`Dhcp_wire.pkt_into_buf` when the pkt.options is the empty list crashes

[hannesm]

The output is Invalid argument: Invalid options. It originates from the code in buf_of_options:

  match options with
  | [] -> invalid_arg "Invalid options"
  ..

Maybe pkt_into_buf should not call buf_of_options?

The log output of dnsvizor 862efdab914961ab846377c77223b956eeaf2230, charrua 3.1.1

console 2026-03-13T14:37:08-00:00:             |      ___|
console 2026-03-13T14:37:08-00:00:   __|  _ \  |  _ \ __ \
console 2026-03-13T14:37:08-00:00: \__ \ (   | | (   |  ) |
console 2026-03-13T14:37:08-00:00: ____/\___/ _|\___/____/
console 2026-03-13T14:37:08-00:00: Solo5: Bindings version v0.10.0
console 2026-03-13T14:37:08-00:00: Solo5: Memory map: 32 MB addressable:
console 2026-03-13T14:37:08-00:00: Solo5:   reserved @ (0x0 - 0xfffff)
console 2026-03-13T14:37:08-00:00: Solo5:       text @ (0x100000 - 0x4e5fff)
console 2026-03-13T14:37:08-00:00: Solo5:     rodata @ (0x4e6000 - 0x674fff)
console 2026-03-13T14:37:08-00:00: Solo5:       data @ (0x675000 - 0xa13fff)
console 2026-03-13T14:37:08-00:00: Solo5:       heap >= 0xa14000 < stack < 0x2000000
console 2026-03-13T14:37:08-00:00: 2026-03-13T14:37:08-00:00: [INFO] [netif] Plugging into service with mac 00:80:41:65:77:07 mtu 1500
console 2026-03-13T14:37:08-00:00: 2026-03-13T14:37:08-00:00: [INFO] [ethernet] Connected Ethernet interface 00:80:41:65:77:07
console 2026-03-13T14:37:08-00:00: 2026-03-13T14:37:08-00:00: [INFO] [ARP] Sending gratuitous ARP for 10.0.42.2 (00:80:41:65:77:07)
console 2026-03-13T14:37:08-00:00: 2026-03-13T14:37:08-00:00: [INFO] [ARP] Sending gratuitous ARP for 10.0.42.2 (00:80:41:65:77:07)
console 2026-03-13T14:37:08-00:00: 2026-03-13T14:37:08-00:00: [INFO] [ipv6] IP6: Starting
console 2026-03-13T14:37:08-00:00: 2026-03-13T14:37:08-00:00: [INFO] [application] Sending reply src MAC 00:80:41:65:77:07 dst MAC ff:ff:ff:ff:ff:ff
console 2026-03-13T14:37:08-00:00: src IP 10.0.42.2 dst IP 255.255.255.255
console 2026-03-13T14:37:08-00:00: src port 67 dst port 68
console 2026-03-13T14:37:08-00:00: operation BOOT REPLY
console 2026-03-13T14:37:08-00:00: htype Ethernet 10MB hlen 6 hops 0
console 2026-03-13T14:37:08-00:00: XID 1362796236 secs 0 flags Broadcast
console 2026-03-13T14:37:08-00:00: ciaddr 0.0.0.0 yiaddr 10.0.42.100
console 2026-03-13T14:37:08-00:00: siaddr 10.0.42.2 giaddr 0.0.0.0 chaddr 00:80:41:70:50:75
console 2026-03-13T14:37:08-00:00: sname charrua-dhcp-server file 
console 2026-03-13T14:37:08-00:00: options Message type DHCP OFFER, IP lease time 7200, Renewal T1 3600, Rebinding T2 5760, Server identifier 10.0.42.2, Subnet mask 255.255.255.0, Routers 10.0.42.1
console 2026-03-13T14:37:08-00:00: 2026-03-13T14:37:08-00:00: [INFO] [application] Handing out lease start 2047 end 9247 addr 10.0.42.100 client id MAC 00:80:41:70:50:75, received options Parameter request Subnet mask, Routers, Message type DHCP REQUEST, Request IP 10.0.42.100, Server identifier 10.0.42.2
console 2026-03-13T14:37:08-00:00: 2026-03-13T14:37:08-00:00: [INFO] [application] Sending reply src MAC 00:80:41:65:77:07 dst MAC ff:ff:ff:ff:ff:ff
console 2026-03-13T14:37:08-00:00: src IP 10.0.42.2 dst IP 255.255.255.255
console 2026-03-13T14:37:08-00:00: src port 67 dst port 68
console 2026-03-13T14:37:08-00:00: operation BOOT REPLY
console 2026-03-13T14:37:08-00:00: htype Ethernet 10MB hlen 6 hops 0
console 2026-03-13T14:37:08-00:00: XID 1362796236 secs 0 flags Broadcast
console 2026-03-13T14:37:08-00:00: ciaddr 0.0.0.0 yiaddr 10.0.42.100
console 2026-03-13T14:37:08-00:00: siaddr 10.0.42.2 giaddr 0.0.0.0 chaddr 00:80:41:70:50:75
console 2026-03-13T14:37:08-00:00: sname charrua-dhcp-server file 
console 2026-03-13T14:37:08-00:00: options 
console 2026-03-13T14:37:08-00:00: Fatal error: exception Invalid_argument("Invalid options")

It may as well be a DNSvizor that slightly early does some DHCP processing (which is a separate issue) -- but I think that charrua raising an exception isn't good either.

[palainp]

Hello @hannesm , returning Cstruct.empty will avoid that exception, and then in pkt_info_buf: let partial_len = Cstruct.length dhcp - Cstruct.length options_end in will equal Cstruct.length dhcp, but I don't really know if that makes sense for the definition of buf_end the line immediately after :(
And I don't know if there is another codebase that makes use of buf_of_options, grep.app tells me no: https://grep.app/search?q=buf_of_options

[reynir]

I don't have the RFC in my head, but I suspect the check is valid enough if it's DHCP. At least, I expect a Message_type telling if it's DCHPOFFER, DHCPACK or DHCPNAK. So I'm curious how we end up in a situation where we try to send back zero options.

[hannesm]

I ran into this issue again today, so either dnsvizor needs something or we need to have charrua dealing with the situation... the failing unikernel:

console 2026-04-09T10:56:58-00:00: Solo5:       heap >= 0xa14000 < stack < 0x2000000
console 2026-04-09T10:56:58-00:00: 2026-04-09T10:56:58-00:00: [INFO] [netif] Plugging into service with mac 00:80:41:65:77:07 mtu 1500
console 2026-04-09T10:56:58-00:00: 2026-04-09T10:56:58-00:00: [INFO] [ethernet] Connected Ethernet interface 00:80:41:65:77:07
console 2026-04-09T10:56:58-00:00: 2026-04-09T10:56:58-00:00: [INFO] [ARP] Sending gratuitous ARP for 10.0.42.2 (00:80:41:65:77:07)
console 2026-04-09T10:56:58-00:00: 2026-04-09T10:56:58-00:00: [INFO] [ARP] Sending gratuitous ARP for 10.0.42.2 (00:80:41:65:77:07)
console 2026-04-09T10:56:58-00:00: 2026-04-09T10:56:58-00:00: [INFO] [ipv6] IP6: Starting
console 2026-04-09T10:56:59-00:00: 2026-04-09T10:56:59-00:00: [INFO] [application] Sending reply src MAC 00:80:41:65:77:07 dst MAC ff:ff:ff:ff:ff:ff
console 2026-04-09T10:56:59-00:00: src IP 10.0.42.2 dst IP 255.255.255.255
...
console 2026-04-09T10:56:59-00:00: sname charrua-dhcp-server file 
console 2026-04-09T10:56:59-00:00: options 
console 2026-04-09T10:56:59-00:00: Fatal error: exception Invalid_argument("Invalid options")
console 2026-04-09T10:56:59-00:00: Raised at Stdlib.invalid_arg in file "stdlib.ml", line 30, characters 20-45
console 2026-04-09T10:56:59-00:00: Called from Dhcp_wire.pkt_into_buf in file "duniverse/charrua/lib/dhcp_wire.ml", line 1441, characters 20-60
console 2026-04-09T10:56:59-00:00: Called from Netif.write_pure in file "duniverse/mirage-net-solo5/src/netif.ml", line 158, characters 12-20
console 2026-04-09T10:56:59-00:00: Called from Netif.write in file "duniverse/mirage-net-solo5/src/netif.ml", line 170, characters 36-61
console 2026-04-09T10:56:59-00:00: Called from Dune__exe__Unikernel.Net.handle_dhcp.(fun) in file "unikernel.ml", line 554, characters 16-150
console 2026-04-09T10:56:59-00:00: Called from Lwt.Concurrent_composition.async in file "duniverse/lwt/src/core/lwt.ml", line 2524, characters 10-14
console 2026-04-09T10:56:59-00:00: Solo5: solo5_exit(2) called

The working unikernel (same image):

console 2026-04-09T10:57:12-00:00: 2026-04-09T10:57:12-00:00: [INFO] [ethernet] Connected Ethernet interface 00:80:41:65:77:07
console 2026-04-09T10:57:12-00:00: 2026-04-09T10:57:12-00:00: [INFO] [ARP] Sending gratuitous ARP for 10.0.42.2 (00:80:41:65:77:07)
console 2026-04-09T10:57:12-00:00: 2026-04-09T10:57:12-00:00: [INFO] [ARP] Sending gratuitous ARP for 10.0.42.2 (00:80:41:65:77:07)
console 2026-04-09T10:57:12-00:00: 2026-04-09T10:57:12-00:00: [INFO] [ipv6] IP6: Starting
console 2026-04-09T10:57:13-00:00: 2026-04-09T10:57:13-00:00: [INFO] [ipv6] IP6: Started with fe80::280:41ff:fe65:7707
console 2026-04-09T10:57:13-00:00: 2026-04-09T10:57:13-00:00: [INFO] [udp] UDP layer connected on 10.0.42.2/24, fe80::280:41ff:fe65:7707/64
console 2026-04-09T10:57:13-00:00: 2026-04-09T10:57:13-00:00: [INFO] [tcp.pcb] TCP layer connected on 10.0.42.2/24, fe80::280:41ff:fe65:7707/64
console 2026-04-09T10:57:13-00:00: 2026-04-09T10:57:13-00:00: [INFO] [tcpip-stack-direct] Dual TCP/IP stack assembled: mac=00:80:41:65:77:07,ip=10.0.42.2/24, fe80::280:41ff:fe65:7707/64
console 2026-04-09T10:57:13-00:00: 2026-04-09T10:57:13-00:00: [WARNING] [happy-eyeballs.mirage] inject was called the 2 times
console 2026-04-09T10:57:13-00:00: 2026-04-09T10:57:13-00:00: [WARNING] [application] No password specified, endpoints requiring authentication won't be accessible.
console 2026-04-09T10:57:13-00:00: 2026-04-09T10:57:13-00:00: [INFO] [application] using a stub resolver, forwarding to udp:141.1.1.1
console 2026-04-09T10:57:13-00:00: 2026-04-09T10:57:13-00:00: [INFO] [application] recording dnsvizor.mirage to 10.0.42.2
console 2026-04-09T10:57:13-00:00: 2026-04-09T10:57:13-00:00: [INFO] [application] recording dnsvizor.mirage to fe80::280:41ff:fe65:7707
console 2026-04-09T10:57:15-00:00: 2026-04-09T10:57:15-00:00: [INFO] [application] Sending reply src MAC 00:80:41:65:77:07 dst MAC ff:ff:ff:ff:ff:ff
console 2026-04-09T10:57:15-00:00: src IP 10.0.42.2 dst IP 255.255.255.255
console 2026-04-09T10:57:15-00:00: src port 67 dst port 68
...

So indeed, there's in the first case already a DHCP request while dnsvizor is not fully initialized.

[reynir]

Are the latter two console outputs also from a DNSvizor instance?

[hannesm]

@reynir yes, and actually from the same DNSvizor, just running at a different time (and there are some other unikernels around which ask for DHCP leases)