Linux 5.7.7

[linux/fpc-iii.git] / net / dccp / feat.c

// SPDX-License-Identifier: GPL-2.0-or-later
/*
 *  net/dccp/feat.c
 *
 *  Feature negotiation for the DCCP protocol (RFC 4340, section 6)
 *
 *  Copyright (c) 2008 Gerrit Renker <gerrit@erg.abdn.ac.uk>
 *  Rewrote from scratch, some bits from earlier code by
 *  Copyright (c) 2005 Andrea Bittau <a.bittau@cs.ucl.ac.uk>
 *
 *  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.
 */
#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)

/**
 * dccp_feat_index  -  Hash function to map feature number into array position
 * Returns consecutive array index or -1 if the feature is not understood.
 */
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);
}

/**
 * dccp_feat_activate  -  Activate feature value on socket
 * @sk: fully connected DCCP socket (after handshake is complete)
 * @feat_num: feature to activate, one of %dccp_feature_numbers
 * @local: whether local (1) or remote (0) @feat_num is meant
 * @fval: the value (SP or NN) to activate, or NULL to use the default value
 *
 * For general use this function is preferable over __dccp_feat_activate().
 */
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;
}

/**
 * dccp_feat_entry_new  -  Central list update routine (called by all others)
 * @head:  list to add to
 * @feat:  feature number
 * @local: whether the local (1) or remote feature with number @feat is meant
 *
 * This is the only constructor and serves to ensure the above invariants.
 */
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;
}

/**
 * dccp_feat_push_change  -  Add/overwrite a Change option in the list
 * @fn_list: feature-negotiation list to update
 * @feat: one of %dccp_feature_numbers
 * @local: whether local (1) or remote (0) @feat_num is meant
 * @mandatory: whether to use Mandatory feature negotiation options
 * @fval: pointer to NN/SP value to be inserted (will be copied)
 */
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;
}

/**
 * dccp_feat_push_confirm  -  Add a Confirm entry to the FN list
 * @fn_list: feature-negotiation list to add to
 * @feat: one of %dccp_feature_numbers
 * @local: whether local (1) or remote (0) @feat_num is being confirmed
 * @fval: pointer to NN/SP value to be inserted or NULL
 *
 * Returns 0 on success, a Reset code for further processing otherwise.
 */
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;
}

/**
 * dccp_feat_valid_nn_length  -  Enforce length constraints on NN options
 * Length is between 0 and %DCCP_OPTVAL_MAXLEN. Used for outgoing packets only,
 * incoming options are accepted as long as their values are valid.
 */
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;
}

/**
 * dccp_feat_insert_opts  -  Generate FN options from current list state
 * @skb: next sk_buff to be sent to the peer
 * @dp: for client during handshake and general negotiation
 * @dreq: used by the server only (all Changes/Confirms in LISTEN/RESPOND)
 */
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;
}

/**
 * __feat_register_nn  -  Register new NN value on socket
 * @fn: feature-negotiation list to register with
 * @feat: an NN feature from %dccp_feature_numbers
 * @mandatory: use Mandatory option if 1
 * @nn_val: value to register (restricted to 4 bytes)
 *
 * Note that NN features are local by definition (RFC 4340, 6.3.2).
 */
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);
}

/**
 * __feat_register_sp  -  Register new SP value/list on socket
 * @fn: feature-negotiation list to register with
 * @feat: an SP feature from %dccp_feature_numbers
 * @is_local: whether the local (1) or the remote (0) @feat is meant
 * @mandatory: use Mandatory option if 1
 * @sp_val: SP value followed by optional preference list
 * @sp_len: length of @sp_val in bytes
 */
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;

        if (dccp_feat_push_change(fn, feat, is_local, mandatory, &fval)) {
                kfree(fval.sp.vec);
                return -ENOMEM;
        }

        return 0;
}

/**
 * dccp_feat_register_sp  -  Register requests to change SP feature values
 * @sk: client or listening socket
 * @feat: one of %dccp_feature_numbers
 * @is_local: whether the local (1) or remote (0) @feat is meant
 * @list: array of preferred values, in descending order of preference
 * @len: length of @list in bytes
 */
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);
}

/**
 * dccp_feat_nn_get  -  Query current/pending value of NN feature
 * @sk: DCCP socket of an established connection
 * @feat: NN feature number from %dccp_feature_numbers
 *
 * For a known NN feature, returns value currently being negotiated, or
 * current (confirmed) value if no negotiation is going on.
 */
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);

/**
 * dccp_feat_signal_nn_change  -  Update NN values for an established connection
 * @sk: DCCP socket of an established connection
 * @feat: NN feature number from %dccp_feature_numbers
 * @nn_val: the new value to use
 *
 * This function is used to communicate NN updates out-of-band.
 */
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;
        }
}

/**
 * dccp_feat_propagate_ccid - Resolve dependencies of features on choice of CCID
 * @fn: feature-negotiation list to update
 * @id: CCID number to track
 * @is_local: whether TX CCID (1) or RX CCID (0) is meant
 *
 * This function needs to be called after registering all other features.
 */
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;
}

/**
 * dccp_feat_finalise_settings  -  Finalise settings before starting negotiation
 * @dp: client or listening socket (settings will be inherited)
 *
 * This is called after all registrations (socket initialisation, sysctls, and
 * sockopt calls), and before sending the first packet containing Change options
 * (ie. client-Request or server-Response), to ensure internal consistency.
 */
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;
}

/**
 * dccp_feat_server_ccid_dependencies  -  Resolve CCID-dependent features
 * It is the server which resolves the dependencies once the CCID has been
 * fully negotiated. If no CCID has been negotiated, it uses the default CCID.
 */
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;
}

/**
 * dccp_feat_prefer  -  Move preferred entry to the start of array
 * Reorder the @array_len elements in @array so that @preferred_value comes
 * first. Returns >0 to indicate that @preferred_value does occur in @array.
 */
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;
}

/**
 * dccp_feat_reconcile  -  Reconcile SP preference lists
 *  @fv: SP list to reconcile into
 *  @arr: received SP preference list
 *  @len: length of @arr in bytes
 *  @is_server: whether this side is the server (and @fv is the server's list)
 *  @reorder: whether to reorder the list in @fv after reconciling with @arr
 * When successful, > 0 is returned and the reconciled list is in @fval.
 * A value of 0 means that negotiation failed (no shared entry).
 */
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);
}

/**
 * dccp_feat_change_recv  -  Process incoming ChangeL/R options
 * @fn: feature-negotiation list to update
 * @is_mandatory: whether the Change was preceded by a Mandatory option
 * @opt: %DCCPO_CHANGE_L or %DCCPO_CHANGE_R
 * @feat: one of %dccp_feature_numbers
 * @val: NN value or SP value/preference list
 * @len: length of @val in bytes
 * @server: whether this node is the server (1) or the client (0)
 */
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;
}

/**
 * dccp_feat_confirm_recv  -  Process received Confirm options
 * @fn: feature-negotiation list to update
 * @is_mandatory: whether @opt was preceded by a Mandatory option
 * @opt: %DCCPO_CONFIRM_L or %DCCPO_CONFIRM_R
 * @feat: one of %dccp_feature_numbers
 * @val: NN value or SP value/preference list
 * @len: length of @val in bytes
 * @server: whether this node is server (1) or client (0)
 */
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;
}

/**
 * dccp_feat_handle_nn_established  -  Fast-path reception of NN options
 * @sk:         socket of an established DCCP connection
 * @mandatory:  whether @opt was preceded by a Mandatory option
 * @opt:        %DCCPO_CHANGE_L | %DCCPO_CONFIRM_R (NN only)
 * @feat:       NN number, one of %dccp_feature_numbers
 * @val:        NN value
 * @len:        length of @val in bytes
 *
 * This function combines the functionality of change_recv/confirm_recv, with
 * the following differences (reset codes are the same):
 *    - cleanup after receiving the Confirm;
 *    - values are directly activated after successful parsing;
 *    - deliberately restricted to NN features.
 * The restriction to NN features is essential since SP features can have non-
 * predictable outcomes (depending on the remote configuration), and are inter-
 * dependent (CCIDs for instance cause further dependencies).
 */
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;
}

/**
 * dccp_feat_parse_options  -  Process Feature-Negotiation Options
 * @sk: for general use and used by the client during connection setup
 * @dreq: used by the server during connection setup
 * @mandatory: whether @opt was preceded by a Mandatory option
 * @opt: %DCCPO_CHANGE_L | %DCCPO_CHANGE_R | %DCCPO_CONFIRM_L | %DCCPO_CONFIRM_R
 * @feat: one of %dccp_feature_numbers
 * @val: value contents of @opt
 * @len: length of @val in bytes
 *
 * Returns 0 on success, a Reset code for ending the connection otherwise.
 */
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 */
}

/**
 * dccp_feat_init  -  Seed feature negotiation with host-specific defaults
 * This initialises global defaults, depending on the value of the sysctls.
 * These can later be overridden by registering changes via setsockopt calls.
 * The last link in the chain is finalise_settings, to make sure that between
 * here and the start of actual feature negotiation no inconsistencies enter.
 *
 * All features not appearing below use either defaults or are otherwise
 * later adjusted through dccp_feat_finalise_settings().
 */
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))
                return -ENOBUFS;
        if (ccid_get_builtin_ccids(&rx.val, &rx.len)) {
                kfree(tx.val);
                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;
}