XDP programs can shrink packets by calling the bpf_xdp_adjust_tail()
helper function. For multi-buffer packets this may lead to reduction of
frag count stored in skb_shared_info area of the xdp_buff struct. This
results in issues with the current handling of XDP_PASS and XDP_DROP
cases.

For XDP_PASS, currently skb is being built using frag count of
xdp_buffer before it was processed by XDP prog and thus will result in
an inconsistent skb when frag count gets reduced by XDP prog. To fix
this, get correct frag count while building the skb instead of using
pre-obtained frag count.

For XDP_DROP, current page recycling logic will not reuse the page but
instead will adjust the pagecnt_bias so that the page can be freed. This
again results in inconsistent behavior as the page refcnt has already
been changed by the helper while freeing the frag(s) as part of
shrinking the packet. To fix this, only adjust pagecnt_bias for buffers
that are stillpart of the packet post-xdp prog run.

Fixes: e213ced1 ("i40e: add support for XDP multi-buffer Rx")
Reported-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Tirthendu Sarkar <tirthendu.sarkar@intel.com>
Link: https://lore.kernel.org/r/20240124191602.566724-6-maciej.fijalkowski@intel.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>