cregit-Linux how code gets into the kernel

Release 4.8 net/dccp/feat.c

Directory: net/dccp
/*
 *  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.
 *
 *  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; }

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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; }

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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; }

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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; }

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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; }

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/* * 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; }

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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; }

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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; }

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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; }

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/* * 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]; }

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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; }

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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); }

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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); }

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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)" : ""); }

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#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); }

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/** * 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); }

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/* 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; }

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/* 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; }

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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)); }

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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; }

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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); } }

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/* * 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; }

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/** * 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; }

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/** * 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; }

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/** * 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; }

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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); }

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

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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); }

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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; }

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/** * 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; }

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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 */ }

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/* 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 */ }

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gerrit renkergerrit renker66100.00%1100.00%
Total66100.00%1100.00%


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; }

Contributors

PersonTokensPropCommitsCommitProp
gerrit renkergerrit renker53100.00%1100.00%
Total53100.00%1100.00%

/** * 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; }

Contributors

PersonTokensPropCommitsCommitProp
gerrit renkergerrit renker27089.40%266.67%
samuel jerosamuel jero3210.60%133.33%
Total302100.00%3100.00%

/** * __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