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: Lorenz Bauer <[email protected]> Signed-off-by: Daniel Borkmann <[email protected]>
torvalds · Nov 19, 2018 · 96b3b6c · 96b3b6c
1 parent 2349944
commit 96b3b6c
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Showing 2 changed files with 14 additions and 2 deletions.
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
@@ -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,

diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
@@ -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);