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Subject[PATCH 4.9 03/51] bpf: convert htab map to hlist_nulls
Date
commit 4fe8435909fddc97b81472026aa954e06dd192a5 upstream.

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: 6c9059817432 ("bpf: pre-allocate hash map elements")
Reported-by: Jonathan Perry <jonperry@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Chenbo Feng <fengc@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
include/linux/list_nulls.h | 5 +++
include/linux/rculist_nulls.h | 14 +++++++
kernel/bpf/hashtab.c | 71 +++++++++++++++++++++++------------
3 files changed, 67 insertions(+), 23 deletions(-)

diff --git a/include/linux/list_nulls.h b/include/linux/list_nulls.h
index b01fe10090843..87ff4f58a2f01 100644
--- a/include/linux/list_nulls.h
+++ b/include/linux/list_nulls.h
@@ -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
diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
index 6224a0ab0b1e8..2720b2fbfb86d 100644
--- a/include/linux/rculist_nulls.h
+++ b/include/linux/rculist_nulls.h
@@ -118,5 +118,19 @@ static inline void hlist_nulls_add_head_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
diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index f9d53ac57f640..8648d7d297081 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -13,10 +13,11 @@
#include <linux/bpf.h>
#include <linux/jhash.h>
#include <linux/filter.h>
+#include <linux/rculist_nulls.h>
#include "percpu_freelist.h"

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

@@ -40,7 +41,7 @@ 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 {
@@ -245,7 +246,7 @@ 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);
}

@@ -282,28 +283,52 @@ 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;

@@ -316,7 +341,7 @@ 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;
}
@@ -335,7 +360,7 @@ static void *htab_map_lookup_elem(struct bpf_map *map, void *key)
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 = 0;
@@ -352,13 +377,13 @@ 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)
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) {
@@ -377,7 +402,7 @@ 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 */
@@ -534,7 +559,7 @@ 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;
@@ -573,9 +598,9 @@ 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;
@@ -590,7 +615,7 @@ 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;
@@ -642,7 +667,7 @@ 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:
@@ -660,7 +685,7 @@ static int htab_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;
@@ -680,7 +705,7 @@ 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;
}
@@ -694,12 +719,12 @@ 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);
}
--
2.20.1


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