netdev - Re: [RFC bpf-next 2/4] bpf: return EOPNOTSUPP when map lookup isn't supported

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Date:   Wed, 19 Sep 2018 08:14:24 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Prashant Bhole <bhole_prashant_q7@....ntt.co.jp>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Quentin Monnet <quentin.monnet@...ronome.com>,
        "David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [RFC bpf-next 2/4] bpf: return EOPNOTSUPP when map lookup isn't
 supported

On Wed, Sep 19, 2018 at 04:51:41PM +0900, Prashant Bhole wrote:
> Return ERR_PTR(-EOPNOTSUPP) from map_lookup_elem() methods of below
> map types:
> - BPF_MAP_TYPE_PROG_ARRAY
> - BPF_MAP_TYPE_STACK_TRACE
> - BPF_MAP_TYPE_XSKMAP
> - BPF_MAP_TYPE_SOCKMAP/BPF_MAP_TYPE_SOCKHASH
> 
> Signed-off-by: Prashant Bhole <bhole_prashant_q7@....ntt.co.jp>
> ---
>  kernel/bpf/arraymap.c | 2 +-
>  kernel/bpf/sockmap.c  | 2 +-
>  kernel/bpf/stackmap.c | 2 +-
>  kernel/bpf/xskmap.c   | 2 +-
>  4 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
> index dded84cbe814..24583da9ffd1 100644
> --- a/kernel/bpf/arraymap.c
> +++ b/kernel/bpf/arraymap.c
> @@ -449,7 +449,7 @@ static void fd_array_map_free(struct bpf_map *map)
>  
>  static void *fd_array_map_lookup_elem(struct bpf_map *map, void *key)
>  {
> -	return NULL;
> +	return ERR_PTR(-EOPNOTSUPP);
>  }

conceptually the set looks good to me.
Please add a test to test_verifier.c to make sure
that these lookup helpers cannot be called from BPF program.
Otherwise this diff may cause crashes.