netdev - Re: [Patch bpf-next v6 4/8] skmsg: move sk_redir from TCP_SKB

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Message-ID: <874ki24omu.fsf@cloudflare.com>
Date:   Tue, 23 Feb 2021 19:36:57 +0100
From:   Jakub Sitnicki <jakub@...udflare.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, duanxiongchun@...edance.com,
        Dongdong Wang <wangdongdong.6@...edance.com>,
        Jiang Wang <jiang.wang@...edance.com>,
        Cong Wang <cong.wang@...edance.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Lorenz Bauer <lmb@...udflare.com>,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [Patch bpf-next v6 4/8] skmsg: move sk_redir from TCP_SKB_CB to
 skb

On Tue, Feb 23, 2021 at 07:04 PM CET, Cong Wang wrote:
> On Tue, Feb 23, 2021 at 9:53 AM Jakub Sitnicki <jakub@...udflare.com> wrote:
>> Based on what I've seen around, mask for sanitizing tagged pointers is
>> usually derived from the flag(s). For instance:
>>
>> #define SKB_DST_NOREF   1UL
>> #define SKB_DST_PTRMASK ~(SKB_DST_NOREF)
>>
>> #define SK_USER_DATA_NOCOPY     1UL
>> #define SK_USER_DATA_BPF        2UL     /* Managed by BPF */
>> #define SK_USER_DATA_PTRMASK    ~(SK_USER_DATA_NOCOPY | SK_USER_DATA_BPF)
>>
>> Using ~(BPF_F_INGRESS) expression would be like substituting mask
>> definition.
>
> Yes, that is why I said we need a mask.

OK

>
>>
>> Alternatively we could clear _skb_refdest after clone, but before
>> enqueuing the skb in ingress_skb. And only for when we're redirecting.
>>
>> I believe that would be in sk_psock_skb_redirect, right before skb_queue_tail.
>
> Hmm? We definitely cannot clear skb->_sk_redir there, as it is used after
> enqueued in ingress_skb, that is in sk_psock_backlog().

You're right. I focused on the sk pointer and forgot it also carries the
ingress flag.