netdev - Re: [bpf-next 1/3] flow_dissector: implements flow dissector BPF hook

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Date:   Sun, 2 Sep 2018 23:03:55 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Petar Penkov <peterpenkov96@...il.com>, netdev@...r.kernel.org
Cc:     davem@...emloft.net, ast@...nel.org, simon.horman@...ronome.com,
        ecree@...arflare.com, songliubraving@...com, tom@...bertland.com,
        Petar Penkov <ppenkov@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [bpf-next 1/3] flow_dissector: implements flow dissector BPF hook

On 08/30/2018 08:22 PM, Petar Penkov wrote:
> From: Petar Penkov <ppenkov@...gle.com>
> 
> Adds a hook for programs of type BPF_PROG_TYPE_FLOW_DISSECTOR and
> attach type BPF_FLOW_DISSECTOR that is executed in the flow dissector
> path. The BPF program is per-network namespace.
> 
> Signed-off-by: Petar Penkov <ppenkov@...gle.com>
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
[...]
> +		err = check_flow_keys_access(env, off, size);
> +		if (!err && t == BPF_READ && value_regno >= 0)
> +			mark_reg_unknown(env, regs, value_regno);
>  	} else {
>  		verbose(env, "R%d invalid mem access '%s'\n", regno,
>  			reg_type_str[reg->type]);
> @@ -1925,6 +1954,8 @@ static int check_helper_mem_access(struct bpf_verifier_env *env, int regno,
>  	case PTR_TO_PACKET_META:
>  		return check_packet_access(env, regno, reg->off, access_size,
>  					   zero_size_allowed);
> +	case PTR_TO_FLOW_KEYS:
> +		return check_flow_keys_access(env, reg->off, access_size);
>  	case PTR_TO_MAP_VALUE:
>  		return check_map_access(env, regno, reg->off, access_size,
>  					zero_size_allowed);
> @@ -3976,6 +4007,7 @@ static bool may_access_skb(enum bpf_prog_type type)
>  	case BPF_PROG_TYPE_SOCKET_FILTER:
>  	case BPF_PROG_TYPE_SCHED_CLS:
>  	case BPF_PROG_TYPE_SCHED_ACT:
> +	case BPF_PROG_TYPE_FLOW_DISSECTOR:
>  		return true;

This one should not be added here. It would allow for LD_ABS to be used, but
you already have direct packet access as well as bpf_skb_load_bytes() helper
enabled. Downside on LD_ABS is that error path will exit the BPF prog with
return 0 for historical reasons w/o user realizing (here: to BPF_OK mapping).
So we should not encourage use of LD_ABS/IND anymore in eBPF context and
avoid surprises.

>  	default:
>  		return false;
> @@ -4451,6 +4483,7 @@ static bool regsafe(struct bpf_reg_state *rold, struct bpf_reg_state *rcur,
>  	case PTR_TO_CTX:
>  	case CONST_PTR_TO_MAP:
>  	case PTR_TO_PACKET_END:
> +	case PTR_TO_FLOW_KEYS:
>  		/* Only valid matches are exact, which memcmp() above
>  		 * would have accepted
>  		 */
> diff --git a/net/core/filter.c b/net/core/filter.c
> index c25eb36f1320..0143b9c0c67e 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -5092,6 +5092,17 @@ sk_skb_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>  	}
>  }
>  
> +static const struct bpf_func_proto *
> +flow_dissector_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> +{
> +	switch (func_id) {
> +	case BPF_FUNC_skb_load_bytes:
> +		return &bpf_skb_load_bytes_proto;

Probably makes sense to also enable bpf_skb_pull_data helper for direct packet
access use to fetch non-linear data from here once.

> +	default:
> +		return bpf_base_func_proto(func_id);
> +	}
> +}
> +
>  static const struct bpf_func_proto *
>  lwt_out_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>  {
> @@ -5207,6 +5218,7 @@ static bool bpf_skb_is_valid_access(int off, int size, enum bpf_access_type type
>  	case bpf_ctx_range(struct __sk_buff, data):
>  	case bpf_ctx_range(struct __sk_buff, data_meta):
>  	case bpf_ctx_range(struct __sk_buff, data_end):
> +	case bpf_ctx_range(struct __sk_buff, flow_keys):
>  		if (size != size_default)
>  			return false;
>  		break;
> @@ -5235,6 +5247,7 @@ static bool sk_filter_is_valid_access(int off, int size,
>  	case bpf_ctx_range(struct __sk_buff, data):
>  	case bpf_ctx_range(struct __sk_buff, data_meta):
>  	case bpf_ctx_range(struct __sk_buff, data_end):
> +	case bpf_ctx_range(struct __sk_buff, flow_keys):
>  	case bpf_ctx_range_till(struct __sk_buff, family, local_port):
[...]
Thanks,
Daniel