[PATCH v2 1/3] bpf: allow zero-initializing hash map seed

[Lorenz Bauer]

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 <l...@cloudflare.com>
---
 include/uapi/linux/bpf.h |  2 ++
 kernel/bpf/hashtab.c     | 13 +++++++++++--
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index f9187b41dff6..2c121f862082 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -253,6 +253,8 @@ enum bpf_attach_type {
 #define BPF_F_NO_COMMON_LRU    (1U << 1)
 /* Specify numa node during map creation */
 #define BPF_F_NUMA_NODE                (1U << 2)
+/* Zero-initialize hash function seed. This should only be used for testing. */
+#define BPF_F_ZERO_SEED                (1U << 6)
 
 /* flags for BPF_PROG_QUERY */
 #define BPF_F_QUERY_EFFECTIVE  (1U << 0)
diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 2c1790288138..4b7c76765d9d 100644
--- 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);
-- 
2.17.1