linux-kernel - Re: [PATCH net] xsk: respect the offsets when copying frags

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Message-ID: <6ed8452b-f370-4443-94ce-f7d65cd51a9e@gmail.com>
Date: Wed, 23 Apr 2025 21:58:16 +0700
From: Bui Quang Minh <minhquangbui99@...il.com>
To: Stanislav Fomichev <stfomichev@...il.com>
Cc: netdev@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>, "David S. Miller"
 <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
 Jesper Dangaard Brouer <hawk@...nel.org>,
 John Fastabend <john.fastabend@...il.com>, Eric Dumazet
 <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
 Simon Horman <horms@...nel.org>,
 Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
 Alexander Lobakin <aleksander.lobakin@...el.com>, bpf@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] xsk: respect the offsets when copying frags

On 4/23/25 21:41, Stanislav Fomichev wrote:
> On 04/23, Bui Quang Minh wrote:
>> Add the missing offsets when copying frags in xdp_copy_frags_from_zc().
> Can you please share more about how you've hit this problem?
> I don't see the caller of this function (xdp_build_skb_from_zc)
> being used at all.
>
> Alexander, do you have plans to use it? Or should we remove it for now?
Hi,

I've been playing around to add support for zerocopy XDP socket with 
multi buffer mergeable buffer in virtio-net (this TODO: 
https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git/tree/drivers/net/virtio_net.c#n1312). 
In that case, I'll have a XDP buff with frags. When we have XDP_PASS 
return, I need to convert the XDP buff with frags to skb with frags, so 
I think the helper is quite helpful. I used it and got packet dropped 
due to checksum error. Debugging the problem, I've found out this issue 
which makes the skb's frag data incorrect.

Thanks,
Quang Minh.