net/ipv4/inetpeer.c

   1 /*
   2  *              INETPEER - A storage for permanent information about peers
   3  *
   4  *  This source is covered by the GNU GPL, the same as all kernel sources.
   5  *
   6  *  Authors:    Andrey V. Savochkin <saw@msu.ru>
   7  */
   8
   9 #include <linux/module.h>
  10 #include <linux/types.h>
  11 #include <linux/slab.h>
  12 #include <linux/interrupt.h>
  13 #include <linux/spinlock.h>
  14 #include <linux/random.h>
  15 #include <linux/timer.h>
  16 #include <linux/time.h>
  17 #include <linux/kernel.h>
  18 #include <linux/mm.h>
  19 #include <linux/net.h>
  20 #include <net/ip.h>
  21 #include <net/inetpeer.h>
  22
  23 /*
  24  *  Theory of operations.
  25  *  We keep one entry for each peer IP address.  The nodes contains long-living
  26  *  information about the peer which doesn't depend on routes.
  27  *  At this moment this information consists only of ID field for the next
  28  *  outgoing IP packet.  This field is incremented with each packet as encoded
  29  *  in inet_getid() function (include/net/inetpeer.h).
  30  *  At the moment of writing this notes identifier of IP packets is generated
  31  *  to be unpredictable using this code only for packets subjected
  32  *  (actually or potentially) to defragmentation.  I.e. DF packets less than
  33  *  PMTU in size uses a constant ID and do not use this code (see
  34  *  ip_select_ident() in include/net/ip.h).
  35  *
  36  *  Route cache entries hold references to our nodes.
  37  *  New cache entries get references via lookup by destination IP address in
  38  *  the avl tree.  The reference is grabbed only when it's needed i.e. only
  39  *  when we try to output IP packet which needs an unpredictable ID (see
  40  *  __ip_select_ident() in net/ipv4/route.c).
  41  *  Nodes are removed only when reference counter goes to 0.
  42  *  When it's happened the node may be removed when a sufficient amount of
  43  *  time has been passed since its last use.  The less-recently-used entry can
  44  *  also be removed if the pool is overloaded i.e. if the total amount of
  45  *  entries is greater-or-equal than the threshold.
  46  *
  47  *  Node pool is organised as an AVL tree.
  48  *  Such an implementation has been chosen not just for fun.  It's a way to
  49  *  prevent easy and efficient DoS attacks by creating hash collisions.  A huge
  50  *  amount of long living nodes in a single hash slot would significantly delay
  51  *  lookups performed with disabled BHs.
  52  *
  53  *  Serialisation issues.
  54  *  1.  Nodes may appear in the tree only with the pool lock held.
  55  *  2.  Nodes may disappear from the tree only with the pool lock held
  56  *      AND reference count being 0.
  57  *  3.  Nodes appears and disappears from unused node list only under
  58  *      "inet_peer_unused_lock".
  59  *  4.  Global variable peer_total is modified under the pool lock.
  60  *  5.  struct inet_peer fields modification:
  61  *              avl_left, avl_right, avl_parent, avl_height: pool lock
  62  *              unused: unused node list lock
  63  *              refcnt: atomically against modifications on other CPU;
  64  *                 usually under some other lock to prevent node disappearing
  65  *              dtime: unused node list lock
  66  *              v4daddr: unchangeable
  67  *              ip_id_count: idlock
  68  */
  69
  70 static struct kmem_cache *peer_cachep __read_mostly;
  71
  72 #define node_height(x) x->avl_height
  73
  74 #define peer_avl_empty ((struct inet_peer *)&peer_fake_node)
  75 static const struct inet_peer peer_fake_node = {
  76         .avl_left       = peer_avl_empty,
  77         .avl_right      = peer_avl_empty,
  78         .avl_height     = 0
  79 };
  80
  81 static struct {
  82         struct inet_peer *root;
  83         spinlock_t      lock;
  84         int             total;
  85 } peers = {
  86         .root           = peer_avl_empty,
  87         .lock           = __SPIN_LOCK_UNLOCKED(peers.lock),
  88         .total          = 0,
  89 };
  90 #define PEER_MAXDEPTH 40 /* sufficient for about 2^27 nodes */
  91
  92 /* Exported for sysctl_net_ipv4.  */
  93 int inet_peer_threshold __read_mostly = 65536 + 128;    /* start to throw entries more
  94                                          * aggressively at this stage */
  95 int inet_peer_minttl __read_mostly = 120 * HZ;  /* TTL under high load: 120 sec */
  96 int inet_peer_maxttl __read_mostly = 10 * 60 * HZ;      /* usual time to live: 10 min */
  97 int inet_peer_gc_mintime __read_mostly = 10 * HZ;
  98 int inet_peer_gc_maxtime __read_mostly = 120 * HZ;
  99
 100 static struct {
 101         struct list_head        list;
 102         spinlock_t              lock;
 103 } unused_peers = {
 104         .list                   = LIST_HEAD_INIT(unused_peers.list),
 105         .lock                   = __SPIN_LOCK_UNLOCKED(unused_peers.lock),
 106 };
 107
 108 static void peer_check_expire(unsigned long dummy);
 109 static DEFINE_TIMER(peer_periodic_timer, peer_check_expire, 0, 0);
 110
 111
 112 /* Called from ip_output.c:ip_init  */
 113 void __init inet_initpeers(void)
 114 {
 115         struct sysinfo si;
 116
 117         /* Use the straight interface to information about memory. */
 118         si_meminfo(&si);
 119         /* The values below were suggested by Alexey Kuznetsov
 120          * <kuznet@ms2.inr.ac.ru>.  I don't have any opinion about the values
 121          * myself.  --SAW
 122          */
 123         if (si.totalram <= (32768*1024)/PAGE_SIZE)
 124                 inet_peer_threshold >>= 1; /* max pool size about 1MB on IA32 */
 125         if (si.totalram <= (16384*1024)/PAGE_SIZE)
 126                 inet_peer_threshold >>= 1; /* about 512KB */
 127         if (si.totalram <= (8192*1024)/PAGE_SIZE)
 128                 inet_peer_threshold >>= 2; /* about 128KB */
 129
 130         peer_cachep = kmem_cache_create("inet_peer_cache",
 131                         sizeof(struct inet_peer),
 132                         0, SLAB_PANIC,
 133                         NULL);
 134
 135         /* All the timers, started at system startup tend
 136            to synchronize. Perturb it a bit.
 137          */
 138         peer_periodic_timer.expires = jiffies
 139                 + net_random() % inet_peer_gc_maxtime
 140                 + inet_peer_gc_maxtime;
 141         add_timer(&peer_periodic_timer);
 142 }
 143
 144 /* Called with or without local BH being disabled. */
 145 static void unlink_from_unused(struct inet_peer *p)
 146 {
 147         if (!list_empty(&p->unused)) {
 148                 spin_lock_bh(&unused_peers.lock);
 149                 list_del_init(&p->unused);
 150                 spin_unlock_bh(&unused_peers.lock);
 151         }
 152 }
 153
 154 /*
 155  * Called with local BH disabled and the pool lock held.
 156  */
 157 #define lookup(_daddr, _stack)                                  \
 158 ({                                                              \
 159         struct inet_peer *u, **v;                               \
 160                                                                 \
 161         stackptr = _stack;                                      \
 162         *stackptr++ = &peers.root;                              \
 163         for (u = peers.root; u != peer_avl_empty; ) {           \
 164                 if (_daddr == u->v4daddr)                       \
 165                         break;                                  \
 166                 if ((__force __u32)_daddr < (__force __u32)u->v4daddr)  \
 167                         v = &u->avl_left;                       \
 168                 else                                            \
 169                         v = &u->avl_right;                      \
 170                 *stackptr++ = v;                                \
 171                 u = *v;                                         \
 172         }                                                       \
 173         u;                                                      \
 174 })
 175
 176 /*
 177  * Called with rcu_read_lock_bh()
 178  * Because we hold no lock against a writer, its quite possible we fall
 179  * in an endless loop.
 180  * But every pointer we follow is guaranteed to be valid thanks to RCU.
 181  * We exit from this function if number of links exceeds PEER_MAXDEPTH
 182  */
 183 static struct inet_peer *lookup_rcu_bh(__be32 daddr)
 184 {
 185         struct inet_peer *u = rcu_dereference_bh(peers.root);
 186         int count = 0;
 187
 188         while (u != peer_avl_empty) {
 189                 if (daddr == u->v4daddr) {
 190                         if (unlikely(!atomic_inc_not_zero(&u->refcnt)))
 191                                 u = NULL;
 192                         return u;
 193                 }
 194                 if ((__force __u32)daddr < (__force __u32)u->v4daddr)
 195                         u = rcu_dereference_bh(u->avl_left);
 196                 else
 197                         u = rcu_dereference_bh(u->avl_right);
 198                 if (unlikely(++count == PEER_MAXDEPTH))
 199                         break;
 200         }
 201         return NULL;
 202 }
 203
 204 /* Called with local BH disabled and the pool lock held. */
 205 #define lookup_rightempty(start)                                \
 206 ({                                                              \
 207         struct inet_peer *u, **v;                               \
 208         *stackptr++ = &start->avl_left;                         \
 209         v = &start->avl_left;                                   \
 210         for (u = *v; u->avl_right != peer_avl_empty; ) {        \
 211                 v = &u->avl_right;                              \
 212                 *stackptr++ = v;                                \
 213                 u = *v;                                         \
 214         }                                                       \
 215         u;                                                      \
 216 })
 217
 218 /* Called with local BH disabled and the pool lock held.
 219  * Variable names are the proof of operation correctness.
 220  * Look into mm/map_avl.c for more detail description of the ideas.
 221  */
 222 static void peer_avl_rebalance(struct inet_peer **stack[],
 223                 struct inet_peer ***stackend)
 224 {
 225         struct inet_peer **nodep, *node, *l, *r;
 226         int lh, rh;
 227
 228         while (stackend > stack) {
 229                 nodep = *--stackend;
 230                 node = *nodep;
 231                 l = node->avl_left;
 232                 r = node->avl_right;
 233                 lh = node_height(l);
 234                 rh = node_height(r);
 235                 if (lh > rh + 1) { /* l: RH+2 */
 236                         struct inet_peer *ll, *lr, *lrl, *lrr;
 237                         int lrh;
 238                         ll = l->avl_left;
 239                         lr = l->avl_right;
 240                         lrh = node_height(lr);
 241                         if (lrh <= node_height(ll)) {   /* ll: RH+1 */
 242                                 node->avl_left = lr;    /* lr: RH or RH+1 */
 243                                 node->avl_right = r;    /* r: RH */
 244                                 node->avl_height = lrh + 1; /* RH+1 or RH+2 */
 245                                 l->avl_left = ll;       /* ll: RH+1 */
 246                                 l->avl_right = node;    /* node: RH+1 or RH+2 */
 247                                 l->avl_height = node->avl_height + 1;
 248                                 *nodep = l;
 249                         } else { /* ll: RH, lr: RH+1 */
 250                                 lrl = lr->avl_left;     /* lrl: RH or RH-1 */
 251                                 lrr = lr->avl_right;    /* lrr: RH or RH-1 */
 252                                 node->avl_left = lrr;   /* lrr: RH or RH-1 */
 253                                 node->avl_right = r;    /* r: RH */
 254                                 node->avl_height = rh + 1; /* node: RH+1 */
 255                                 l->avl_left = ll;       /* ll: RH */
 256                                 l->avl_right = lrl;     /* lrl: RH or RH-1 */
 257                                 l->avl_height = rh + 1; /* l: RH+1 */
 258                                 lr->avl_left = l;       /* l: RH+1 */
 259                                 lr->avl_right = node;   /* node: RH+1 */
 260                                 lr->avl_height = rh + 2;
 261                                 *nodep = lr;
 262                         }
 263                 } else if (rh > lh + 1) { /* r: LH+2 */
 264                         struct inet_peer *rr, *rl, *rlr, *rll;
 265                         int rlh;
 266                         rr = r->avl_right;
 267                         rl = r->avl_left;
 268                         rlh = node_height(rl);
 269                         if (rlh <= node_height(rr)) {   /* rr: LH+1 */
 270                                 node->avl_right = rl;   /* rl: LH or LH+1 */
 271                                 node->avl_left = l;     /* l: LH */
 272                                 node->avl_height = rlh + 1; /* LH+1 or LH+2 */
 273                                 r->avl_right = rr;      /* rr: LH+1 */
 274                                 r->avl_left = node;     /* node: LH+1 or LH+2 */
 275                                 r->avl_height = node->avl_height + 1;
 276                                 *nodep = r;
 277                         } else { /* rr: RH, rl: RH+1 */
 278                                 rlr = rl->avl_right;    /* rlr: LH or LH-1 */
 279                                 rll = rl->avl_left;     /* rll: LH or LH-1 */
 280                                 node->avl_right = rll;  /* rll: LH or LH-1 */
 281                                 node->avl_left = l;     /* l: LH */
 282                                 node->avl_height = lh + 1; /* node: LH+1 */
 283                                 r->avl_right = rr;      /* rr: LH */
 284                                 r->avl_left = rlr;      /* rlr: LH or LH-1 */
 285                                 r->avl_height = lh + 1; /* r: LH+1 */
 286                                 rl->avl_right = r;      /* r: LH+1 */
 287                                 rl->avl_left = node;    /* node: LH+1 */
 288                                 rl->avl_height = lh + 2;
 289                                 *nodep = rl;
 290                         }
 291                 } else {
 292                         node->avl_height = (lh > rh ? lh : rh) + 1;
 293                 }
 294         }
 295 }
 296
 297 /* Called with local BH disabled and the pool lock held. */
 298 #define link_to_pool(n)                                         \
 299 do {                                                            \
 300         n->avl_height = 1;                                      \
 301         n->avl_left = peer_avl_empty;                           \
 302         n->avl_right = peer_avl_empty;                          \
 303         smp_wmb(); /* lockless readers can catch us now */      \
 304         **--stackptr = n;                                       \
 305         peer_avl_rebalance(stack, stackptr);                    \
 306 } while (0)
 307
 308 static void inetpeer_free_rcu(struct rcu_head *head)
 309 {
 310         kmem_cache_free(peer_cachep, container_of(head, struct inet_peer, rcu));
 311 }
 312
 313 /* May be called with local BH enabled. */
 314 static void unlink_from_pool(struct inet_peer *p)
 315 {
 316         int do_free;
 317
 318         do_free = 0;
 319
 320         spin_lock_bh(&peers.lock);
 321         /* Check the reference counter.  It was artificially incremented by 1
 322          * in cleanup() function to prevent sudden disappearing.  If we can
 323          * atomically (because of lockless readers) take this last reference,
 324          * it's safe to remove the node and free it later.
 325          */
 326         if (atomic_cmpxchg(&p->refcnt, 1, 0) == 1) {
 327                 struct inet_peer **stack[PEER_MAXDEPTH];
 328                 struct inet_peer ***stackptr, ***delp;
 329                 if (lookup(p->v4daddr, stack) != p)
 330                         BUG();
 331                 delp = stackptr - 1; /* *delp[0] == p */
 332                 if (p->avl_left == peer_avl_empty) {
 333                         *delp[0] = p->avl_right;
 334                         --stackptr;
 335                 } else {
 336                         /* look for a node to insert instead of p */
 337                         struct inet_peer *t;
 338                         t = lookup_rightempty(p);
 339                         BUG_ON(*stackptr[-1] != t);
 340                         **--stackptr = t->avl_left;
 341                         /* t is removed, t->v4daddr > x->v4daddr for any
 342                          * x in p->avl_left subtree.
 343                          * Put t in the old place of p. */
 344                         *delp[0] = t;
 345                         t->avl_left = p->avl_left;
 346                         t->avl_right = p->avl_right;
 347                         t->avl_height = p->avl_height;
 348                         BUG_ON(delp[1] != &p->avl_left);
 349                         delp[1] = &t->avl_left; /* was &p->avl_left */
 350                 }
 351                 peer_avl_rebalance(stack, stackptr);
 352                 peers.total--;
 353                 do_free = 1;
 354         }
 355         spin_unlock_bh(&peers.lock);
 356
 357         if (do_free)
 358                 call_rcu_bh(&p->rcu, inetpeer_free_rcu);
 359         else
 360                 /* The node is used again.  Decrease the reference counter
 361                  * back.  The loop "cleanup -> unlink_from_unused
 362                  *   -> unlink_from_pool -> putpeer -> link_to_unused
 363                  *   -> cleanup (for the same node)"
 364                  * doesn't really exist because the entry will have a
 365                  * recent deletion time and will not be cleaned again soon.
 366                  */
 367                 inet_putpeer(p);
 368 }
 369
 370 /* May be called with local BH enabled. */
 371 static int cleanup_once(unsigned long ttl)
 372 {
 373         struct inet_peer *p = NULL;
 374
 375         /* Remove the first entry from the list of unused nodes. */
 376         spin_lock_bh(&unused_peers.lock);
 377         if (!list_empty(&unused_peers.list)) {
 378                 __u32 delta;
 379
 380                 p = list_first_entry(&unused_peers.list, struct inet_peer, unused);
 381                 delta = (__u32)jiffies - p->dtime;
 382
 383                 if (delta < ttl) {
 384                         /* Do not prune fresh entries. */
 385                         spin_unlock_bh(&unused_peers.lock);
 386                         return -1;
 387                 }
 388
 389                 list_del_init(&p->unused);
 390
 391                 /* Grab an extra reference to prevent node disappearing
 392                  * before unlink_from_pool() call. */
 393                 atomic_inc(&p->refcnt);
 394         }
 395         spin_unlock_bh(&unused_peers.lock);
 396
 397         if (p == NULL)
 398                 /* It means that the total number of USED entries has
 399                  * grown over inet_peer_threshold.  It shouldn't really
 400                  * happen because of entry limits in route cache. */
 401                 return -1;
 402
 403         unlink_from_pool(p);
 404         return 0;
 405 }
 406
 407 /* Called with or without local BH being disabled. */
 408 struct inet_peer *inet_getpeer(__be32 daddr, int create)
 409 {
 410         struct inet_peer *p;
 411         struct inet_peer **stack[PEER_MAXDEPTH], ***stackptr;
 412
 413         /* Look up for the address quickly, lockless.
 414          * Because of a concurrent writer, we might not find an existing entry.
 415          */
 416         rcu_read_lock_bh();
 417         p = lookup_rcu_bh(daddr);
 418         rcu_read_unlock_bh();
 419
 420         if (p) {
 421                 /* The existing node has been found.
 422                  * Remove the entry from unused list if it was there.
 423                  */
 424                 unlink_from_unused(p);
 425                 return p;
 426         }
 427
 428         /* retry an exact lookup, taking the lock before.
 429          * At least, nodes should be hot in our cache.
 430          */
 431         spin_lock_bh(&peers.lock);
 432         p = lookup(daddr, stack);
 433         if (p != peer_avl_empty) {
 434                 atomic_inc(&p->refcnt);
 435                 spin_unlock_bh(&peers.lock);
 436                 /* Remove the entry from unused list if it was there. */
 437                 unlink_from_unused(p);
 438                 return p;
 439         }
 440         p = create ? kmem_cache_alloc(peer_cachep, GFP_ATOMIC) : NULL;
 441         if (p) {
 442                 p->v4daddr = daddr;
 443                 atomic_set(&p->refcnt, 1);
 444                 atomic_set(&p->rid, 0);
 445                 atomic_set(&p->ip_id_count, secure_ip_id(daddr));
 446                 p->tcp_ts_stamp = 0;
 447                 INIT_LIST_HEAD(&p->unused);
 448
 449
 450                 /* Link the node. */
 451                 link_to_pool(p);
 452                 peers.total++;
 453         }
 454         spin_unlock_bh(&peers.lock);
 455
 456         if (peers.total >= inet_peer_threshold)
 457                 /* Remove one less-recently-used entry. */
 458                 cleanup_once(0);
 459
 460         return p;
 461 }
 462
 463 /* Called with local BH disabled. */
 464 static void peer_check_expire(unsigned long dummy)
 465 {
 466         unsigned long now = jiffies;
 467         int ttl;
 468
 469         if (peers.total >= inet_peer_threshold)
 470                 ttl = inet_peer_minttl;
 471         else
 472                 ttl = inet_peer_maxttl
 473                                 - (inet_peer_maxttl - inet_peer_minttl) / HZ *
 474                                         peers.total / inet_peer_threshold * HZ;
 475         while (!cleanup_once(ttl)) {
 476                 if (jiffies != now)
 477                         break;
 478         }
 479
 480         /* Trigger the timer after inet_peer_gc_mintime .. inet_peer_gc_maxtime
 481          * interval depending on the total number of entries (more entries,
 482          * less interval). */
 483         if (peers.total >= inet_peer_threshold)
 484                 peer_periodic_timer.expires = jiffies + inet_peer_gc_mintime;
 485         else
 486                 peer_periodic_timer.expires = jiffies
 487                         + inet_peer_gc_maxtime
 488                         - (inet_peer_gc_maxtime - inet_peer_gc_mintime) / HZ *
 489                                 peers.total / inet_peer_threshold * HZ;
 490         add_timer(&peer_periodic_timer);
 491 }
 492
 493 void inet_putpeer(struct inet_peer *p)
 494 {
 495         local_bh_disable();
 496
 497         if (atomic_dec_and_lock(&p->refcnt, &unused_peers.lock)) {
 498                 list_add_tail(&p->unused, &unused_peers.list);
 499                 p->dtime = (__u32)jiffies;
 500                 spin_unlock(&unused_peers.lock);
 501         }
 502
 503         local_bh_enable();
 504 }