn))
            goto unknown_feature_or_value;

        /* 6.3.2: "The feature remote MUST accept any valid value..." */
        fval.nn = dccp_decode_value_var(val, len);
        if (!dccp_feat_is_valid_nn_val(feat, fval.nn))
            goto unknown_feature_or_value;

        return dccp_feat_push_confirm(fn, feat, local, &fval);
    }

    /*
     *  Unidirectional/simultaneous negotiation of SP features (6.3.1)
     */
    entry = dccp_feat_list_lookup(fn, feat, local);
    if (entry == NULL) {
        /*
         * No particular preferences have been registered. We deal with
         * this situation by assuming that all valid values are equally
         * acceptable, and apply the following checks:
         * - if the peer's list is a singleton, we accept a valid value;
         * - if we are the server, we first try to see if the peer (the
         *   client) advertises the default value. If yes, we use it,
         *   otherwise we accept the preferred value;
         * - else if we are the client, we use the first list element.
         */
        if (dccp_feat_clone_sp_val(&fval, val, 1))
            return DCCP_RESET_CODE_TOO_BUSY;

        if (len > 1 && server) {
            defval = dccp_feat_default_value(feat);
            if (dccp_feat_preflist_match(&defval, 1, val, len) > -1)
                fval.sp.vec[0] = defval;
        } else if (!dccp_feat_is_valid_sp_val(feat, fval.sp.vec[0])) {
            kfree(fval.sp.vec);
            goto unknown_feature_or_value;
        }

        /* Treat unsupported CCIDs like invalid values */
        if (feat == DCCPF_CCID && !ccid_support_check(fval.sp.vec, 1)) {
            kfree(fval.sp.vec);
            goto not_valid_or_not_known;
        }

        return dccp_feat_push_confirm(fn, feat, local, &fval);

    } else if (entry->state == FEAT_UNSTABLE) { /* 6.6.2 */
        return 0;
    }

    if (dccp_feat_reconcile(&entry->val, val, len, server, true)) {
        entry->empty_confirm = false;
    } else if (is_mandatory) {
        return DCCP_RESET_CODE_MANDATORY_ERROR;
    } else if (entry->state == FEAT_INITIALISING) {
        /*
         * Failed simultaneous negotiation (server only): try to `save'
         * the connection by checking whether entry contains the default
         * value for @feat. If yes, send an empty Confirm to signal that
         * the received Change was not understood - which implies using
         * the default value.
         * If this also fails, we use Reset as the last resort.
         */
        WARN_ON(!server);
        defval = dccp_feat_default_value(feat);
        if (!dccp_feat_reconcile(&entry->val, &defval, 1, server, true))
            return DCCP_RESET_CODE_OPTION_ERROR;
        entry->empty_confirm = true;
    }
    entry->needs_confirm   = true;
    entry->needs_mandatory = false;
    entry->state           = FEAT_STABLE;
    return 0;

unknown_feature_or_value:
    if (!is_mandatory)
        return dccp_push_empty_confirm(fn, feat, local);

not_valid_or_not_known:
    return is_mandatory ? DCCP_RESET_CODE_MANDATORY_ERROR
                : DCCP_RESET_CODE_OPTION_ERROR;
}

static u8 dccp_feat_confirm_recv(struct list_head *fn, u8 is_mandatory, u8 opt,
                 u8 feat, u8 *val, u8 len, const bool server)
{
    u8 *plist, plen, type = dccp_feat_type(feat);
    const bool local = (opt == DCCPO_CONFIRM_R);
    struct dccp_feat_entry *entry = dccp_feat_list_lookup(fn, feat, local);

    dccp_feat_print_opt(opt, feat, val, len, is_mandatory);

    if (entry == NULL) {    /* nothing queued: ignore or handle error */
        if (is_mandatory && type == FEAT_UNKNOWN)
            return DCCP_RESET_CODE_MANDATORY_ERROR;

        if (!local && type == FEAT_NN)      /* 6.3.2 */
            goto confirmation_failed;
        return 0;
    }

    if (entry->state != FEAT_CHANGING)      /* 6.6.2 */
        return 0;

    if (len == 0) {
        if (dccp_feat_must_be_understood(feat)) /* 6.6.7 */
            goto confirmation_failed;
        /*
         * Empty Confirm during connection setup: this means reverting
         * to the `old' value, which in this case is the default. Since
         * we handle default values automatically when no other values
         * have been set, we revert to the old value by removing this
         * entry from the list.
         */
        dccp_feat_list_pop(entry);
        return 0;
    }

    if (type == FEAT_NN) {
        if (len > sizeof(entry->val.nn))
            goto confirmation_failed;

        if (entry->val.nn == dccp_decode_value_var(val, len))
            goto confirmation_succeeded;

        DCCP_WARN("Bogus Confirm for non-existing value\n");
        goto confirmation_failed;
    }

    /*
     * Parsing SP Confirms: the first element of @val is the preferred
     * SP value which the peer confirms, the remainder depends on @len.
     * Note that only the confirmed value need to be a valid SP value.
     */
    if (!dccp_feat_is_valid_sp_val(feat, *val))
        goto confirmation_failed;

    if (len == 1) {     /* peer didn't supply a preference list */
        plist = val;
        plen  = len;
    } else {        /* preferred value + preference list */
        plist = val + 1;
        plen  = len - 1;
    }

    /* Check whether the peer got the reconciliation right (6.6.8) */
    if (dccp_feat_reconcile(&entry->val, plist, plen, server, 0) != *val) {
        DCCP_WARN("Confirm selected the wrong value %u\n", *val);
        return DCCP_RESET_CODE_OPTION_ERROR;
    }
    entry->val.sp.vec[0] = *val;

confirmation_succeeded:
    entry->state = FEAT_STABLE;
    return 0;

confirmation_failed:
    DCCP_WARN("Confirmation failed\n");
    return is_mandatory ? DCCP_RESET_CODE_MANDATORY_ERROR
                : DCCP_RESET_CODE_OPTION_ERROR;
}

static u8 dccp_feat_handle_nn_established(struct sock *sk, u8 mandatory, u8 opt,
                      u8 feat, u8 *val, u8 len)
{
    struct list_head *fn = &dccp_sk(sk)->dccps_featneg;
    const bool local = (opt == DCCPO_CONFIRM_R);
    struct dccp_feat_entry *entry;
    u8 type = dccp_feat_type(feat);
    dccp_feat_val fval;

    dccp_feat_print_opt(opt, feat, val, len, mandatory);

    /* Ignore non-mandatory unknown and non-NN features */
    if (type == FEAT_UNKNOWN) {
        if (local && !mandatory)
            return 0;
        goto fast_path_unknown;
    } else if (type != FEAT_NN) {
        return 0;
    }

    /*
     * We don't accept empty Confirms, since in fast-path feature
     * negotiation the values are enabled immediately after sending
     * the Change option.
     * Empty Changes on the other hand are invalid (RFC 4340, 6.1).
     */
    if (len == 0 || len > sizeof(fval.nn))
        goto fast_path_unknown;

    if (opt == DCCPO_CHANGE_L) {
        fval.nn = dccp_decode_value_var(val, len);
        if (!dccp_feat_is_valid_nn_val(feat, fval.nn))
            goto fast_path_unknown;

        if (dccp_feat_push_confirm(fn, feat, local, &fval) ||
            dccp_feat_activate(sk, feat, local, &fval))
            return DCCP_RESET_CODE_TOO_BUSY;

        /* set the `Ack Pending' flag to piggyback a Confirm */
        inet_csk_schedule_ack(sk);

    } else if (opt == DCCPO_CONFIRM_R) {
        entry = dccp_feat_list_lookup(fn, feat, local);
        if (entry == NULL || entry->state != FEAT_CHANGING)
            return 0;

        fval.nn = dccp_decode_value_var(val, len);
        /*
         * Just ignore a value that doesn't match our current value.
         * If the option changes twice within two RTTs, then at least
         * one CONFIRM will be received for the old value after a
         * new CHANGE was sent.
         */
        if (fval.nn != entry->val.nn)
            return 0;

        /* Only activate after receiving the Confirm option (6.6.1). */
        dccp_feat_activate(sk, feat, local, &fval);

        /* It has been confirmed - so remove the entry */
        dccp_feat_list_pop(entry);

    } else {
        DCCP_WARN("Received illegal option %u\n", opt);
        goto fast_path_failed;
    }
    return 0;

fast_path_unknown:
    if (!mandatory)
        return dccp_push_empty_confirm(fn, feat, local);

fast_path_failed:
    return mandatory ? DCCP_RESET_CODE_MANDATORY_ERROR
             : DCCP_RESET_CODE_OPTION_ERROR;
}

int dccp_feat_parse_options(struct sock *sk, struct dccp_request_sock *dreq,
                u8 mandatory, u8 opt, u8 feat, u8 *val, u8 len)
{
    struct dccp_sock *dp = dccp_sk(sk);
    struct list_head *fn = dreq ? &dreq->dreq_featneg : &dp->dccps_featneg;
    bool server = false;

    switch (sk->sk_state) {
    /*
     *  Negotiation during connection setup
     */
    case DCCP_LISTEN:
        server = true;          /* fall through */
    case DCCP_REQUESTING:
        switch (opt) {
        case DCCPO_CHANGE_L:
        case DCCPO_CHANGE_R:
            return dccp_feat_change_recv(fn, mandatory, opt, feat,
                             val, len, server);
        case DCCPO_CONFIRM_R:
        case DCCPO_CONFIRM_L:
            return dccp_feat_confirm_recv(fn, mandatory, opt, feat,
                              val, len, server);
        }
        break;
    /*
     *  Support for exchanging NN options on an established connection.
     */
    case DCCP_OPEN:
    case DCCP_PARTOPEN:
        return dccp_feat_handle_nn_established(sk, mandatory, opt, feat,
                               val, len);
    }
    return 0;   /* ignore FN options in all other states */
}

int dccp_feat_init(struct sock *sk)
{
    struct list_head *fn = &dccp_sk(sk)->dccps_featneg;
    u8 on = 1, off = 0;
    int rc;
    struct {
        u8 *val;
        u8 len;
    } tx, rx;

    /* Non-negotiable (NN) features */
    rc = __feat_register_nn(fn, DCCPF_SEQUENCE_WINDOW, 0,
                    sysctl_dccp_sequence_window);
    if (rc)
        return rc;

    /* Server-priority (SP) features */

    /* Advertise that short seqnos are not supported (7.6.1) */
    rc = __feat_register_sp(fn, DCCPF_SHORT_SEQNOS, true, true, &off, 1);
    if (rc)
        return rc;

    /* RFC 4340 12.1: "If a DCCP is not ECN capable, ..." */
    rc = __feat_register_sp(fn, DCCPF_ECN_INCAPABLE, true, true, &on, 1);
    if (rc)
        return rc;

    /*
     * We advertise the available list of CCIDs and reorder according to
     * preferences, to avoid failure resulting from negotiating different
     * singleton values (which always leads to failure).
     * These settings can still (later) be overridden via sockopts.
     */
    if (ccid_get_builtin_ccids(&tx.val, &tx.len) ||
        ccid_get_builtin_ccids(&rx.val, &rx.len))
        return -ENOBUFS;

    if (!dccp_feat_prefer(sysctl_dccp_tx_ccid, tx.val, tx.len) ||
        !dccp_feat_prefer(sysctl_dccp_rx_ccid, rx.val, rx.len))
        goto free_ccid_lists;

    rc = __feat_register_sp(fn, DCCPF_CCID, true, false, tx.val, tx.len);
    if (rc)
        goto free_ccid_lists;

    rc = __feat_register_sp(fn, DCCPF_CCID, false, false, rx.val, rx.len);

free_ccid_lists:
    kfree(tx.val);
    kfree(rx.val);
    return rc;
}

int dccp_feat_activate_values(struct sock *sk, struct list_head *fn_list)
{
    struct dccp_sock *dp = dccp_sk(sk);
    struct dccp_feat_entry *cur, *next;
    int idx;
    dccp_feat_val *fvals[DCCP_FEAT_SUPPORTED_MAX][2] = {
         [0 ... DCCP_FEAT_SUPPORTED_MAX-1] = { NULL, NULL }
    };

    list_for_each_entry(cur, fn_list, node) {
        /*
         * An empty Confirm means that either an unknown feature type
         * or an invalid value was present. In the first case there is
         * nothing to activate, in the other the default value is used.
         */
        if (cur->empty_confirm)
            continue;

        idx = dccp_feat_index(cur->feat_num);
        if (idx < 0) {
            DCCP_BUG("Unknown feature %u", cur->feat_num);
            goto activation_failed;
        }
        if (cur->state != FEAT_STABLE) {
            DCCP_CRIT("Negotiation of %s %s failed in state %s",
                  cur->is_local ? "local" : "remote",
                  dccp_feat_fname(cur->feat_num),
                  dccp_feat_sname[cur->state]);
            goto activation_failed;
        }
        fvals[idx][cur->is_local] = &cur->val;
    }

    /*
     * Activate in decreasing order of index, so that the CCIDs are always
     * activated as the last feature. This avoids the case where a CCID
     * relies on the initialisation of one or more features that it depends
     * on (e.g. Send NDP Count, Send Ack Vector, and Ack Ratio features).
     */
    for (idx = DCCP_FEAT_SUPPORTED_MAX; --idx >= 0;)
        if (__dccp_feat_activate(sk, idx, 0, fvals[idx][0]) ||
            __dccp_feat_activate(sk, idx, 1, fvals[idx][1])) {
            DCCP_CRIT("Could not activate %d", idx);
            goto activation_failed;
        }

    /* Clean up Change options which have been confirmed already */
    list_for_each_entry_safe(cur, next, fn_list, node)
        if (!cur->needs_confirm)
            dccp_feat_list_pop(cur);

    dccp_pr_debug("Activation OK\n");
    return 0;

activation_failed:
    /*
     * We clean up everything that may have been allocated, since
     * it is difficult to track at which stage negotiation failed.
     * This is ok, since all allocation functions below are robust
     * against NULL arguments.
     */
    ccid_hc_rx_delete(dp->dccps_hc_rx_ccid, sk);
    ccid_hc_tx_delete(dp->dccps_hc_tx_ccid, sk);
    dp->dccps_hc_rx_ccid = dp->dccps_hc_tx_ccid = NULL;
    dccp_ackvec_free(dp->dccps_hc_rx_ackvec);
    dp->dccps_hc_rx_ackvec = NULL;
    return -1;
}