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bpf: convert htab map to hlist_nulls
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when all map elements are pre-allocated one cpu can delete and reuse htab_elem
while another cpu is still walking the hlist. In such case the lookup may
miss the element. Convert hlist to hlist_nulls to avoid such scenario.
When bucket lock is taken there is no need to take such precautions,
so only convert map_lookup and map_get_next to nulls.
The race window is extremely small and only reproducible with explicit
udelay() inside lookup_nulls_elem_raw()

Similar to hlist add hlist_nulls_for_each_entry_safe() and
hlist_nulls_entry_safe() helpers.

Fixes: 6c90598 ("bpf: pre-allocate hash map elements")
Reported-by: Jonathan Perry <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
Acked-by: Daniel Borkmann <[email protected]>
Signed-off-by: David S. Miller <[email protected]>
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@4ast @davem330
4ast authored and davem330 committed Mar 9, 2017
1 parent 9f69154 commit 4fe8435
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Showing 3 changed files with 79 additions and 34 deletions.
5 changes: 5 additions & 0 deletions include/linux/list_nulls.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -29,6 +29,11 @@ struct hlist_nulls_node {
((ptr)->first = (struct hlist_nulls_node *) NULLS_MARKER(nulls))

#define hlist_nulls_entry(ptr, type, member) container_of(ptr,type,member)

#define hlist_nulls_entry_safe(ptr, type, member) \
({ typeof(ptr) ____ptr = (ptr); \
!is_a_nulls(____ptr) ? hlist_nulls_entry(____ptr, type, member) : NULL; \
})
/**
* ptr_is_a_nulls - Test if a ptr is a nulls
* @ptr: ptr to be tested
Expand Down
14 changes: 14 additions & 0 deletions include/linux/rculist_nulls.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -156,5 +156,19 @@ static inline void hlist_nulls_add_tail_rcu(struct hlist_nulls_node *n,
({ tpos = hlist_nulls_entry(pos, typeof(*tpos), member); 1; }); \
pos = rcu_dereference_raw(hlist_nulls_next_rcu(pos)))

/**
* hlist_nulls_for_each_entry_safe -
* iterate over list of given type safe against removal of list entry
* @tpos: the type * to use as a loop cursor.
* @pos: the &struct hlist_nulls_node to use as a loop cursor.
* @head: the head for your list.
* @member: the name of the hlist_nulls_node within the struct.
*/
#define hlist_nulls_for_each_entry_safe(tpos, pos, head, member) \
for (({barrier();}), \
pos = rcu_dereference_raw(hlist_nulls_first_rcu(head)); \
(!is_a_nulls(pos)) && \
({ tpos = hlist_nulls_entry(pos, typeof(*tpos), member); \
pos = rcu_dereference_raw(hlist_nulls_next_rcu(pos)); 1; });)
#endif
#endif
94 changes: 60 additions & 34 deletions kernel/bpf/hashtab.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -13,11 +13,12 @@
#include <linux/bpf.h>
#include <linux/jhash.h>
#include <linux/filter.h>
#include <linux/rculist_nulls.h>
#include "percpu_freelist.h"
#include "bpf_lru_list.h"

struct bucket {
struct hlist_head head;
struct hlist_nulls_head head;
raw_spinlock_t lock;
};

Expand All @@ -44,7 +45,7 @@ enum extra_elem_state {
/* each htab element is struct htab_elem + key + value */
struct htab_elem {
union {
struct hlist_node hash_node;
struct hlist_nulls_node hash_node;
struct {
void *padding;
union {
Expand Down Expand Up @@ -337,7 +338,7 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
goto free_htab;

for (i = 0; i < htab->n_buckets; i++) {
INIT_HLIST_HEAD(&htab->buckets[i].head);
INIT_HLIST_NULLS_HEAD(&htab->buckets[i].head, i);
raw_spin_lock_init(&htab->buckets[i].lock);
}

Expand Down Expand Up @@ -377,28 +378,52 @@ static inline struct bucket *__select_bucket(struct bpf_htab *htab, u32 hash)
return &htab->buckets[hash & (htab->n_buckets - 1)];
}

static inline struct hlist_head *select_bucket(struct bpf_htab *htab, u32 hash)
static inline struct hlist_nulls_head *select_bucket(struct bpf_htab *htab, u32 hash)
{
return &__select_bucket(htab, hash)->head;
}

static struct htab_elem *lookup_elem_raw(struct hlist_head *head, u32 hash,
/* this lookup function can only be called with bucket lock taken */
static struct htab_elem *lookup_elem_raw(struct hlist_nulls_head *head, u32 hash,
void *key, u32 key_size)
{
struct hlist_nulls_node *n;
struct htab_elem *l;

hlist_for_each_entry_rcu(l, head, hash_node)
hlist_nulls_for_each_entry_rcu(l, n, head, hash_node)
if (l->hash == hash && !memcmp(&l->key, key, key_size))
return l;

return NULL;
}

/* can be called without bucket lock. it will repeat the loop in
* the unlikely event when elements moved from one bucket into another
* while link list is being walked
*/
static struct htab_elem *lookup_nulls_elem_raw(struct hlist_nulls_head *head,
u32 hash, void *key,
u32 key_size, u32 n_buckets)
{
struct hlist_nulls_node *n;
struct htab_elem *l;

again:
hlist_nulls_for_each_entry_rcu(l, n, head, hash_node)
if (l->hash == hash && !memcmp(&l->key, key, key_size))
return l;

if (unlikely(get_nulls_value(n) != (hash & (n_buckets - 1))))
goto again;

return NULL;
}

/* Called from syscall or from eBPF program */
static void *__htab_map_lookup_elem(struct bpf_map *map, void *key)
{
struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
struct hlist_head *head;
struct hlist_nulls_head *head;
struct htab_elem *l;
u32 hash, key_size;

Expand All @@ -411,7 +436,7 @@ static void *__htab_map_lookup_elem(struct bpf_map *map, void *key)

head = select_bucket(htab, hash);

l = lookup_elem_raw(head, hash, key, key_size);
l = lookup_nulls_elem_raw(head, hash, key, key_size, htab->n_buckets);

return l;
}
Expand Down Expand Up @@ -444,8 +469,9 @@ static void *htab_lru_map_lookup_elem(struct bpf_map *map, void *key)
static bool htab_lru_map_delete_node(void *arg, struct bpf_lru_node *node)
{
struct bpf_htab *htab = (struct bpf_htab *)arg;
struct htab_elem *l, *tgt_l;
struct hlist_head *head;
struct htab_elem *l = NULL, *tgt_l;
struct hlist_nulls_head *head;
struct hlist_nulls_node *n;
unsigned long flags;
struct bucket *b;

Expand All @@ -455,9 +481,9 @@ static bool htab_lru_map_delete_node(void *arg, struct bpf_lru_node *node)

raw_spin_lock_irqsave(&b->lock, flags);

hlist_for_each_entry_rcu(l, head, hash_node)
hlist_nulls_for_each_entry_rcu(l, n, head, hash_node)
if (l == tgt_l) {
hlist_del_rcu(&l->hash_node);
hlist_nulls_del_rcu(&l->hash_node);
break;
}

Expand All @@ -470,7 +496,7 @@ static bool htab_lru_map_delete_node(void *arg, struct bpf_lru_node *node)
static int htab_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
{
struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
struct hlist_head *head;
struct hlist_nulls_head *head;
struct htab_elem *l, *next_l;
u32 hash, key_size;
int i;
Expand All @@ -484,15 +510,15 @@ static int htab_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
head = select_bucket(htab, hash);

/* lookup the key */
l = lookup_elem_raw(head, hash, key, key_size);
l = lookup_nulls_elem_raw(head, hash, key, key_size, htab->n_buckets);

if (!l) {
i = 0;
goto find_first_elem;
}

/* key was found, get next key in the same bucket */
next_l = hlist_entry_safe(rcu_dereference_raw(hlist_next_rcu(&l->hash_node)),
next_l = hlist_nulls_entry_safe(rcu_dereference_raw(hlist_nulls_next_rcu(&l->hash_node)),
struct htab_elem, hash_node);

if (next_l) {
Expand All @@ -511,7 +537,7 @@ static int htab_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
head = select_bucket(htab, i);

/* pick first element in the bucket */
next_l = hlist_entry_safe(rcu_dereference_raw(hlist_first_rcu(head)),
next_l = hlist_nulls_entry_safe(rcu_dereference_raw(hlist_nulls_first_rcu(head)),
struct htab_elem, hash_node);
if (next_l) {
/* if it's not empty, just return it */
Expand Down Expand Up @@ -676,7 +702,7 @@ static int htab_map_update_elem(struct bpf_map *map, void *key, void *value,
{
struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
struct htab_elem *l_new = NULL, *l_old;
struct hlist_head *head;
struct hlist_nulls_head *head;
unsigned long flags;
struct bucket *b;
u32 key_size, hash;
Expand Down Expand Up @@ -715,9 +741,9 @@ static int htab_map_update_elem(struct bpf_map *map, void *key, void *value,
/* add new element to the head of the list, so that
* concurrent search will find it before old elem
*/
hlist_add_head_rcu(&l_new->hash_node, head);
hlist_nulls_add_head_rcu(&l_new->hash_node, head);
if (l_old) {
hlist_del_rcu(&l_old->hash_node);
hlist_nulls_del_rcu(&l_old->hash_node);
free_htab_elem(htab, l_old);
}
ret = 0;
Expand All @@ -731,7 +757,7 @@ static int htab_lru_map_update_elem(struct bpf_map *map, void *key, void *value,
{
struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
struct htab_elem *l_new, *l_old = NULL;
struct hlist_head *head;
struct hlist_nulls_head *head;
unsigned long flags;
struct bucket *b;
u32 key_size, hash;
Expand Down Expand Up @@ -772,10 +798,10 @@ static int htab_lru_map_update_elem(struct bpf_map *map, void *key, void *value,
/* add new element to the head of the list, so that
* concurrent search will find it before old elem
*/
hlist_add_head_rcu(&l_new->hash_node, head);
hlist_nulls_add_head_rcu(&l_new->hash_node, head);
if (l_old) {
bpf_lru_node_set_ref(&l_new->lru_node);
hlist_del_rcu(&l_old->hash_node);
hlist_nulls_del_rcu(&l_old->hash_node);
}
ret = 0;

Expand All @@ -796,7 +822,7 @@ static int __htab_percpu_map_update_elem(struct bpf_map *map, void *key,
{
struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
struct htab_elem *l_new = NULL, *l_old;
struct hlist_head *head;
struct hlist_nulls_head *head;
unsigned long flags;
struct bucket *b;
u32 key_size, hash;
Expand Down Expand Up @@ -835,7 +861,7 @@ static int __htab_percpu_map_update_elem(struct bpf_map *map, void *key,
ret = PTR_ERR(l_new);
goto err;
}
hlist_add_head_rcu(&l_new->hash_node, head);
hlist_nulls_add_head_rcu(&l_new->hash_node, head);
}
ret = 0;
err:
Expand All @@ -849,7 +875,7 @@ static int __htab_lru_percpu_map_update_elem(struct bpf_map *map, void *key,
{
struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
struct htab_elem *l_new = NULL, *l_old;
struct hlist_head *head;
struct hlist_nulls_head *head;
unsigned long flags;
struct bucket *b;
u32 key_size, hash;
Expand Down Expand Up @@ -897,7 +923,7 @@ static int __htab_lru_percpu_map_update_elem(struct bpf_map *map, void *key,
} else {
pcpu_copy_value(htab, htab_elem_get_ptr(l_new, key_size),
value, onallcpus);
hlist_add_head_rcu(&l_new->hash_node, head);
hlist_nulls_add_head_rcu(&l_new->hash_node, head);
l_new = NULL;
}
ret = 0;
Expand Down Expand Up @@ -925,7 +951,7 @@ static int htab_lru_percpu_map_update_elem(struct bpf_map *map, void *key,
static int htab_map_delete_elem(struct bpf_map *map, void *key)
{
struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
struct hlist_head *head;
struct hlist_nulls_head *head;
struct bucket *b;
struct htab_elem *l;
unsigned long flags;
Expand All @@ -945,7 +971,7 @@ static int htab_map_delete_elem(struct bpf_map *map, void *key)
l = lookup_elem_raw(head, hash, key, key_size);

if (l) {
hlist_del_rcu(&l->hash_node);
hlist_nulls_del_rcu(&l->hash_node);
free_htab_elem(htab, l);
ret = 0;
}
Expand All @@ -957,7 +983,7 @@ static int htab_map_delete_elem(struct bpf_map *map, void *key)
static int htab_lru_map_delete_elem(struct bpf_map *map, void *key)
{
struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
struct hlist_head *head;
struct hlist_nulls_head *head;
struct bucket *b;
struct htab_elem *l;
unsigned long flags;
Expand All @@ -977,7 +1003,7 @@ static int htab_lru_map_delete_elem(struct bpf_map *map, void *key)
l = lookup_elem_raw(head, hash, key, key_size);

if (l) {
hlist_del_rcu(&l->hash_node);
hlist_nulls_del_rcu(&l->hash_node);
ret = 0;
}

Expand All @@ -992,12 +1018,12 @@ static void delete_all_elements(struct bpf_htab *htab)
int i;

for (i = 0; i < htab->n_buckets; i++) {
struct hlist_head *head = select_bucket(htab, i);
struct hlist_node *n;
struct hlist_nulls_head *head = select_bucket(htab, i);
struct hlist_nulls_node *n;
struct htab_elem *l;

hlist_for_each_entry_safe(l, n, head, hash_node) {
hlist_del_rcu(&l->hash_node);
hlist_nulls_for_each_entry_safe(l, n, head, hash_node) {
hlist_nulls_del_rcu(&l->hash_node);
if (l->state != HTAB_EXTRA_ELEM_USED)
htab_elem_free(htab, l);
}
Expand Down

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