netdev - Re: [PATCH v2 net-next 2/6] udp: Constify skb argument in lookup functions

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Date:   Tue, 29 Aug 2017 17:58:32 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     tom@...ntonium.net
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH v2 net-next 2/6] udp: Constify skb argument in lookup
 functions

From: Tom Herbert <tom@...ntonium.net>
Date: Tue, 29 Aug 2017 16:27:07 -0700

> For UDP socket lookup functions, and associateed functions that take an
> skbuf as argument, declare the skb argument as constant.
> 
> One caveat is that reuseport_select_sock can be called from the UDP
> lookup functions with an skb argument. This function temporarily
> modifies the skbuff data pointer (in bpf_run via a pull/push sequence).
> To resolve compiler warning I added a local skbuf declaration that is
> not const and assigned to the skb argument with an explicit cast.
> 
> Signed-off-by: Tom Herbert <tom@...ntonium.net>

Please don't do this.

If reuseport_select_sock() modifies anything in the SKB, especially
skb->data, it infects the entire call chain.  So you can't mark it
const in this family of calls.