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Linux/net/dccp/ackvec.c

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  1 // SPDX-License-Identifier: GPL-2.0-only
  2 /*
  3  *  net/dccp/ackvec.c
  4  *
  5  *  An implementation of Ack Vectors for the DCCP protocol
  6  *  Copyright (c) 2007 University of Aberdeen, Scotland, UK
  7  *  Copyright (c) 2005 Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
  8  */
  9 #include "dccp.h"
 10 #include <linux/kernel.h>
 11 #include <linux/slab.h>
 12 #include <linux/export.h>
 13 
 14 static struct kmem_cache *dccp_ackvec_slab;
 15 static struct kmem_cache *dccp_ackvec_record_slab;
 16 
 17 struct dccp_ackvec *dccp_ackvec_alloc(const gfp_t priority)
 18 {
 19         struct dccp_ackvec *av = kmem_cache_zalloc(dccp_ackvec_slab, priority);
 20 
 21         if (av != NULL) {
 22                 av->av_buf_head = av->av_buf_tail = DCCPAV_MAX_ACKVEC_LEN - 1;
 23                 INIT_LIST_HEAD(&av->av_records);
 24         }
 25         return av;
 26 }
 27 
 28 static void dccp_ackvec_purge_records(struct dccp_ackvec *av)
 29 {
 30         struct dccp_ackvec_record *cur, *next;
 31 
 32         list_for_each_entry_safe(cur, next, &av->av_records, avr_node)
 33                 kmem_cache_free(dccp_ackvec_record_slab, cur);
 34         INIT_LIST_HEAD(&av->av_records);
 35 }
 36 
 37 void dccp_ackvec_free(struct dccp_ackvec *av)
 38 {
 39         if (likely(av != NULL)) {
 40                 dccp_ackvec_purge_records(av);
 41                 kmem_cache_free(dccp_ackvec_slab, av);
 42         }
 43 }
 44 
 45 /**
 46  * dccp_ackvec_update_records  -  Record information about sent Ack Vectors
 47  * @av:         Ack Vector records to update
 48  * @seqno:      Sequence number of the packet carrying the Ack Vector just sent
 49  * @nonce_sum:  The sum of all buffer nonces contained in the Ack Vector
 50  */
 51 int dccp_ackvec_update_records(struct dccp_ackvec *av, u64 seqno, u8 nonce_sum)
 52 {
 53         struct dccp_ackvec_record *avr;
 54 
 55         avr = kmem_cache_alloc(dccp_ackvec_record_slab, GFP_ATOMIC);
 56         if (avr == NULL)
 57                 return -ENOBUFS;
 58 
 59         avr->avr_ack_seqno  = seqno;
 60         avr->avr_ack_ptr    = av->av_buf_head;
 61         avr->avr_ack_ackno  = av->av_buf_ackno;
 62         avr->avr_ack_nonce  = nonce_sum;
 63         avr->avr_ack_runlen = dccp_ackvec_runlen(av->av_buf + av->av_buf_head);
 64         /*
 65          * When the buffer overflows, we keep no more than one record. This is
 66          * the simplest way of disambiguating sender-Acks dating from before the
 67          * overflow from sender-Acks which refer to after the overflow; a simple
 68          * solution is preferable here since we are handling an exception.
 69          */
 70         if (av->av_overflow)
 71                 dccp_ackvec_purge_records(av);
 72         /*
 73          * Since GSS is incremented for each packet, the list is automatically
 74          * arranged in descending order of @ack_seqno.
 75          */
 76         list_add(&avr->avr_node, &av->av_records);
 77 
 78         dccp_pr_debug("Added Vector, ack_seqno=%llu, ack_ackno=%llu (rl=%u)\n",
 79                       (unsigned long long)avr->avr_ack_seqno,
 80                       (unsigned long long)avr->avr_ack_ackno,
 81                       avr->avr_ack_runlen);
 82         return 0;
 83 }
 84 
 85 static struct dccp_ackvec_record *dccp_ackvec_lookup(struct list_head *av_list,
 86                                                      const u64 ackno)
 87 {
 88         struct dccp_ackvec_record *avr;
 89         /*
 90          * Exploit that records are inserted in descending order of sequence
 91          * number, start with the oldest record first. If @ackno is `before'
 92          * the earliest ack_ackno, the packet is too old to be considered.
 93          */
 94         list_for_each_entry_reverse(avr, av_list, avr_node) {
 95                 if (avr->avr_ack_seqno == ackno)
 96                         return avr;
 97                 if (before48(ackno, avr->avr_ack_seqno))
 98                         break;
 99         }
100         return NULL;
101 }
102 
103 /*
104  * Buffer index and length computation using modulo-buffersize arithmetic.
105  * Note that, as pointers move from right to left, head is `before' tail.
106  */
107 static inline u16 __ackvec_idx_add(const u16 a, const u16 b)
108 {
109         return (a + b) % DCCPAV_MAX_ACKVEC_LEN;
110 }
111 
112 static inline u16 __ackvec_idx_sub(const u16 a, const u16 b)
113 {
114         return __ackvec_idx_add(a, DCCPAV_MAX_ACKVEC_LEN - b);
115 }
116 
117 u16 dccp_ackvec_buflen(const struct dccp_ackvec *av)
118 {
119         if (unlikely(av->av_overflow))
120                 return DCCPAV_MAX_ACKVEC_LEN;
121         return __ackvec_idx_sub(av->av_buf_tail, av->av_buf_head);
122 }
123 
124 /**
125  * dccp_ackvec_update_old  -  Update previous state as per RFC 4340, 11.4.1
126  * @av:         non-empty buffer to update
127  * @distance:   negative or zero distance of @seqno from buf_ackno downward
128  * @seqno:      the (old) sequence number whose record is to be updated
129  * @state:      state in which packet carrying @seqno was received
130  */
131 static void dccp_ackvec_update_old(struct dccp_ackvec *av, s64 distance,
132                                    u64 seqno, enum dccp_ackvec_states state)
133 {
134         u16 ptr = av->av_buf_head;
135 
136         BUG_ON(distance > 0);
137         if (unlikely(dccp_ackvec_is_empty(av)))
138                 return;
139 
140         do {
141                 u8 runlen = dccp_ackvec_runlen(av->av_buf + ptr);
142 
143                 if (distance + runlen >= 0) {
144                         /*
145                          * Only update the state if packet has not been received
146                          * yet. This is OK as per the second table in RFC 4340,
147                          * 11.4.1; i.e. here we are using the following table:
148                          *                     RECEIVED
149                          *                      0   1   3
150                          *              S     +---+---+---+
151                          *              T   0 | 0 | 0 | 0 |
152                          *              O     +---+---+---+
153                          *              R   1 | 1 | 1 | 1 |
154                          *              E     +---+---+---+
155                          *              D   3 | 0 | 1 | 3 |
156                          *                    +---+---+---+
157                          * The "Not Received" state was set by reserve_seats().
158                          */
159                         if (av->av_buf[ptr] == DCCPAV_NOT_RECEIVED)
160                                 av->av_buf[ptr] = state;
161                         else
162                                 dccp_pr_debug("Not changing %llu state to %u\n",
163                                               (unsigned long long)seqno, state);
164                         break;
165                 }
166 
167                 distance += runlen + 1;
168                 ptr       = __ackvec_idx_add(ptr, 1);
169 
170         } while (ptr != av->av_buf_tail);
171 }
172 
173 /* Mark @num entries after buf_head as "Not yet received". */
174 static void dccp_ackvec_reserve_seats(struct dccp_ackvec *av, u16 num)
175 {
176         u16 start = __ackvec_idx_add(av->av_buf_head, 1),
177             len   = DCCPAV_MAX_ACKVEC_LEN - start;
178 
179         /* check for buffer wrap-around */
180         if (num > len) {
181                 memset(av->av_buf + start, DCCPAV_NOT_RECEIVED, len);
182                 start = 0;
183                 num  -= len;
184         }
185         if (num)
186                 memset(av->av_buf + start, DCCPAV_NOT_RECEIVED, num);
187 }
188 
189 /**
190  * dccp_ackvec_add_new  -  Record one or more new entries in Ack Vector buffer
191  * @av:          container of buffer to update (can be empty or non-empty)
192  * @num_packets: number of packets to register (must be >= 1)
193  * @seqno:       sequence number of the first packet in @num_packets
194  * @state:       state in which packet carrying @seqno was received
195  */
196 static void dccp_ackvec_add_new(struct dccp_ackvec *av, u32 num_packets,
197                                 u64 seqno, enum dccp_ackvec_states state)
198 {
199         u32 num_cells = num_packets;
200 
201         if (num_packets > DCCPAV_BURST_THRESH) {
202                 u32 lost_packets = num_packets - 1;
203 
204                 DCCP_WARN("Warning: large burst loss (%u)\n", lost_packets);
205                 /*
206                  * We received 1 packet and have a loss of size "num_packets-1"
207                  * which we squeeze into num_cells-1 rather than reserving an
208                  * entire byte for each lost packet.
209                  * The reason is that the vector grows in O(burst_length); when
210                  * it grows too large there will no room left for the payload.
211                  * This is a trade-off: if a few packets out of the burst show
212                  * up later, their state will not be changed; it is simply too
213                  * costly to reshuffle/reallocate/copy the buffer each time.
214                  * Should such problems persist, we will need to switch to a
215                  * different underlying data structure.
216                  */
217                 for (num_packets = num_cells = 1; lost_packets; ++num_cells) {
218                         u8 len = min_t(u32, lost_packets, DCCPAV_MAX_RUNLEN);
219 
220                         av->av_buf_head = __ackvec_idx_sub(av->av_buf_head, 1);
221                         av->av_buf[av->av_buf_head] = DCCPAV_NOT_RECEIVED | len;
222 
223                         lost_packets -= len;
224                 }
225         }
226 
227         if (num_cells + dccp_ackvec_buflen(av) >= DCCPAV_MAX_ACKVEC_LEN) {
228                 DCCP_CRIT("Ack Vector buffer overflow: dropping old entries");
229                 av->av_overflow = true;
230         }
231 
232         av->av_buf_head = __ackvec_idx_sub(av->av_buf_head, num_packets);
233         if (av->av_overflow)
234                 av->av_buf_tail = av->av_buf_head;
235 
236         av->av_buf[av->av_buf_head] = state;
237         av->av_buf_ackno            = seqno;
238 
239         if (num_packets > 1)
240                 dccp_ackvec_reserve_seats(av, num_packets - 1);
241 }
242 
243 /**
244  * dccp_ackvec_input  -  Register incoming packet in the buffer
245  */
246 void dccp_ackvec_input(struct dccp_ackvec *av, struct sk_buff *skb)
247 {
248         u64 seqno = DCCP_SKB_CB(skb)->dccpd_seq;
249         enum dccp_ackvec_states state = DCCPAV_RECEIVED;
250 
251         if (dccp_ackvec_is_empty(av)) {
252                 dccp_ackvec_add_new(av, 1, seqno, state);
253                 av->av_tail_ackno = seqno;
254 
255         } else {
256                 s64 num_packets = dccp_delta_seqno(av->av_buf_ackno, seqno);
257                 u8 *current_head = av->av_buf + av->av_buf_head;
258 
259                 if (num_packets == 1 &&
260                     dccp_ackvec_state(current_head) == state &&
261                     dccp_ackvec_runlen(current_head) < DCCPAV_MAX_RUNLEN) {
262 
263                         *current_head   += 1;
264                         av->av_buf_ackno = seqno;
265 
266                 } else if (num_packets > 0) {
267                         dccp_ackvec_add_new(av, num_packets, seqno, state);
268                 } else {
269                         dccp_ackvec_update_old(av, num_packets, seqno, state);
270                 }
271         }
272 }
273 
274 /**
275  * dccp_ackvec_clear_state  -  Perform house-keeping / garbage-collection
276  * This routine is called when the peer acknowledges the receipt of Ack Vectors
277  * up to and including @ackno. While based on on section A.3 of RFC 4340, here
278  * are additional precautions to prevent corrupted buffer state. In particular,
279  * we use tail_ackno to identify outdated records; it always marks the earliest
280  * packet of group (2) in 11.4.2.
281  */
282 void dccp_ackvec_clear_state(struct dccp_ackvec *av, const u64 ackno)
283 {
284         struct dccp_ackvec_record *avr, *next;
285         u8 runlen_now, eff_runlen;
286         s64 delta;
287 
288         avr = dccp_ackvec_lookup(&av->av_records, ackno);
289         if (avr == NULL)
290                 return;
291         /*
292          * Deal with outdated acknowledgments: this arises when e.g. there are
293          * several old records and the acks from the peer come in slowly. In
294          * that case we may still have records that pre-date tail_ackno.
295          */
296         delta = dccp_delta_seqno(av->av_tail_ackno, avr->avr_ack_ackno);
297         if (delta < 0)
298                 goto free_records;
299         /*
300          * Deal with overlapping Ack Vectors: don't subtract more than the
301          * number of packets between tail_ackno and ack_ackno.
302          */
303         eff_runlen = delta < avr->avr_ack_runlen ? delta : avr->avr_ack_runlen;
304 
305         runlen_now = dccp_ackvec_runlen(av->av_buf + avr->avr_ack_ptr);
306         /*
307          * The run length of Ack Vector cells does not decrease over time. If
308          * the run length is the same as at the time the Ack Vector was sent, we
309          * free the ack_ptr cell. That cell can however not be freed if the run
310          * length has increased: in this case we need to move the tail pointer
311          * backwards (towards higher indices), to its next-oldest neighbour.
312          */
313         if (runlen_now > eff_runlen) {
314 
315                 av->av_buf[avr->avr_ack_ptr] -= eff_runlen + 1;
316                 av->av_buf_tail = __ackvec_idx_add(avr->avr_ack_ptr, 1);
317 
318                 /* This move may not have cleared the overflow flag. */
319                 if (av->av_overflow)
320                         av->av_overflow = (av->av_buf_head == av->av_buf_tail);
321         } else {
322                 av->av_buf_tail = avr->avr_ack_ptr;
323                 /*
324                  * We have made sure that avr points to a valid cell within the
325                  * buffer. This cell is either older than head, or equals head
326                  * (empty buffer): in both cases we no longer have any overflow.
327                  */
328                 av->av_overflow = 0;
329         }
330 
331         /*
332          * The peer has acknowledged up to and including ack_ackno. Hence the
333          * first packet in group (2) of 11.4.2 is the successor of ack_ackno.
334          */
335         av->av_tail_ackno = ADD48(avr->avr_ack_ackno, 1);
336 
337 free_records:
338         list_for_each_entry_safe_from(avr, next, &av->av_records, avr_node) {
339                 list_del(&avr->avr_node);
340                 kmem_cache_free(dccp_ackvec_record_slab, avr);
341         }
342 }
343 
344 /*
345  *      Routines to keep track of Ack Vectors received in an skb
346  */
347 int dccp_ackvec_parsed_add(struct list_head *head, u8 *vec, u8 len, u8 nonce)
348 {
349         struct dccp_ackvec_parsed *new = kmalloc(sizeof(*new), GFP_ATOMIC);
350 
351         if (new == NULL)
352                 return -ENOBUFS;
353         new->vec   = vec;
354         new->len   = len;
355         new->nonce = nonce;
356 
357         list_add_tail(&new->node, head);
358         return 0;
359 }
360 EXPORT_SYMBOL_GPL(dccp_ackvec_parsed_add);
361 
362 void dccp_ackvec_parsed_cleanup(struct list_head *parsed_chunks)
363 {
364         struct dccp_ackvec_parsed *cur, *next;
365 
366         list_for_each_entry_safe(cur, next, parsed_chunks, node)
367                 kfree(cur);
368         INIT_LIST_HEAD(parsed_chunks);
369 }
370 EXPORT_SYMBOL_GPL(dccp_ackvec_parsed_cleanup);
371 
372 int __init dccp_ackvec_init(void)
373 {
374         dccp_ackvec_slab = kmem_cache_create("dccp_ackvec",
375                                              sizeof(struct dccp_ackvec), 0,
376                                              SLAB_HWCACHE_ALIGN, NULL);
377         if (dccp_ackvec_slab == NULL)
378                 goto out_err;
379 
380         dccp_ackvec_record_slab = kmem_cache_create("dccp_ackvec_record",
381                                              sizeof(struct dccp_ackvec_record),
382                                              0, SLAB_HWCACHE_ALIGN, NULL);
383         if (dccp_ackvec_record_slab == NULL)
384                 goto out_destroy_slab;
385 
386         return 0;
387 
388 out_destroy_slab:
389         kmem_cache_destroy(dccp_ackvec_slab);
390         dccp_ackvec_slab = NULL;
391 out_err:
392         DCCP_CRIT("Unable to create Ack Vector slab cache");
393         return -ENOBUFS;
394 }
395 
396 void dccp_ackvec_exit(void)
397 {
398         kmem_cache_destroy(dccp_ackvec_slab);
399         dccp_ackvec_slab = NULL;
400         kmem_cache_destroy(dccp_ackvec_record_slab);
401         dccp_ackvec_record_slab = NULL;
402 }
403 

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