On Mon, Oct 1, 2018 at 12:47 PM Lorenz Bauer <lmb@xxxxxxxxxxxxxx> wrote:
>
> Add a new flag BPF_F_ZERO_SEED, which forces a hash map
> to initialize the seed to zero.
> ---
> include/uapi/linux/bpf.h | 2 ++
> kernel/bpf/hashtab.c | 8 ++++++--
> 2 files changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index aa5ccd2385ed..9d15c8f179ac 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -252,6 +252,8 @@ 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 */
> +#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..a79e123dae62 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;
> @@ -373,7 +373,11 @@ 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();
> +
If this is for testing only, you can slap a capable(CAP_SYS_ADMIN)
check in here, right? I doubt it matters, but I don't really like
seeing something like this exposed to unprivileged userspace just
because you need it for kernel testing.
