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Linux/kernel/bpf/lpm_trie.c

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  1 /*
  2  * Longest prefix match list implementation
  3  *
  4  * Copyright (c) 2016,2017 Daniel Mack
  5  * Copyright (c) 2016 David Herrmann
  6  *
  7  * This file is subject to the terms and conditions of version 2 of the GNU
  8  * General Public License.  See the file COPYING in the main directory of the
  9  * Linux distribution for more details.
 10  */
 11 
 12 #include <linux/bpf.h>
 13 #include <linux/err.h>
 14 #include <linux/slab.h>
 15 #include <linux/spinlock.h>
 16 #include <linux/vmalloc.h>
 17 #include <net/ipv6.h>
 18 
 19 /* Intermediate node */
 20 #define LPM_TREE_NODE_FLAG_IM BIT(0)
 21 
 22 struct lpm_trie_node;
 23 
 24 struct lpm_trie_node {
 25         struct rcu_head rcu;
 26         struct lpm_trie_node __rcu      *child[2];
 27         u32                             prefixlen;
 28         u32                             flags;
 29         u8                              data[0];
 30 };
 31 
 32 struct lpm_trie {
 33         struct bpf_map                  map;
 34         struct lpm_trie_node __rcu      *root;
 35         size_t                          n_entries;
 36         size_t                          max_prefixlen;
 37         size_t                          data_size;
 38         raw_spinlock_t                  lock;
 39 };
 40 
 41 /* This trie implements a longest prefix match algorithm that can be used to
 42  * match IP addresses to a stored set of ranges.
 43  *
 44  * Data stored in @data of struct bpf_lpm_key and struct lpm_trie_node is
 45  * interpreted as big endian, so data[0] stores the most significant byte.
 46  *
 47  * Match ranges are internally stored in instances of struct lpm_trie_node
 48  * which each contain their prefix length as well as two pointers that may
 49  * lead to more nodes containing more specific matches. Each node also stores
 50  * a value that is defined by and returned to userspace via the update_elem
 51  * and lookup functions.
 52  *
 53  * For instance, let's start with a trie that was created with a prefix length
 54  * of 32, so it can be used for IPv4 addresses, and one single element that
 55  * matches 192.168.0.0/16. The data array would hence contain
 56  * [0xc0, 0xa8, 0x00, 0x00] in big-endian notation. This documentation will
 57  * stick to IP-address notation for readability though.
 58  *
 59  * As the trie is empty initially, the new node (1) will be places as root
 60  * node, denoted as (R) in the example below. As there are no other node, both
 61  * child pointers are %NULL.
 62  *
 63  *              +----------------+
 64  *              |       (1)  (R) |
 65  *              | 192.168.0.0/16 |
 66  *              |    value: 1    |
 67  *              |   [0]    [1]   |
 68  *              +----------------+
 69  *
 70  * Next, let's add a new node (2) matching 192.168.0.0/24. As there is already
 71  * a node with the same data and a smaller prefix (ie, a less specific one),
 72  * node (2) will become a child of (1). In child index depends on the next bit
 73  * that is outside of what (1) matches, and that bit is 0, so (2) will be
 74  * child[0] of (1):
 75  *
 76  *              +----------------+
 77  *              |       (1)  (R) |
 78  *              | 192.168.0.0/16 |
 79  *              |    value: 1    |
 80  *              |   [0]    [1]   |
 81  *              +----------------+
 82  *                   |
 83  *    +----------------+
 84  *    |       (2)      |
 85  *    | 192.168.0.0/24 |
 86  *    |    value: 2    |
 87  *    |   [0]    [1]   |
 88  *    +----------------+
 89  *
 90  * The child[1] slot of (1) could be filled with another node which has bit #17
 91  * (the next bit after the ones that (1) matches on) set to 1. For instance,
 92  * 192.168.128.0/24:
 93  *
 94  *              +----------------+
 95  *              |       (1)  (R) |
 96  *              | 192.168.0.0/16 |
 97  *              |    value: 1    |
 98  *              |   [0]    [1]   |
 99  *              +----------------+
100  *                   |      |
101  *    +----------------+  +------------------+
102  *    |       (2)      |  |        (3)       |
103  *    | 192.168.0.0/24 |  | 192.168.128.0/24 |
104  *    |    value: 2    |  |     value: 3     |
105  *    |   [0]    [1]   |  |    [0]    [1]    |
106  *    +----------------+  +------------------+
107  *
108  * Let's add another node (4) to the game for 192.168.1.0/24. In order to place
109  * it, node (1) is looked at first, and because (4) of the semantics laid out
110  * above (bit #17 is 0), it would normally be attached to (1) as child[0].
111  * However, that slot is already allocated, so a new node is needed in between.
112  * That node does not have a value attached to it and it will never be
113  * returned to users as result of a lookup. It is only there to differentiate
114  * the traversal further. It will get a prefix as wide as necessary to
115  * distinguish its two children:
116  *
117  *                      +----------------+
118  *                      |       (1)  (R) |
119  *                      | 192.168.0.0/16 |
120  *                      |    value: 1    |
121  *                      |   [0]    [1]   |
122  *                      +----------------+
123  *                           |      |
124  *            +----------------+  +------------------+
125  *            |       (4)  (I) |  |        (3)       |
126  *            | 192.168.0.0/23 |  | 192.168.128.0/24 |
127  *            |    value: ---  |  |     value: 3     |
128  *            |   [0]    [1]   |  |    [0]    [1]    |
129  *            +----------------+  +------------------+
130  *                 |      |
131  *  +----------------+  +----------------+
132  *  |       (2)      |  |       (5)      |
133  *  | 192.168.0.0/24 |  | 192.168.1.0/24 |
134  *  |    value: 2    |  |     value: 5   |
135  *  |   [0]    [1]   |  |   [0]    [1]   |
136  *  +----------------+  +----------------+
137  *
138  * 192.168.1.1/32 would be a child of (5) etc.
139  *
140  * An intermediate node will be turned into a 'real' node on demand. In the
141  * example above, (4) would be re-used if 192.168.0.0/23 is added to the trie.
142  *
143  * A fully populated trie would have a height of 32 nodes, as the trie was
144  * created with a prefix length of 32.
145  *
146  * The lookup starts at the root node. If the current node matches and if there
147  * is a child that can be used to become more specific, the trie is traversed
148  * downwards. The last node in the traversal that is a non-intermediate one is
149  * returned.
150  */
151 
152 static inline int extract_bit(const u8 *data, size_t index)
153 {
154         return !!(data[index / 8] & (1 << (7 - (index % 8))));
155 }
156 
157 /**
158  * longest_prefix_match() - determine the longest prefix
159  * @trie:       The trie to get internal sizes from
160  * @node:       The node to operate on
161  * @key:        The key to compare to @node
162  *
163  * Determine the longest prefix of @node that matches the bits in @key.
164  */
165 static size_t longest_prefix_match(const struct lpm_trie *trie,
166                                    const struct lpm_trie_node *node,
167                                    const struct bpf_lpm_trie_key *key)
168 {
169         size_t prefixlen = 0;
170         size_t i;
171 
172         for (i = 0; i < trie->data_size; i++) {
173                 size_t b;
174 
175                 b = 8 - fls(node->data[i] ^ key->data[i]);
176                 prefixlen += b;
177 
178                 if (prefixlen >= node->prefixlen || prefixlen >= key->prefixlen)
179                         return min(node->prefixlen, key->prefixlen);
180 
181                 if (b < 8)
182                         break;
183         }
184 
185         return prefixlen;
186 }
187 
188 /* Called from syscall or from eBPF program */
189 static void *trie_lookup_elem(struct bpf_map *map, void *_key)
190 {
191         struct lpm_trie *trie = container_of(map, struct lpm_trie, map);
192         struct lpm_trie_node *node, *found = NULL;
193         struct bpf_lpm_trie_key *key = _key;
194 
195         /* Start walking the trie from the root node ... */
196 
197         for (node = rcu_dereference(trie->root); node;) {
198                 unsigned int next_bit;
199                 size_t matchlen;
200 
201                 /* Determine the longest prefix of @node that matches @key.
202                  * If it's the maximum possible prefix for this trie, we have
203                  * an exact match and can return it directly.
204                  */
205                 matchlen = longest_prefix_match(trie, node, key);
206                 if (matchlen == trie->max_prefixlen) {
207                         found = node;
208                         break;
209                 }
210 
211                 /* If the number of bits that match is smaller than the prefix
212                  * length of @node, bail out and return the node we have seen
213                  * last in the traversal (ie, the parent).
214                  */
215                 if (matchlen < node->prefixlen)
216                         break;
217 
218                 /* Consider this node as return candidate unless it is an
219                  * artificially added intermediate one.
220                  */
221                 if (!(node->flags & LPM_TREE_NODE_FLAG_IM))
222                         found = node;
223 
224                 /* If the node match is fully satisfied, let's see if we can
225                  * become more specific. Determine the next bit in the key and
226                  * traverse down.
227                  */
228                 next_bit = extract_bit(key->data, node->prefixlen);
229                 node = rcu_dereference(node->child[next_bit]);
230         }
231 
232         if (!found)
233                 return NULL;
234 
235         return found->data + trie->data_size;
236 }
237 
238 static struct lpm_trie_node *lpm_trie_node_alloc(const struct lpm_trie *trie,
239                                                  const void *value)
240 {
241         struct lpm_trie_node *node;
242         size_t size = sizeof(struct lpm_trie_node) + trie->data_size;
243 
244         if (value)
245                 size += trie->map.value_size;
246 
247         node = kmalloc(size, GFP_ATOMIC | __GFP_NOWARN);
248         if (!node)
249                 return NULL;
250 
251         node->flags = 0;
252 
253         if (value)
254                 memcpy(node->data + trie->data_size, value,
255                        trie->map.value_size);
256 
257         return node;
258 }
259 
260 /* Called from syscall or from eBPF program */
261 static int trie_update_elem(struct bpf_map *map,
262                             void *_key, void *value, u64 flags)
263 {
264         struct lpm_trie *trie = container_of(map, struct lpm_trie, map);
265         struct lpm_trie_node *node, *im_node = NULL, *new_node = NULL;
266         struct lpm_trie_node __rcu **slot;
267         struct bpf_lpm_trie_key *key = _key;
268         unsigned long irq_flags;
269         unsigned int next_bit;
270         size_t matchlen = 0;
271         int ret = 0;
272 
273         if (unlikely(flags > BPF_EXIST))
274                 return -EINVAL;
275 
276         if (key->prefixlen > trie->max_prefixlen)
277                 return -EINVAL;
278 
279         raw_spin_lock_irqsave(&trie->lock, irq_flags);
280 
281         /* Allocate and fill a new node */
282 
283         if (trie->n_entries == trie->map.max_entries) {
284                 ret = -ENOSPC;
285                 goto out;
286         }
287 
288         new_node = lpm_trie_node_alloc(trie, value);
289         if (!new_node) {
290                 ret = -ENOMEM;
291                 goto out;
292         }
293 
294         trie->n_entries++;
295 
296         new_node->prefixlen = key->prefixlen;
297         RCU_INIT_POINTER(new_node->child[0], NULL);
298         RCU_INIT_POINTER(new_node->child[1], NULL);
299         memcpy(new_node->data, key->data, trie->data_size);
300 
301         /* Now find a slot to attach the new node. To do that, walk the tree
302          * from the root and match as many bits as possible for each node until
303          * we either find an empty slot or a slot that needs to be replaced by
304          * an intermediate node.
305          */
306         slot = &trie->root;
307 
308         while ((node = rcu_dereference_protected(*slot,
309                                         lockdep_is_held(&trie->lock)))) {
310                 matchlen = longest_prefix_match(trie, node, key);
311 
312                 if (node->prefixlen != matchlen ||
313                     node->prefixlen == key->prefixlen ||
314                     node->prefixlen == trie->max_prefixlen)
315                         break;
316 
317                 next_bit = extract_bit(key->data, node->prefixlen);
318                 slot = &node->child[next_bit];
319         }
320 
321         /* If the slot is empty (a free child pointer or an empty root),
322          * simply assign the @new_node to that slot and be done.
323          */
324         if (!node) {
325                 rcu_assign_pointer(*slot, new_node);
326                 goto out;
327         }
328 
329         /* If the slot we picked already exists, replace it with @new_node
330          * which already has the correct data array set.
331          */
332         if (node->prefixlen == matchlen) {
333                 new_node->child[0] = node->child[0];
334                 new_node->child[1] = node->child[1];
335 
336                 if (!(node->flags & LPM_TREE_NODE_FLAG_IM))
337                         trie->n_entries--;
338 
339                 rcu_assign_pointer(*slot, new_node);
340                 kfree_rcu(node, rcu);
341 
342                 goto out;
343         }
344 
345         /* If the new node matches the prefix completely, it must be inserted
346          * as an ancestor. Simply insert it between @node and *@slot.
347          */
348         if (matchlen == key->prefixlen) {
349                 next_bit = extract_bit(node->data, matchlen);
350                 rcu_assign_pointer(new_node->child[next_bit], node);
351                 rcu_assign_pointer(*slot, new_node);
352                 goto out;
353         }
354 
355         im_node = lpm_trie_node_alloc(trie, NULL);
356         if (!im_node) {
357                 ret = -ENOMEM;
358                 goto out;
359         }
360 
361         im_node->prefixlen = matchlen;
362         im_node->flags |= LPM_TREE_NODE_FLAG_IM;
363         memcpy(im_node->data, node->data, trie->data_size);
364 
365         /* Now determine which child to install in which slot */
366         if (extract_bit(key->data, matchlen)) {
367                 rcu_assign_pointer(im_node->child[0], node);
368                 rcu_assign_pointer(im_node->child[1], new_node);
369         } else {
370                 rcu_assign_pointer(im_node->child[0], new_node);
371                 rcu_assign_pointer(im_node->child[1], node);
372         }
373 
374         /* Finally, assign the intermediate node to the determined spot */
375         rcu_assign_pointer(*slot, im_node);
376 
377 out:
378         if (ret) {
379                 if (new_node)
380                         trie->n_entries--;
381 
382                 kfree(new_node);
383                 kfree(im_node);
384         }
385 
386         raw_spin_unlock_irqrestore(&trie->lock, irq_flags);
387 
388         return ret;
389 }
390 
391 static int trie_delete_elem(struct bpf_map *map, void *key)
392 {
393         /* TODO */
394         return -ENOSYS;
395 }
396 
397 #define LPM_DATA_SIZE_MAX       256
398 #define LPM_DATA_SIZE_MIN       1
399 
400 #define LPM_VAL_SIZE_MAX        (KMALLOC_MAX_SIZE - LPM_DATA_SIZE_MAX - \
401                                  sizeof(struct lpm_trie_node))
402 #define LPM_VAL_SIZE_MIN        1
403 
404 #define LPM_KEY_SIZE(X)         (sizeof(struct bpf_lpm_trie_key) + (X))
405 #define LPM_KEY_SIZE_MAX        LPM_KEY_SIZE(LPM_DATA_SIZE_MAX)
406 #define LPM_KEY_SIZE_MIN        LPM_KEY_SIZE(LPM_DATA_SIZE_MIN)
407 
408 static struct bpf_map *trie_alloc(union bpf_attr *attr)
409 {
410         struct lpm_trie *trie;
411         u64 cost = sizeof(*trie), cost_per_node;
412         int ret;
413 
414         if (!capable(CAP_SYS_ADMIN))
415                 return ERR_PTR(-EPERM);
416 
417         /* check sanity of attributes */
418         if (attr->max_entries == 0 ||
419             attr->map_flags != BPF_F_NO_PREALLOC ||
420             attr->key_size < LPM_KEY_SIZE_MIN ||
421             attr->key_size > LPM_KEY_SIZE_MAX ||
422             attr->value_size < LPM_VAL_SIZE_MIN ||
423             attr->value_size > LPM_VAL_SIZE_MAX)
424                 return ERR_PTR(-EINVAL);
425 
426         trie = kzalloc(sizeof(*trie), GFP_USER | __GFP_NOWARN);
427         if (!trie)
428                 return ERR_PTR(-ENOMEM);
429 
430         /* copy mandatory map attributes */
431         trie->map.map_type = attr->map_type;
432         trie->map.key_size = attr->key_size;
433         trie->map.value_size = attr->value_size;
434         trie->map.max_entries = attr->max_entries;
435         trie->map.map_flags = attr->map_flags;
436         trie->data_size = attr->key_size -
437                           offsetof(struct bpf_lpm_trie_key, data);
438         trie->max_prefixlen = trie->data_size * 8;
439 
440         cost_per_node = sizeof(struct lpm_trie_node) +
441                         attr->value_size + trie->data_size;
442         cost += (u64) attr->max_entries * cost_per_node;
443         if (cost >= U32_MAX - PAGE_SIZE) {
444                 ret = -E2BIG;
445                 goto out_err;
446         }
447 
448         trie->map.pages = round_up(cost, PAGE_SIZE) >> PAGE_SHIFT;
449 
450         ret = bpf_map_precharge_memlock(trie->map.pages);
451         if (ret)
452                 goto out_err;
453 
454         raw_spin_lock_init(&trie->lock);
455 
456         return &trie->map;
457 out_err:
458         kfree(trie);
459         return ERR_PTR(ret);
460 }
461 
462 static void trie_free(struct bpf_map *map)
463 {
464         struct lpm_trie *trie = container_of(map, struct lpm_trie, map);
465         struct lpm_trie_node __rcu **slot;
466         struct lpm_trie_node *node;
467 
468         raw_spin_lock(&trie->lock);
469 
470         /* Always start at the root and walk down to a node that has no
471          * children. Then free that node, nullify its reference in the parent
472          * and start over.
473          */
474 
475         for (;;) {
476                 slot = &trie->root;
477 
478                 for (;;) {
479                         node = rcu_dereference_protected(*slot,
480                                         lockdep_is_held(&trie->lock));
481                         if (!node)
482                                 goto unlock;
483 
484                         if (rcu_access_pointer(node->child[0])) {
485                                 slot = &node->child[0];
486                                 continue;
487                         }
488 
489                         if (rcu_access_pointer(node->child[1])) {
490                                 slot = &node->child[1];
491                                 continue;
492                         }
493 
494                         kfree(node);
495                         RCU_INIT_POINTER(*slot, NULL);
496                         break;
497                 }
498         }
499 
500 unlock:
501         raw_spin_unlock(&trie->lock);
502 }
503 
504 static int trie_get_next_key(struct bpf_map *map, void *key, void *next_key)
505 {
506         return -ENOTSUPP;
507 }
508 
509 const struct bpf_map_ops trie_map_ops = {
510         .map_alloc = trie_alloc,
511         .map_free = trie_free,
512         .map_get_next_key = trie_get_next_key,
513         .map_lookup_elem = trie_lookup_elem,
514         .map_update_elem = trie_update_elem,
515         .map_delete_elem = trie_delete_elem,
516 };
517 

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