Hello!

Actually ip_gre is probably right.  It seems that CHECKSUM_UNNECESSARY
should not be set for PPPOE/GRE packets at all.  So it would be a bug in
the sk* driver.

Alexey/Dave, is this interpretation of ip_summed correct?

What's about CHECKSUM_UNNECESSARY, yes, it is set only for TCP/UDP packets,
when device verifies the checksum itself. It is not the case for
PPOE frames. CHECKSUM_HW is more flxible, ipip/gre tunnels use this adjusting
skb->csum by checksum of stripped headers.

ppp_input() could do the same thing. It does not, hence suggested patch
is corrrect minimal solution.

Alexey