netdev - Re: [PATCH net v2] net: llc: explicitly set skb->transport

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Message-ID: <38130786-702c-4089-a518-cba7857448ca@gmail.com>
Date: Tue, 24 Dec 2024 20:35:36 -0500
From: Antonio Pastor <antonio.pastor@...il.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: netdev@...r.kernel.org, pabeni@...hat.com, horms@...nel.org,
 kuba@...nel.org, "David S. Miller" <davem@...emloft.net>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH net v2] net: llc: explicitly set skb->transport_header

On 2024-12-23 13:18, Eric Dumazet wrote:
> I see skb->transport_header being advanced at line 61 :
>
>      /* Pass the frame on. */
>      skb->transport_header += 5;

Same treatment as before? Reset after pull?

@@ -58,8 +58,8 @@ static int snap_rcv(struct sk_buff *skb, struct 
net_device *dev,
         proto = find_snap_client(skb_transport_header(skb));
         if (proto) {
                 /* Pass the frame on. */
-               skb->transport_header += 5;
                 skb_pull_rcsum(skb, 5);
+               skb_reset_transport_header(skb);
                 rc = proto->rcvfunc(skb, dev, &snap_packet_type, orig_dev);
         }
         rcu_read_unlock();

I'll submit that as a separate patch.

> Note that setting the transport header (conditionally or not) in
> __netif_receive_skb()
> is probably a mistake. Only network (ipv4, ipv6) handlers can possibly
> know the concept
> of transport header.
Not too sure about that, but I don't have specifics. Non-IP stacks are 
probably all ancient, but some might care.
> Hopefully at some point we can remove this defensive code.
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index c7f3dea3e0eb9eb05865e49dd7a8535afb974149..b6722e11ee4e171e6a2f379fc1d0197dfcb1a842
> 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -5474,8 +5474,6 @@ static int __netif_receive_skb_core(struct
> sk_buff **pskb, bool pfmemalloc,
>          orig_dev = skb->dev;
>
>          skb_reset_network_header(skb);
> -       if (!skb_transport_header_was_set(skb))
> -               skb_reset_transport_header(skb);
>          skb_reset_mac_len(skb);
>
>          pt_prev = NULL;

I don't know the code well enough to identify all possible paths after 
this point to ensure all of them set transport header. I'd expect 
IPv4/IPv6 are OK and we are making LLC whole, but don't know what else 
to test beyond that. My testing is limited to SNAP... taking that out 
requires regression testing at a level I'm not comfortable running.