netdev - Re: [bpf-next PATCH v2 07/18] bpf: sockmap, add msg_cork

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Message-Id: <20180315.144147.81123006304884551.davem@davemloft.net>
Date:   Thu, 15 Mar 2018 14:41:47 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     john.fastabend@...il.com
Cc:     ast@...nel.org, daniel@...earbox.net, davejwatson@...com,
        netdev@...r.kernel.org
Subject: Re: [bpf-next PATCH v2 07/18] bpf: sockmap, add msg_cork_bytes()
 helper

From: John Fastabend <john.fastabend@...il.com>
Date: Mon, 12 Mar 2018 12:23:39 -0700

> In the case where we need a specific number of bytes before a
> verdict can be assigned, even if the data spans multiple sendmsg
> or sendfile calls. The BPF program may use msg_cork_bytes().
> 
> The extreme case is a user can call sendmsg repeatedly with
> 1-byte msg segments. Obviously, this is bad for performance but
> is still valid. If the BPF program needs N bytes to validate
> a header it can use msg_cork_bytes to specify N bytes and the
> BPF program will not be called again until N bytes have been
> accumulated. The infrastructure will attempt to coalesce data
> if possible so in many cases (most my use cases at least) the
> data will be in a single scatterlist element with data pointers
> pointing to start/end of the element. However, this is dependent
> on available memory so is not guaranteed. So BPF programs must
> validate data pointer ranges, but this is the case anyways to
> convince the verifier the accesses are valid.
> 
> Signed-off-by: John Fastabend <john.fastabend@...il.com>

Acked-by: David S. Miller <davem@...emloft.net>