Re: [Xen-devel] [PATCH net-next] xen-netfront: avoid packet loss when ethernet header crosses page boundary

Vitaly Kuznetsov [off-list ref] writes:

David Vrabel [off-list ref] writes:

quoted

On 22/08/16 16:42, Vitaly Kuznetsov wrote:

quoted

I see two ways to fix the issue:
- Change the 'wire' protocol between netfront and netback to start keeping
  the original SKB structure. We'll have to add a flag indicating the fact
  that the particular request is a part of the original linear part and not
  a frag. We'll need to know the length of the linear part to pre-allocate
  memory.

I don't think there needs to be a protocol change.  I think the check in
netback is bogus -- it's the total packet length that must be >
HLEN_ETH.  The upper layers will pull any headers from the frags as
needed

I'm afraid this is not always true, just removing the check leads us to
the following:

[  495.442186] kernel BUG at ./include/linux/skbuff.h:1927! 
[  495.468789] invalid opcode: 0000 [#1] SMP 

What I wanted to say here is that this test makes me think the
description of the patch I suggested is correct: an SKB can't have its
linear part shorter than ETH_HLEN as the header is being pointed directly,
upper network layers don't assemble it from frags, the check in netback
is valid.

So, how can we proceed here?

-- 
  Vitaly