netdev - Re: [PATCH] packet: Allow packets with only a header (but no payload)

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Message-ID: <CA+FuTScr0o0ZmwWxVP4FZoQosRRiQG6+MDvCahMCWmjLkg+33g@mail.gmail.com>
Date:	Tue, 21 Jul 2015 12:28:44 -0400
From:	Willem de Bruijn <willemb@...gle.com>
To:	Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Cc:	Network Development <netdev@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Daniel Borkmann <dborkman@...hat.com>,
	David Miller <davem@...emloft.net>, mostrows@...thlink.net
Subject: Re: [PATCH] packet: Allow packets with only a header (but no payload)

On Tue, Jul 21, 2015 at 12:14 PM, Martin Blumenstingl
<martin.blumenstingl@...glemail.com> wrote:
> 9c70776 added validation for the packet size in packet_snd. This change
> enforced that every packet needs a long enough header and at least one
> byte payload.
>
> However, when trying to establish a PPPoE connection the following message
> is printed every time a PPPoE discovery packet is sent:
> pppd: packet size is too short (24 <= 24)
>
> From what I can see in the PPPoE code the "PADI" discovery packet can
> consist of only a header with no payload (when there is neither a service
> name nor a Host-Uniq configured).

Interesting. 9c7077622dd9 only extended the check from tpacket_snd to
packet_snd to make the two paths equivalent. The existing check had the
ominous statement

    /* net device doesn't like empty head */

so allowing a header-only packet while correct in your case may not be
safe in some edge cases (specific device drivers?).

This was also discussed previously

  http://www.spinics.net/lists/netdev/msg309677.html

In any case, I don't think that reverting the patch and restoring the old
inconsistent state is a fix.
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