gus
Hi all,
I have a Raspberry Pi Nerves device which I have attached a MCP2515 SPI to CAN transceiver board. I can get this board to work fine with regular raspbian, and I just send commands using candump and cansend utilities from the canutils tool suite. I’ve got to the point where I have a slightly customized buildroot image of the standard rpi3 firmware to enable the MCP2515 board and include the SocketCAN kernel modules (heavily based off of the work done by @brien and his 2018 talk titled “Customize Your Car: An Adventure in Using Elixir and Nerves to Hack Your Vehicle’s Electronics Network”. Huge thank you
)
So far, so good. I was testing using the ng_can library, and indeed able to read the data. However, I have found the library to be a little bit less flexible than I need for my application. Particularly, it sets a fixed can bus bitrate at initialization. Additionally, the library hasn’t been updated in several years, and I’d like to avoid pulling in the C NIF dependencies. Plus, I’ve been looking for an excuse to finally create and publish my own library, and it is one that help me better cement my understanding of several OTP concepts.
So I’m thinking about trying to create a pure-elixir library for SocketCAN. I was browsing this thread on community automotive projects, which hints that this should be possible with just the :socket module from Erlang.
Here’s where my trouble starts: I’ll be the first to admit that the Erlang documentation is daunting to me, having spent all my time in Elixir thus far. I can’t understand just quite why I can’t get the socket connection to the system SocketCAN to work, or if I am even approaching it in the right way. Maybe the problem is that it isn’t even supported on my hardware for some reason - though theoretically it should, since I have the kernel modules properly installed (ng_can does work).
Some reference: the SocketCAN docs say that the socket connection can be opened in C with s = socket(PF_CAN, SOCK_RAW, CAN_RAW);. This is pretty much the same thing for the python-can library implementation, which is sock = socket.socket(constants.PF_CAN, socket.SOCK_RAW, constants.CAN_RAW)
I think my main confusion is how to emulate these parameters in :socket.open/3 (though maybe I need :socket.open/4?).
Below is a couple of things I’ve tried/some extra info.
Also, I tried to join the Elixir slack to be part of the Nerves community there, but I can’t find any valid join link. If someone could shoot one my way I would be grateful ![]()
Thank you!
Gus
iex(nerves@nerves.local)1> :socket.open(:local, :raw, :pf_can)
{:error, {:invalid, {:protocol, :pf_can}}}
iex(nerves@nerves.local)2> cat "/etc/protocols"
# Internet (IP) protocols
#
# Updated from http://www.iana.org/assignments/protocol-numbers and other
# sources.
ip 0 IP # internet protocol, pseudo protocol number
hopopt 0 HOPOPT # IPv6 Hop-by-Hop Option [RFC1883]
icmp 1 ICMP # internet control message protocol
igmp 2 IGMP # Internet Group Management
ggp 3 GGP # gateway-gateway protocol
ipencap 4 IP-ENCAP # IP encapsulated in IP (officially ``IP'')
st 5 ST # ST datagram mode
tcp 6 TCP # transmission control protocol
egp 8 EGP # exterior gateway protocol
igp 9 IGP # any private interior gateway (Cisco)
pup 12 PUP # PARC universal packet protocol
udp 17 UDP # user datagram protocol
hmp 20 HMP # host monitoring protocol
xns-idp 22 XNS-IDP # Xerox NS IDP
rdp 27 RDP # "reliable datagram" protocol
iso-tp4 29 ISO-TP4 # ISO Transport Protocol class 4 [RFC905]
dccp 33 DCCP # Datagram Congestion Control Prot. [RFC4340]
xtp 36 XTP # Xpress Transfer Protocol
ddp 37 DDP # Datagram Delivery Protocol
idpr-cmtp 38 IDPR-CMTP # IDPR Control Message Transport
ipv6 41 IPv6 # Internet Protocol, version 6
ipv6-route 43 IPv6-Route # Routing Header for IPv6
ipv6-frag 44 IPv6-Frag # Fragment Header for IPv6
idrp 45 IDRP # Inter-Domain Routing Protocol
rsvp 46 RSVP # Reservation Protocol
gre 47 GRE # General Routing Encapsulation
esp 50 IPSEC-ESP # Encap Security Payload [RFC2406]
ah 51 IPSEC-AH # Authentication Header [RFC2402]
skip 57 SKIP # SKIP
ipv6-icmp 58 IPv6-ICMP # ICMP for IPv6
ipv6-nonxt 59 IPv6-NoNxt # No Next Header for IPv6
ipv6-opts 60 IPv6-Opts # Destination Options for IPv6
rspf 73 RSPF CPHB # Radio Shortest Path First (officially CPHB)
vmtp 81 VMTP # Versatile Message Transport
eigrp 88 EIGRP # Enhanced Interior Routing Protocol (Cisco)
ospf 89 OSPFIGP # Open Shortest Path First IGP
ax.25 93 AX.25 # AX.25 frames
ipip 94 IPIP # IP-within-IP Encapsulation Protocol
etherip 97 ETHERIP # Ethernet-within-IP Encapsulation [RFC3378]
encap 98 ENCAP # Yet Another IP encapsulation [RFC1241]
# 99 # any private encryption scheme
pim 103 PIM # Protocol Independent Multicast
ipcomp 108 IPCOMP # IP Payload Compression Protocol
vrrp 112 VRRP # Virtual Router Redundancy Protocol [RFC5798]
l2tp 115 L2TP # Layer Two Tunneling Protocol [RFC2661]
isis 124 ISIS # IS-IS over IPv4
sctp 132 SCTP # Stream Control Transmission Protocol
fc 133 FC # Fibre Channel
mobility-header 135 Mobility-Header # Mobility Support for IPv6 [RFC3775]
udplite 136 UDPLite # UDP-Lite [RFC3828]
mpls-in-ip 137 MPLS-in-IP # MPLS-in-IP [RFC4023]
manet 138 # MANET Protocols [RFC5498]
hip 139 HIP # Host Identity Protocol
shim6 140 Shim6 # Shim6 Protocol [RFC5533]
wesp 141 WESP # Wrapped Encapsulating Security Payload
rohc 142 ROHC # Robust Header Compression
iex(nerves@nerves.local)3> :socket.supports()
[
ioctl_requests: [
siftxqlen: true,
sifmtu: true,
sifdstaddr: true,
sifbrdaddr: true,
sifaddr: true,
giftxqlen: true,
gifnetmask: true,
gifname: true,
gifmtu: true,
gifmap: true,
gifindex: true,
gifhwaddr: true,
gifflags: true,
gifdstaddr: true,
gifconf: true,
gifbrdaddr: true,
gifaddr: true,
sifflags: true
],
ioctl_flags: [
staticarp: false,
slave: true,
simplex: false,
renaming: false,
promisc: true,
ppromisc: false,
portsel: true,
pointopoint: true,
oactive: false,
notrailers: true,
noarp: true,
master: true,
lower_up: false,
link2: false,
link1: false,
link0: false,
knowsepoch: false,
echo: false,
dynamic: true,
dying: false,
dormant: false,
cantconfig: false,
automedia: true,
allmulti: true,
nogroup: false,
multicast: true,
up: true,
loopback: true,
broadcast: true,
debug: true,
running: true,
monitor: false
],
options: [
{{:ipv6, :authhdr}, true},
{{:ipv6, :use_min_mtu}, false},
{{:socket, :busy_poll}, false},
{{:IPv6, :pktoptions}, false},
{{:IP, :options}, false},
{{:IP, :transparent}, true},
{{:ipv6, :recvpktinfo}, true},
{{:socket, :acceptconn}, true},
{{:IP, :dontfrag}, false},
{{:IP, :recvif}, false},
{{:IPv6, :router_alert}, true},
{{:ip, :multicast_if}, true},
{{:IPv6, :recvtclass}, true},
{{:ip, :freebind}, true},
{{:socket, :domain}, true},
{{:socket, :reuseaddr}, true},
{{:ipv6, :portrange}, false},
{{:tcp, :maxseg}, true},
{{:IPv6, :drop_membership}, true},
{{:ipv6, :unicast_hops}, true},
{{:IP, :recvdstaddr}, false},
{{:tcp, :info}, false},
{{:IP, :multicast_if}, true},
{{:IP, :hdrincl}, true},
{{:ipv6, :faith}, false},
{{:IPv6, :tclass}, true},
{{:ipv6, :rthdr}, true},
{{:TCP, :maxseg}, true},
{{:ip, :sendsrcaddr}, false},
{{:ipv6, :addrform}, true},
{{:socket, :rxq_ovfl}, false},
{{:ipv6, :flowinfo}, false},
{{:ip, :dontfrag}, false},
{{:socket, :dontroute}, true},
{{:IPv6, :recvpktinfo}, true},
{{:IP, :tos}, true},
{{:ipv6, :recvhoplimit}, true},
{{:ipv6, :add_membership}, true},
{{:IPv6, :checksum}, false},
{{:IPv6, :rthdr}, true},
{{:IPv6, :dstopts}, true},
{{:ipv6, :multicast_if}, true},
{{:socket, :priority}, true},
{{:IPv6, :v6only}, true},
{{:ipv6, ...}, true},
{{...}, ...},
{...},
...
],
msg_flags: [
peek: true,
oob: true,
nosignal: true,
errqueue: true,
eor: true,
ctrunc: true,
confirm: true,
cmsg_cloexec: true,
dontroute: true,
trunc: true,
more: true
],
protocols: [
DCCP: true,
hopopt: true,
egp: true,
icmp: true,
EGP: true,
wesp: true,
udp: true,
RSPF: true,
HOPOPT: true,
skip: true,
CPHB: true,
vmtp: true,
"IPv6-Opts": true,
ROHC: true,
"mpls-in-ip": true,
RDP: true,
fc: true,
IPv6: true,
PUP: true,
PIM: true,
ipcomp: true,
IDRP: true,
igp: true,
GGP: true,
L2TP: true,
"IPSEC-ESP": true,
ah: true,
EIGRP: true,
sctp: true,
WESP: true,
"ipv6-icmp": true,
ISIS: true,
UDPLite: true,
"IPv6-NoNxt": true,
idrp: true,
"IPv6-ICMP": true,
ICMP: true,
ST: true,
TCP: true,
vrrp: true,
rsvp: true,
RSVP: true,
ggp: true,
"iso-tp4": true,
DDP: true,
...
],
sctp: false,
ipv6: true,
local: true,
netns: true,
sendfile: true
]
iex(nerves@nerves.local)4> :socket.open(29, :raw)
{:error, :eprotonosupport}
iex(nerves@nerves.local)5> "29 is the value defined for PF_CAN in socket.h"
"29 is the value defined for PF_CAN in socket.h"
iex(nerves@nerves.local)6> :socket.open(29, :raw, :local)
{:error, {:invalid, {:protocol, :local}}}
iex(nerves@nerves.local)7> :socket.open(29, :raw, :ip)
{:error, :eprotonosupport}
iex(nerves@nerves.local)8> :socket.open(29, :raw, :ipv6)
{:error, :einval}
iex(nerves@nerves.local)9> :socket.open(29, :raw, :tcp)
{:error, :eprotonosupport}
iex(nerves@nerves.local)10> :socket.open(29, :raw, :udp)
{:error, :einval}
iex(nerves@nerves.local)11>
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hauleth
So few things:
PF_CANfor you. You need to check your OS and extract respective integer value on your own. I know this can be daunting, but with Python you should be able to do it. I also have looked up that in the SocketCAN project and found that it is equal to29.CAN_RAWprotocol value as well, so you need to use integer value there. Some digging in Linux headers shows that it is value1.Now we need to apply it to our code:
gus
:socket.open(29, :raw, 1)does the trick!I tried so many options for the protocol as the third argument, but didn’t realize that I should be looking at the integer values to replace
CAN_RAW.Thanks for the suggestion!
gus
Any chance you would also be able to help me understand the Erlang errors when trying to bind the socket? In the SocketCAN docs, it shows the following C example:
I can successfully open the socket as per above, but I’ve been struggling as well to get the socket to bind. The Erlang docs show the following for
bind:bind(Socket, Addr) -> ok | {error, Reason}spec for
Addr:I’m pretty certain I need to use
sockaddr_native, and the integer value should be the same asAF_CANas per the C example (value is 29). So I’ve tried the following:The error in question for each:
From my understanding, the
sockaddr_nativeis an Erlang record, which in Elixir is a tagged tuple. The code below seems to confirm that I’ve got that part right.Record.is_record({:sockaddr_native, 29, "can0"}) # trueSome help to enlighten me on what I am missing with this Erlang function call (and more generally, how to better understand how to use Erlang from Elixir) would be helpful, thanks!
Gus
gus
Some more digging:
socket:bind/2seems to callprim_socket:bind/2, which is defined below:Presumably the
enc_sockaddr/1call is throwing the error…jhogberg
#{ ... snip ... }is a map, not a record. Use%{ :family => 29, :addr => address }instead.Note that
addresshas to be a validstruct sockaddr_canin binary form, just as it would appear in memory for thebind(2)call.__kernel_sa_family_tseems to be ashort, so 16 bits. You then have anintwhich is 32 bits, but alignment restrictions most likely mean that it’s most likely preceded by 16 bits of padding. There’s also tail padding to consider but I’m not sure how much that will be in this case.gus
Wow, this got a lot deeper into the weeds than I was expecting/realized! Big thanks for the reply and pointing me in the right direction.
For my future reference, can you clarify what is the difference between records and maps in the Erlang spec syntax? I was getting really tripped up on that part. If I were to guess, it would be that records have the name after the
#but before the braces? And also the=>operator.Map:
#{Field1=>Value1, ..., FieldN=>ValueN}Record:
#Name{Field1=Value1, ..., FieldN=ValueN}I also guess part of my original confusion is that I’m now seeing that map pattern matching syntax is the
:=operator, which is different from Elixir. Maybe that’s why I started to look at records instead.#{family := integer(), addr := binary()}Anyways, I also realized that I was having other syntax issues in the code posted above: I was pattern matching and extracting the individual fields of the socket returned by
:socket.open/3, like so:Incorrect:
{:ok, {:"$socket", sock}} = :socket.open(29, :raw, 1)Correct:
{:ok, sock} = :socket.open(29, :raw, 1)This is where my argument error was coming from. However, you definitely enlightened me that the
addrfor thesockaddr_nativetype was a binary, but not to pass in the"can0"binary, but rather the wholestruct sockaddr_candata!I played with it a little bit, and was able to get it to bind correctly (the difference was the padding, which I guess is not going to be the same on every device depending on struct packing? For debugging, I actually compiled an example C program and printed the
sockaddr_canstruct to see the data to ensure I got it right). The code is below:Remaining issue
If I actually run the code exactly as above, it does not work, I get
{:error, :enodev}from the:socket.bind/2call. I played around with it and discovered that I can only correctly bind and get the return of:okif theifindexis set to0.This is the part I’m confused about - why does
:socket.ioctl(sock, :gifindex, 'can0')return the wrong index? How can I debug this?Again, huge thank you!
gus
Hmm I am realizing that the C example uses
SIOCGIFINDEX, which might be different than the Erlang:gifindexoption:ioctl(s, SIOCGIFINDEX, &ifr);This might be the reason
jhogberg
Pretty much.
It should be the same on every device with the same ABI, so it may differ between x86 and ARM, but should be the same for the same architecture (broadly speaking).
What does it return if you execute the C example? Edit: if it’s the same value and works fine, how does the
struct sockaddr_canlook in memory, byte-by-byte?That’s the one it should be using. Can you check if
config.hcontains#define ESOCK_USE_IFINDEX?(edit: you should be able to find it at
erts/$TARGET/config.hin the source tree)gus
The output is
0x04. One thing to note is that I’ve been using Nerves to test everything Elixir, but Raspbian OS Lite to test this C program, python, plus other things. So I’m not sure if the return will be the same, since the configuration might be different.Happy to check this, but not sure where to look? In my Nerves rpi3 custom image directory? I’m not compiling Erlang myself (that I’m aware of, maybe that’s happening under the Nerves hood)
Thanks!
Gus
jhogberg
I’d be surprised if the ABI is different, so try checking how
struct sockaddr_canlooks in memory if you set all the fields to recognizable values, maybe we’ve missed something when constructing the binary address.Ah, that will make things more difficult.
What happens if you ask for the
gifnameof the returned interface index?