On Tue, Jul 21, 2015 at 12:14 PM, Martin Blumenstingl
[off-list ref] 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.