netdev - Re: [PATCH v4 bpf-next 1/2] bpf: add cg_skb_is_valid_access for BPF_PROG_TYPE_CGROUP

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Date:   Fri, 19 Oct 2018 02:03:25 +0000
From:   Alexei Starovoitov <ast@...com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Song Liu <songliubraving@...com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     "ast@...nel.org" <ast@...nel.org>, Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v4 bpf-next 1/2] bpf: add cg_skb_is_valid_access for
 BPF_PROG_TYPE_CGROUP_SKB

On 10/18/18 6:33 PM, Alexei Starovoitov wrote:
> On 10/18/18 5:14 PM, Daniel Borkmann wrote:
>>> +    case bpf_ctx_range(struct __sk_buff, data_meta):
>>> +    case bpf_ctx_range(struct __sk_buff, flow_keys):
>>> +        return false;
>> ... if it's disallowed anyway (disallowing it is the right thing to do,
>> but no need to save/restore then..)?
>>
>
> that's a good point.
> why shouldn't we allow cg_skb to access data_meta?
> xdp can set it and cgroup_skb_ingress will consume it here.

I'll take it back.
When xdp doesn't set meta_data it will be zero and
bpf_compute_data_pointers() will point data_meta to skb->data.
On ingress that's eth header, but for tx it will point
to reserved space for future eth header.
So we cannot do that.
Let's keep it disabled and adjust
bpf_compute_and_save_data_pointers() to save only 'data' pointer.