On Tue, 27 Oct 2020 11:15:31 -0600
David Ahern [off-list ref] wrote:

On 10/27/20 10:26 AM, Jesper Dangaard Brouer wrote:

quoted

The BPF-helpers for FIB lookup (bpf_xdp_fib_lookup and bpf_skb_fib_lookup)
can perform MTU check and return BPF_FIB_LKUP_RET_FRAG_NEEDED.  The BPF-prog
don't know the MTU value that caused this rejection.

If the BPF-prog wants to implement PMTU (Path MTU Discovery) (rfc1191) it
need to know this MTU value for the ICMP packet.

Patch change lookup and result struct bpf_fib_lookup, to contain this MTU
value as output via a union with 'tot_len' as this is the value used for
the MTU lookup.

Signed-off-by: Jesper Dangaard Brouer <redacted>
---
 include/uapi/linux/bpf.h       |   11 +++++++++--
 net/core/filter.c              |   17 ++++++++++++-----
 tools/include/uapi/linux/bpf.h |   11 +++++++++--
 3 files changed, 30 insertions(+), 9 deletions(-)

Reviewed-by: David Ahern <dsahern@kernel.org>

Thanks a lot for the review.  I didn't propagate-it-over in V5 of this
patch, as I changed the name of the output member from mtu to
mtu_result in V5.  Please review V5 and give your review consent.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer