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bpf: allow zero-initializing hash map seed

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Add a new flag BPF_F_ZERO_SEED, which forces a hash map
to initialize the seed to zero. This is useful when doing
performance analysis both on individual BPF programs, as
well as the kernel's hash table implementation.

Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
This commit is contained in:
Lorenz Bauer 2018-11-16 11:41:08 +00:00 committed by Daniel Borkmann
parent 23499442c3
commit 96b3b6c909
2 changed files with 14 additions and 2 deletions
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3
include/uapi/linux/bpf.h
View file

@ -269,6 +269,9 @@ enum bpf_attach_type {
/* Flag for stack_map, store build_id+offset instead of pointer */
#define BPF_F_STACK_BUILD_ID (1U << 5)
/* Zero-initialize hash function seed. This should only be used for testing. */
#define BPF_F_ZERO_SEED (1U << 6)
enum bpf_stack_build_id_status {
/* user space need an empty entry to identify end of a trace */
BPF_STACK_BUILD_ID_EMPTY = 0,

13
kernel/bpf/hashtab.c
View file

@ -23,7 +23,7 @@
#define HTAB_CREATE_FLAG_MASK \
(BPF_F_NO_PREALLOC | BPF_F_NO_COMMON_LRU | BPF_F_NUMA_NODE | \
BPF_F_RDONLY | BPF_F_WRONLY)
BPF_F_RDONLY | BPF_F_WRONLY | BPF_F_ZERO_SEED)
struct bucket {
struct hlist_nulls_head head;
@ -244,6 +244,7 @@ static int htab_map_alloc_check(union bpf_attr *attr)
*/
bool percpu_lru = (attr->map_flags & BPF_F_NO_COMMON_LRU);
bool prealloc = !(attr->map_flags & BPF_F_NO_PREALLOC);
bool zero_seed = (attr->map_flags & BPF_F_ZERO_SEED);
int numa_node = bpf_map_attr_numa_node(attr);
BUILD_BUG_ON(offsetof(struct htab_elem, htab) !=
@ -257,6 +258,10 @@ static int htab_map_alloc_check(union bpf_attr *attr)
*/
return -EPERM;
if (zero_seed && !capable(CAP_SYS_ADMIN))
/* Guard against local DoS, and discourage production use. */
return -EPERM;
if (attr->map_flags & ~HTAB_CREATE_FLAG_MASK)
/* reserved bits should not be used */
return -EINVAL;
@ -373,7 +378,11 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
if (!htab->buckets)
goto free_htab;
htab->hashrnd = get_random_int();
if (htab->map.map_flags & BPF_F_ZERO_SEED)
htab->hashrnd = 0;
else
htab->hashrnd = get_random_int();
for (i = 0; i < htab->n_buckets; i++) {
INIT_HLIST_NULLS_HEAD(&htab->buckets[i].head, i);
raw_spin_lock_init(&htab->buckets[i].lock);