netdev - Re: [PATCH v2 bpf-next 1/2] bpf: add cg_skb_is_valid_access for BPF_PROG_TYPE_CGROUP

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Date:   Thu, 18 Oct 2018 05:06:11 +0000
From:   Song Liu <songliubraving@...com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 1/2] bpf: add cg_skb_is_valid_access for
 BPF_PROG_TYPE_CGROUP_SKB



> On Oct 17, 2018, at 9:44 PM, Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> 
> On Wed, Oct 17, 2018 at 04:36:15PM -0700, Song Liu wrote:
>> BPF programs of BPF_PROG_TYPE_CGROUP_SKB need to access headers in the
>> skb. This patch enables direct access of skb for these programs.
>> 
>> In __cgroup_bpf_run_filter_skb(), bpf_compute_data_pointers() is called
>> to compute proper data_end for the BPF program.
>> 
>> Signed-off-by: Song Liu <songliubraving@...com>
>> ---
>> kernel/bpf/cgroup.c |  4 ++++
>> net/core/filter.c   | 36 +++++++++++++++++++++++++++++++++++-
>> 2 files changed, 39 insertions(+), 1 deletion(-)
>> 
>> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
>> index 00f6ed2e4f9a..340d496f35bd 100644
>> --- a/kernel/bpf/cgroup.c
>> +++ b/kernel/bpf/cgroup.c
>> @@ -566,6 +566,10 @@ int __cgroup_bpf_run_filter_skb(struct sock *sk,
>> 	save_sk = skb->sk;
>> 	skb->sk = sk;
>> 	__skb_push(skb, offset);
>> +
>> +	/* compute pointers for the bpf prog */
>> +	bpf_compute_data_pointers(skb);
> 
> cg_skb_is_valid_access() below looks good to me now,
> but I just realized that above change is not safe for all sockets.
> After sk_filter_trim_cap() is called in udp_queue_rcv_skb()
> it needs to see valid UDP_SKB_CB.
> But sizeof(struct udp_skb_cb)==28, so bpf_compute_data_pointers()
> would mangle the end of it.
> So we have to save/restore data_end/data_meta pointers as well.
> 
> I'm thinking that new helper like:
>  bpf_compute_and_save_data_pointers(skb, &buffer_of_16_bytes);
>  BPF_PROG_RUN_ARRAY();
>  bpf_restore_data_pointers(skb, &buffer_of_16_bytes);
> would be decent interface.

Thanks Alexei!

Will send v3 shortly.

Song