Commit 96b3b6c9 authored by Lorenz Bauer's avatar Lorenz Bauer Committed by Daniel Borkmann
Browse files

bpf: allow zero-initializing hash map seed 



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: default avatarLorenz Bauer <lmb@cloudflare.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
parent 23499442

include/uapi/linux/bpf.h

+3 −0
Original line number Diff line number Diff line
@@ -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,

kernel/bpf/hashtab.c

+11 −2
Original line number Diff line number Diff line
@@ -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;



	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);

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