6LoWPAN: ICMP Ping Zephyr -> Linux broken in master [regression, bisected]

[benpicco]

Describe the bug
When trying to ping a Linux host over 6LoWPAN/802.15.4, current Zephyr master fails with an error. This used to work in a previous revision. Linux can still ping Zephyr .

To Reproduce
Steps to reproduce the behavior:

0. cd samples/net/sockets/echo_server
1. mkdir build; cd build
2. cmake -DBOARD=nrf52840_pca10056 -DCONF_FILE="prj.conf overlay-802154.conf" ..
3. make flash
4. on the device: net ping fe80::a04c:1116:6337:a465 (the IP of the raspberry pi)

Expected behavior

uart:~$  net ping fe80::a04c:1116:6337:a465
Sent a ping to fe80::a04c:1116:6337:a465
Received echo reply from fe80::a04c:1116:6337:a465 to fe80::7841:4946:59b8:3d9c

Impact
I did only test ICMP ping, I did not check if 6LoWPAN is currently broken or only ICMP.

Screenshots or console output

uart:~$ net ping fe80::a04c:1116:6337:a465
Sent a ping to fe80::a04c:1116:6337:a465
Ping timeout
[00:00:05.329,315] <err> net_pkt: Uncontiguous data cannot be linearized

Environment (please complete the following information):

• OS: Linux 4.19.20+ (Raspbian)
• Toolchain: arm-none-eabi-gcc (15:7-2018-q2-6) 7.3.1 20180622 (release) [ARM/embedded-7-branch revision 261907]
• Raspberry Pi with mrf24j40 module connected over SPI¹

Additional context
I did a bisect based on the known good commit 8f8c601

Unfortunately, some commits that might be related to the issue did not build and had to be skipped.

There are only 'skip'ped commits left to test.
The first bad commit could be any of:
2ad71c7
7e25568
fa4c52b
6ea5a12
c0db7ef
070b4d2
7d0ed71
1be2706
30e5c51
86ac993
We cannot bisect more!

[1] no need to compile the kernel, nowadays Raspbian includes all necessary drivers - you only need the overlay.
Note that fragmentation is broken in Linux 4.14, but everything works fine with 4.19 - you can install the newer kernel with sudo BRANCH=next rpi-update