netdev - Re: [PATCH] [RFC] net: bpf: make __bpf_skb_max_len(skb) an skb-independent constant

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Message-ID: <20200421102719.06bdfe02@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Tue, 21 Apr 2020 10:27:19 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Maciej Żenczykowski <zenczykowski@...il.com>
Cc:     Maciej Żenczykowski <maze@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Linux Network Development Mailing List 
        <netdev@...r.kernel.org>, linux-kernel@...r.kernel.org,
        "David S . Miller" <davem@...emloft.net>
Subject: Re: [PATCH] [RFC] net: bpf: make __bpf_skb_max_len(skb) an
 skb-independent constant

On Mon, 20 Apr 2020 16:14:27 -0700 Maciej Żenczykowski wrote:
> From: Maciej Żenczykowski <maze@...gle.com>
> 
> This function is used from:
>   bpf_skb_adjust_room
>   __bpf_skb_change_tail
>   __bpf_skb_change_head
> 
> but in the case of forwarding we're likely calling these functions
> during receive processing on ingress and bpf_redirect()'ing at
> a later point in time to egress on another interface, thus these
> mtu checks are for the wrong device.

Interesting. Without redirecting there should also be no reason
to do this check at ingress, right? So at ingress it's either 
incorrect or unnecessary?