netdev - Re: [PATCH bpf-next 2/3] veth: rework veth_xdp_rcv

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Message-ID: <YgeUFb4LIP7VfeL9@lore-desk>
Date:   Sat, 12 Feb 2022 12:03:49 +0100
From:   Lorenzo Bianconi <lorenzo@...nel.org>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org, davem@...emloft.net,
        ast@...nel.org, daniel@...earbox.net, brouer@...hat.com,
        toke@...hat.com, pabeni@...hat.com, echaudro@...hat.com,
        lorenzo.bianconi@...hat.com, toshiaki.makita1@...il.com,
        andrii@...nel.org
Subject: Re: [PATCH bpf-next 2/3] veth: rework veth_xdp_rcv_skb in order to
 accept non-linear skb

On Feb 11, Jakub Kicinski wrote:
> On Fri, 11 Feb 2022 02:20:31 +0100 Lorenzo Bianconi wrote:
> > +	if (skb_shared(skb) || skb_head_is_locked(skb)) {
> 
> Is this sufficient to guarantee that the frags can be written?
> skb_cow_data() tells a different story.

Do you mean to consider paged part of the skb always not writable, right?
In other words, we should check something like:

	if (skb_shared(skb) || skb_head_is_locked(skb) ||
	    skb_shinfo(skb)->nr_frags) {
	    ...
	}

Regards,
Lorenzo

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