linux-kernel - Re: [RFC PATCH v1 1/3] virtio/vsock: fix header length on skb merging

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Message-ID: <20230320145718.5gytg6t5pcz5rpnm@sgarzare-redhat>
Date:   Mon, 20 Mar 2023 15:57:18 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Arseniy Krasnov <avkrasnov@...rdevices.ru>
Cc:     Stefan Hajnoczi <stefanha@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Bobby Eshleman <bobby.eshleman@...edance.com>,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel@...rdevices.ru, oxffffaa@...il.com
Subject: Re: [RFC PATCH v1 1/3] virtio/vsock: fix header length on skb merging

On Sun, Mar 19, 2023 at 09:51:06PM +0300, Arseniy Krasnov wrote:
>This fixes header length calculation of skbuff during data appending to
>it. When such skbuff is processed in dequeue callbacks, e.g. 'skb_pull()'
>is called on it, 'skb->len' is dynamic value, so it is impossible to use
>it in header, because value from header must be permanent for valid
>credit calculation ('rx_bytes'/'fwd_cnt').
>
>Fixes: 077706165717 ("virtio/vsock: don't use skbuff state to account credit")

I don't understand how this commit introduced this problem, can you
explain it better?

Is it related more to the credit than to the size in the header itself?

Anyway, the patch LGTM, but we should explain better the issue.

Thanks,
Stefano

>Signed-off-by: Arseniy Krasnov <AVKrasnov@...rdevices.ru>
>---
> net/vmw_vsock/virtio_transport_common.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>index 6d15cd4d090a..3c75986e16c2 100644
>--- a/net/vmw_vsock/virtio_transport_common.c
>+++ b/net/vmw_vsock/virtio_transport_common.c
>@@ -1091,7 +1091,7 @@ virtio_transport_recv_enqueue(struct vsock_sock *vsk,
> 			memcpy(skb_put(last_skb, skb->len), skb->data, skb->len);
> 			free_pkt = true;
> 			last_hdr->flags |= hdr->flags;
>-			last_hdr->len = cpu_to_le32(last_skb->len);
>+			le32_add_cpu(&last_hdr->len, len);
> 			goto out;
> 		}
> 	}
>-- 
>2.25.1
>