Invalid Decoding JoinRequest on mipsel

[devleesch001]

Hello,

I have an importante issue.

What happened?

When the MQTT Forwarder deserialize JoinRequest Frame from Semtech UDP GWMP Frame, the MIC code is badly decoded.

For exemple the Semtech UDP GWMP JoinRequest have this MIC code [1 192 29 90] but after decoding by MQTT Forwarder have this [1 192 28 90].

All my JoinRequest with MQTT Packet Forwarder are in MIC Error.

I am sur the problem appears during json deserialization, especially when decoding the payload in base64.

How I found?

I added in code a debug print (only for me), before and After this line https://github.com/chirpstack/chirpstack-mqtt-forwarder/blob/8b2ed3d2a2bc1aab8d11a6a78a2d09929be983b9/src/backend/semtech_udp/mod.rs#L342C5-L342C51

Like this.

debug!("--- RAW UPLINK --- {:?}", data);

let pl = structs::PushData::from_slice(data)?;

debug!("--- DECODED PAYLOAD --- {:?}", pl.payload);

And I obtained this :

RAW UDP GWMP Frame :

7b 22 72 78 70 6b 22 3a 5b 7b 22 6a 76 65 72 22 3a 31 2c 22 74 6d 73 74 22 3a 38 36 33 34 32 32 31 33 2c 22 63 68 61 6e 22 3a 37 2c 22 72 66 63 68 22 3a 31 2c 22 66 72 65 71 22 3a 38 36 38 2e 35 30 30 30 30 30 2c 22 6d 69 64 22 3a 20 30 2c 22 73 74 61 74 22 3a 31 2c 22 6d 6f 64 75 22 3a 22 4c 4f 52 41 22 2c 22 64 61 74 72 22 3a 22 53 46 31 32 42 57 31 32 35 22 2c 22 63 6f 64 72 22 3a 22 34 2f 35 22 2c 22 72 73 73 69 73 22 3a 2d 35 32 2c 22 6c 73 6e 72 22 3a 36 2e 32 2c 22 66 6f 66 66 22 3a 2d 37 34 37 36 2c 22 72 73 73 69 22 3a 2d 35 31 2c 22 73 69 7a 65 22 3a 32 33 2c 22 64 61 74 61 22 3a 22 41 41 49 41 41 41 44 77 37 4a 41 41 2b 52 30 78 46 67 48 73 6b 41 41 6c 4c 77 48 41 48 56 6f 3d 22 7d 5d 7d

Manual decoding I optain this Payload in JSON

{"rxpk":[{"jver":1,"tmst":86342213,"chan":7,"rfch":1,"freq":868.500000,"mid": 0,"stat":1,"modu":"LORA","datr":"SF12BW125","codr":"4/5","rssis":-52,"lsnr":6.2,"foff":-7476,"rssi":-51,"size":23,"data":"AAIAAADw7JAA+R0xFgHskAAlLwHAHVo="}]}

This LoRaWAN DATA

AAIAAADw7JAA+R0xFgHskAAlLwHAHVo=

00 02 00 00 00 f0 ec 90 00 f9 1d 31 16 01 ec 90 00 25 2f 01 c0 1d 5a

Contain this MIC

MIC : [1 192 29 90], 01c01d5a

But after let pl = structs::PushData::from_slice(data)?;

When MQTT Forwarder deserialize the RAW UDP GWMP and decode the DATA of Payload Frame I obtaine this LoRaWAN Frame

00 02 00 00 00 f0 ec 90 00 f9 1d 31 16 01 ec 90 00 25 2f 01 c0 1c 5a

MIC : [1 192 28 90], 01c01c5a

The mic code does not correspond to the UDP frame.

GOOD 01c01d5a, BAD 01c01c5a

Material

Gateway : RAK7268CV2

uname : Linux RAK7268CV2 5.4.154 #0 Fri Nov 24 07:30:08 2023 mips GNU/Linux

cpuinfo :

system type		: MediaTek MT7628AN ver:1 eco:2
machine			: Rakwireless RAK636
processor		: 0
cpu model		: MIPS 24KEc V5.5
BogoMIPS		: 385.02
wait instruction	: yes
microsecond timers	: yes
tlb_entries		: 32
extra interrupt vector	: yes
hardware watchpoint	: yes, count: 4, address/irw mask: [0x0ffc, 0x0ffc, 0x0ffb, 0x0ffb]
isa			: mips1 mips2 mips32r1 mips32r2
ASEs implemented	: mips16 dsp
Options implemented	: tlb 4kex 4k_cache prefetch mcheck ejtag llsc pindexed_dcache userlocal vint perf_cntr_intr_bit perf
shadow register sets	: 1
kscratch registers	: 0
package			: 0
core			: 0
VCED exceptions		: not available
VCEI exceptions		: not available

OS : WisGateOS_2.2.1_RAK based on OpenWRT 21.2
Toolchain : make dist-mipsel-unknown-linux-musl

[devleesch001]

The problem seems to come from the base64 library, i made little poc to simple decode base64 string, follwoing oficial documention I obtain this

use base64::prelude::*;

fn main() {
    assert_eq!(BASE64_STANDARD.decode(b"+uwgVQA=").expect("panic"), b"\xFA\xEC\x20\x55\0");
    assert_eq!(BASE64_STANDARD.encode(b"\xFF\xEC\x20\x55\0"), "/+wgVQA=");
}

on my pc x86_64-unknown-linux-musl :

./base64_test
echo $?
0

All it's ok

But on gateway with mipsel-unknown-linux-musl :

./base64_test
thread 'main' panicked at src/main.rs:5:5:
assertion `left == right` failed
  left: [250, 236, 32, 84, 0]
 right: [250, 236, 32, 85, 0]

echo $?
101

This is exactly the same probleme.

The problem could be come cross-rs, but I don't know.

[devleesch001]

Maybe the compiler.

rust-lang/compiler-team#648

Maybe we should develop our custom base64 decoding for this architecture

[brocaar]

Thanks for reporting. This sound very similar to #50.

[brocaar]

I can confirm that I was able to reproduce this issue. After compiling using the latest Rust toolchain, everything works as expected. I think this was a toolchain related issue. As mips has been moved to a tier 3 target, I have made the following change in the Makefile: a9d9ebb

I'm creating a new release right now, which should be available shortly.

[brocaar]

This was released as v4.2.0.

[devleesch001]

I just compiled it and it works on my gateway.
Thanks you very much

[brocaar]

Sorry, v4.2.1 re-introduces this issue because I downgraded the Rust toolchain because of nightly build issues (I forgot about that this might cause this issue). I will release a v4.2.2 shortly.