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SubjectRe: [PATCH 5/6] bpf: hash: avoid to call kmalloc() in eBPF prog
On 12/15/2015 12:21 PM, Ming Lei wrote:
...
> +static int htab_init_elems_allocator(struct bpf_htab *htab)
> +{
> + int ret = htab_pre_alloc_elems(htab);
> +
> + if (ret)
> + return ret;
> +
> + ret = percpu_ida_init(&htab->elems_pool, htab->map.max_entries);
> + if (ret)
> + htab_destroy_elems(htab);
> + return ret;
> +}
> +
> +static void htab_deinit_elems_allocator(struct bpf_htab *htab)
> +{
> + htab_destroy_elems(htab);
> + percpu_ida_destroy(&htab->elems_pool);
> +}
> +
> +static struct htab_elem *htab_alloc_elem(struct bpf_htab *htab)
> +{
> + int tag = percpu_ida_alloc(&htab->elems_pool, TASK_RUNNING);
> + struct htab_elem *elem;
> +
> + if (tag < 0)
> + return NULL;
> +
> + elem = htab->elems[tag];
> + elem->tag = tag;
> + return elem;
> +}
....
> @@ -285,12 +424,8 @@ static int htab_map_update_elem(struct bpf_map *map, void *key, void *value,
> * search will find it before old elem
> */
> hlist_add_head_rcu_lock(&l_new->hash_node, head);
> - if (l_old) {
> - hlist_del_rcu_lock(&l_old->hash_node);
> - kfree_rcu(l_old, rcu);
> - } else {
> - atomic_inc(&htab->count);
> - }
> + if (l_old)
> + htab_free_elem_rcu(htab, l_old);
> bit_spin_unlock(HLIST_LOCK_BIT, (unsigned long *)&head->first);
> raw_local_irq_restore(flags);

On a quick look, you are using the ida to keep track of elements, right? What happens
if you have a hash-table of max_entry size 1, fill that one slot and later on try to
replace it with a different element.

Old behaviour (htab->count) doesn't increase htab count and would allow the replacement
of that element to happen.

Looks like in your case, we'd get -E2BIG from htab_alloc_elem(), no? ... as preallocated
pool is already used up then?


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