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

[Song Liu <liu.song.a23@gmail.com>]

On Mon, Oct 8, 2018 at 3:34 AM Lorenz Bauer <lmb@cloudflare.com> wrote:
>
> 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>
> ---
>  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)

Please add this line after
#define BPF_F_STACK_BUILD_ID    (1U << 5)

Other than this
Acked-by: Song Liu <songliubraving@fb.com>


>
>  /* 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
>