netdev - Re: [PATCH bpf-next] bpf: hashtab - allow BPF_MAP_LOOKUP{,_AND_DELETE}

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Message-ID: <6df59861-8334-49ac-8dca-2b0bac82f2d7@linux.dev>
Date: Mon, 18 Aug 2025 13:58:08 -0700
From: Yonghong Song <yonghong.song@...ux.dev>
To: Maciej Żenczykowski <maze@...gle.com>,
 Maciej Żenczykowski <zenczykowski@...il.com>,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>
Cc: Linux Network Development Mailing List <netdev@...r.kernel.org>,
 "David S . Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>,
 Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
 BPF Mailing List <bpf@...r.kernel.org>, Stanislav Fomichev <sdf@...ichev.me>
Subject: Re: [PATCH bpf-next] bpf: hashtab - allow
 BPF_MAP_LOOKUP{,_AND_DELETE}_BATCH with NULL keys/values.

On 8/13/25 12:39 AM, Maciej Żenczykowski wrote:
> BPF_MAP_LOOKUP_AND_DELETE_BATCH keys & values == NULL
> seems like a nice way to simply quickly clear a map.

This will change existing API as users will expect
some error (e.g., -EFAULT) return when keys or values is NULL.

We have a 'flags' field in uapi header in

         struct { /* struct used by BPF_MAP_*_BATCH commands */
                 __aligned_u64   in_batch;       /* start batch,
                                                  * NULL to start from beginning
                                                  */
                 __aligned_u64   out_batch;      /* output: next start batch */
                 __aligned_u64   keys;
                 __aligned_u64   values;
                 __u32           count;          /* input/output:
                                                  * input: # of key/value
                                                  * elements
                                                  * output: # of filled elements
                                                  */
                 __u32           map_fd;
                 __u64           elem_flags;
                 __u64           flags;
         } batch;

we can add a flag in 'flags' like BPF_F_CLEAR_MAP_IF_KV_NULL with a comment
that if keys or values is NULL, the batched elements will be cleared.

>
> BPF_MAP_LOOKUP keys/values == NULL might be useful if we just want
> the values/keys and don't want to bother copying the keys/values...
>
> BPF_MAP_LOOKUP keys & values == NULL might be useful to count
> the number of populated entries.

bpf_map_lookup_elem() does not have flags field, so we probably should not
change existins semantics.

>
> Cc: Alexei Starovoitov <ast@...nel.org>
> Cc: Daniel Borkmann <daniel@...earbox.net>
> Cc: Stanislav Fomichev <sdf@...ichev.me>
> Signed-off-by: Maciej Żenczykowski <maze@...gle.com>
> ---
>   kernel/bpf/hashtab.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index 5001131598e5..8fbdd000d9e0 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -1873,9 +1873,9 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
>   
>   	rcu_read_unlock();
>   	bpf_enable_instrumentation();
> -	if (bucket_cnt && (copy_to_user(ukeys + total * key_size, keys,
> +	if (bucket_cnt && (ukeys && copy_to_user(ukeys + total * key_size, keys,
>   	    key_size * bucket_cnt) ||
> -	    copy_to_user(uvalues + total * value_size, values,
> +	    uvalues && copy_to_user(uvalues + total * value_size, values,
>   	    value_size * bucket_cnt))) {
>   		ret = -EFAULT;
>   		goto after_loop;